environmental Science: BIO_104_

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Fresh Water, Oceans, and Coasts Upon completing this chapter, you will be able to:

➤ Explain water’s importance to people and ecosystems, and describe the distribution of fresh water on Earth ➤ Describe the freshwater, marine, and coastal portions of the interconnected aquatic system ➤ Discuss how we use water and how human activities affect aquatic systems ➤ Assess problems of water supply and propose solutions to address depletion of fresh water ➤ Describe the major classes of water pollution and compare and contrast point sources and non-point sources

of water pollution ➤ Describe legislation in the United States that addresses water quality ➤ Explain how we treat drinking water and wastewater ➤ Review the state of ocean fisheries and reasons for their decline ➤ Evaluate marine protected areas and reserves as innovative solutions

Louisiana’s vanishing coastal wetlands support a diversity of wildlife, such as this Great Egret.

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Louisiana’s coastal wet lands transition from commu- nities of salt-tolerant grasses at the ocean’s edge to fresh- water bald cypress swamps further inland. These eco- systems support a diversity of animals, including ea- gles, pelicans, shrimp, oys- ters, black bears, alligators, and sea turtles. The state’s coastal wetlands also pro- tect cities such as New Or- leans and Baton Rouge from storms. Vegetation in these wetlands acts as a windbreak on strong winds and as a water break on waves com- ing inland from the Gulf.

Louisiana’s millions of acres of coastal wetlands were created over the past 7,000 years as the Missis- sippi River fanned out and deposited its sediments at its delta before emptying into the Gulf of Mexico. The Mississippi River accumulates large quantities of sediment from its 3.2-million-km2 (1.2-million-mi2) watershed (FIGURE 12.1B). Much of this sediment origi- nates from the Missouri River basin that drains Ameri- ca’s agricultural heartland.

The salt marshes in the river’s delta naturally com- pact over time. This compaction lowers the level of

the marsh bottom and sub- merges vegetation under increasingly deeper waters. When waters become too deep, the vegetation dies, and soils are then washed away by the ocean. The natural compaction is off- set, however, by inputs of sediments from the river and from the deposition of organic matter from marsh grasses. These additions keep soil levels high, water depths relatively stable, and vegetation healthy.

So why are Louisiana’s wetlands being swallowed by the sea? It’s because people have modified the Mississippi River so extensively that its sediments no longer reach the wetlands that need them. The river’s basin contains roughly 2,000 dams, which slow river flow and allow sediments suspended in the water to settle in reservoirs. This not only prevents sediments from reaching the river’s delta, but also slowly fills in each dam’s reservoir, decreasing its volume and shortening its life span. Therefore, dams in Minne- sota and other locations throughout the Mississippi basin affect the Louisiana coastline hundreds of miles downriver.

CENTRAL CASE STUDY

Starving the Louisiana Coast of Sediment “The Louisiana and Mississippi coastal region is critical to the economic, cultural, and environmental

integrity of the nation.” —Nancy Sutley, Chair of the White House Council on Environmental Quality

“What really screwed up the marsh is when they put the levees on the river. They should take the levees out and let the water run; that’s what built the land.” —Frank “Blackie” Campo, Resident of Shell Beach, Louisiana

T he state of Louisiana is shrinking. Its coastal wetlands straddle the boundary between

the land and the ocean, and these wetlands are disappearing beneath the waters of the

Gulf of Mexico. Louisiana loses 65 km2 (25 mi2) of coastal wetlands each year, and com-

parisons of wetland area from the mid-1800s to the early 1990s show drastic losses (FIGURE 12.1A).

Since the 1930s alone, Louisiana has lost nearly 4,900 km2 (1,900 mi2) of coastal wetlands—an

area roughly the size of Delaware.

NORTH AMERICA

Gulf of Mexico

SOUTH AMERICA

Pacific Ocean

Atlantic Ocean

Mississippi River Louisiana

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exploration. These canals fragment the wetlands and increase erosion rates. They also enable salty ocean water to penetrate inland and damage freshwater marshes. The 2010 Deepwater Horizon oil spill in the Gulf of Mexico (pp. 326–327) also affected Louisiana’s marshes. Oil from the spill washed into coastal wet- lands, coating marsh grasses and impairing their ability to secure oxygen.

Proposed solutions for coastal erosion center on restoring the system to its natural state by diverting large quantities of water from the Mississippi River into coastal wetlands. Proponents of this approach point to the Atchafalaya River, which currently drains one- third of the Mississippi River’s volume. The Atchafalaya delta, fed by this water and sediment, is gaining land area of healthy coastal wetlands. In March 2010, the Obama administration announced the creation of a

The Mississippi River is also lined with thousands of miles of levees. These structures prevent small- scale flooding, and the mouth of the Mississippi is lined with levees to provide a deep river channel for shipping into the Gulf of Mexico. These levees pre- vent the river from fanning out into its delta and turn the lower Mississippi into a “barrel” that shoots sedi- ments off the continental shelf into the deep waters of the Gulf (FIGURE 12.1C).

Although Louisiana’s economy has benefited from oil and gas extraction, these activities have also pro- moted wetland losses. The extraction of large quanti- ties of oil, natural gas, and saline groundwater asso- ciated with oil deposits causes the land to compact, lowering soil levels. Additionally, engineers have cut nearly 13,000 km (8,000 mi) of canals through coastal wetlands to facilitate shipping and oil and gas

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Sioux Falls Des Moines

Memphis

St. Louis Louisville

Columbus

Pittsburgh

New Orleans

Oklahoma City

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Chicago

Helena

Mississippi River watershed

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(a) Coastal wetland area in 1839 and 1993 (b) Mississippi River watershed

FIGURE 12.1  The size of Louisiana’s coastal wetlands shrunk substantially (a) from 1839 to 1993 because people modified sediment deposition patterns in the Mississippi River’s delta by con- structing extensive levees along the river and blocking sediments upriver behind dams. The Mississippi River system (b) is the largest in the United States, draining over 40% of the land area of the lower 48 states. A satellite image of south Louisiana (c) shows the brown plumes of sediments being released into the deep waters of the Gulf of Mexico from the Mississippi River (plume on the right) and the Atchafalaya River (plume on the left). (a) Adapted from Environmental Defense Fund.

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tied up in glaciers, icecaps, and underground aquifers, just over 1 part in 10,000 of Earth’s water is easily accessible for human use.

Water is renewed and recycled as it moves through the water cycle (pp. 37–38). Precipitation falling from the sky either sinks into the ground or acts as runoff to form rivers, which carry water to the oceans or large inland lakes. As they flow, rivers can interact with ponds, wetlands, and coastal aquatic ecosystems. Underground aquifers exchange water with rivers, ponds, and lakes through the sediments on the bottoms of these water bodies. The movement of water in the water cycle creates a web of interconnected aquatic systems (FIGURE 12.3) that exchange water, organisms, sediments, pollutants, and other dissolved substances. What happens in one system therefore affects other systems—even those that are far away. Let’s examine the components of this intercon- nected system, beginning with groundwater.

Groundwater plays key roles in the water cycle Some of the precipitation reaching Earth’s land surface in- filtrates the surface and percolates downward through the soil to become groundwater, water beneath the surface held within pores in soil or rock. Groundwater flows slowly be- neath the surface and can remain underground for long peri- ods. Groundwater makes up one-fifth of Earth’s fresh water supply and plays a key role in meeting human water needs.

Groundwater is contained within aquifers: porous, spongelike formations of rock, sand, or gravel that hold wa- ter (p. 38) (FIGURE 12.4). An aquifer’s upper layer, or zone of

“roadmap” for restoring the Gulf Coast that empha- sizes coastal ecosystem restoration, supported by $63 million in funding in 2009–2011. Some residents op- pose water diversions from the Mississippi River, how- ever, because they do not wish to see the land they own submerged by floodwaters. Residents also fear contamination of local water supplies from the pollut- ants carried by the Mississippi River.

Given the conflicting demands we put on wa- terways for water withdrawal, shipping, and flood control, there are no easy solutions to the problems faced in the Mississippi River and southern Louisiana. But how we tackle problems like those in Louisiana’s coastal wetlands will help determine the long-term sustainability of our most precious natural resource— water. ■

FRESHWATER SYSTEMS “Water, water, everywhere, nor any drop to drink.” The well- known line from the poem The Rime of the Ancient Mariner describes the situation on our planet well. Water may seem abundant, but water that we can drink is quite rare and lim- ited (FIGURE 12.2). About 97.5% of Earth’s water resides in the oceans and is too salty to drink or to use to water crops. Only 2.5% is considered fresh water, water that is relatively pure with few dissolved salts. Because most fresh water is

Surface fresh water Fresh water

All water

Fresh water (2.5%) Surface fresh water (1%) Water within organisms (1%)

Rivers (1%)

Atmospheric water vapor (8%)

Soil moisture (38%)

Lakes (52%)

Oceans (97.5%)

Groundwater (20%)

Ice caps and glaciers (79%)

FIGURE 12.2  Only 2.5% of Earth’s water is fresh water. Of that 2.5%, most is tied up in glaciers and ice caps. Of the 1% that is surface water, most is in lakes and soil moisture. Data from United Nations Environment Programme (UNEP) and World Resources Institute.

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Groundwater flowing into ocean

Dam blocks river flows and traps sediments in reservoir

Urban and industrial pollutants

Agricultural pollutants and eroded soil

Water withdrawals for irrigation reduce river flow

Levees facilitate shipping but prevent deposition of river sediments to coastal wetlands

Ocean

River

Reservoir

Dam

Salt marsh

Freshwater wetlands

Groundwater springs feed river water

Pesticides and fertilizer enter groundwater and surface water

FIGURE 12.3 Water flows through freshwater systems and marine and coastal aquatic systems that interact exten- sively with one another. People affect the components of the system by constructing dams and levees, withdraw- ing water for human use, and introducing pollutants. Because the systems are closely connected, these impacts can cascade through the system and cause effects far from where they originated.

Artesian well

Aquifer recharge area

Water table

Ground- water

Unconfined aquifer

Upper confining layer (clay)

Confined aquifer

Lower confining layer (clay)

Spring

Well

FIGURE 12.4  Groundwater may occur in unconfined aquifers above impermeable layers or in confined aquifers under pressure between impermeable layers. Water may rise naturally to the surface at springs and through the wells we dig. Artesian wells tap into confined aquifers to mine water under pressure.

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streams f lowing or wetlands moist when surface conditions are otherwise dry. Each day in the United States, 1.9 trillion L (492 billion gal) of groundwater are released into surface waters—nearly as much as the daily f low of the Mississippi River.

Water that falls from the sky as rain, emerges from springs, or melts from snow or a glacier and then flows over the land surface is called runoff. Runoff converges as it flows downhill and forms streams. These small watercourses may merge into rivers, whose water eventually reaches a lake or ocean. A smaller river flowing into a larger one is called a tributary. The area of land drained by a river and all its tribu- taries is the river’s watershed (p. 21). If you could trace ev- ery drop of water in the Mississippi River back to the spot it first fell as precipitation, you would have delineated the river’s watershed.

Landscapes determine where rivers flow, but rivers shape the landscapes through which they run. Over thousands or millions of years, a meandering river may shift from one course to another, back and forth over a large area, carving out a flat valley (FIGURE 12.6). Areas nearest a river’s course that are flooded periodically are said to be within the river’s floodplain. Frequent deposition of silt from flooding makes floodplain soils especially fertile. As a result, agriculture thrives in floodplains, and riparian (riverside) forests are pro- ductive and species-rich. A river’s meandering is often driven by large-scale flooding events that scour new channels dur- ing periods of high flow. However, extensive damming on the Mississippi and other rivers has reduced the rate of river me- andering 66–83% from its historic rate. This occurs because floodwaters are trapped by dams and held in reservoirs rather than coursing down the river.

Rivers and streams host diverse biological communi- ties. Algae and detritus (p. 68) support many types of inver- tebrates, from water beetles to crayfish. Fish and amphib- ians consume aquatic invertebrates and plants, and birds such as kingfishers, herons, and ospreys dine on fish and amphibians.

aeration, contains pore spaces partly filled with water. In the lower layer, or zone of saturation, the spaces are completely filled with water. The boundary between these two zones is the water table.

The largest known aquifer is the Ogallala Aquifer, which underlies the Great Plains of the United States (FIGURE 12.5). Water from this massive aquifer has en- abled American farmers to create the most bountiful grain- producing region in the world. However, unsustainable water withdrawals are threatening the long-term use of the aquifer for agriculture.

Surface water converges in river and stream ecosystems Surface water, liquid fresh water located atop Earth’s sur- face, accounts for just 1% of fresh water, but it is vital for our survival and for the planet’s ecological systems. Groundwa- ter and surface water interact, and water can f low from one type of system to the other. Surface water becomes ground- water by infiltration. Groundwater becomes surface water through springs (and human-drilled wells), often keeping

Wyoming South Dakota

Texas

Saturated thickness, in feet

Miles

Kilometers

Colorado

Kansas

Nebraska

New Mexico Oklahoma

0 100

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0–100 100–400 400–800 800–1,200

Ogallala Aquifer

FIGURE 12.5  The Ogallala Aquifer is the world’s largest aquifer, and it held 3,700 km3 (881 mi3) of water before pumping began. This aquifer underlies 453,000 km2 (175,000 mi2) of the Great Plains beneath eight U.S. states. Overpumping for irrigation is currently reducing the volume and extent of this aquifer.

FIGURE 12.6  Rivers, such as the Wood River in Alaska shown here, shape the landscapes through which they flow.

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light, so it has no photosynthetic life and has lower levels of dissolved oxygen than upper waters.

Ponds and lakes change over time as streams and run- off bring them sediment and nutrients. Oligotrophic lakes and ponds, which are low in nutrients and high in oxygen, may slowly give way to the high-nutrient, low-oxygen conditions of eutrophic water bodies (p. 25). Eventually, water bodies may fill in completely by the process of aquatic succession (p. 74). These changes occur naturally, but eutrophication can also result from human-caused nutrient pollution, as is hap- pening in the Chesapeake Bay (pp. 23–25).

Freshwater wetlands include marshes, swamps, bogs, and vernal pools Wetlands are systems in which the soil is saturated with wa- ter, and they generally feature shallow standing water with ample vegetation. There are many types of freshwater wet- lands, and most are enormously rich and productive. In fresh- water marshes, shallow water allows plants such as cattails and bulrushes to grow above the water surface. Swamps also consist of shallow water rich in vegetation, but they occur in

Lakes and ponds are ecologically diverse systems Lakes and ponds are bodies of standing surface water. The largest lakes, such as North America’s Great Lakes, are some- times known as inland seas. Although lakes and ponds can vary greatly in size, scientists have described several zones common to these waters (FIGURE 12.7).

Around the nutrient-rich edges of a water body, the wa- ter is shallow enough that aquatic plants grow from the mud and reach above the water’s surface. This region, named the littoral zone, abounds in invertebrates—such as insect lar- vae, snails, and crayfish—on which fish, birds, turtles, and amphibians feed. The benthic zone extends along the bottom of the lake or pond, from shore to the deepest point, and is home to many invertebrates. In the open portion of a lake or pond, far from shore, sunlight penetrates shallow waters of the limnetic zone. Because light enables photosynthesis (p. 30), the limnetic zone supports phytoplankton (algae, protists, and cyanobacteria; p. 21), which in turn support zooplankton (p. 25), both of which are eaten by fish. The open water below the limnetic zone does not receive sun-

Limnetic zone

Littoral zone

Benthic zone

Sunlight

FIGURE 12.7  In lakes and ponds, emergent plants grow along the shoreline in the littoral zone. The limnetic zone is the layer of open, sunlit water where photosynthesis takes place. The benthic zone, at the bottom of the water body, often is muddy, rich in detritus and nutrients, and low in oxygen.

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The physical makeup of the ocean is complex The world’s five major oceans—Pacific, Atlantic, Indian, Arctic, and Antarctic—are all connected, comprising a single vast body of water that covers 71% of Earth’s surface. Ocean water contains roughly 96.5% H2O by mass; most of the re- mainder consists of ions from dissolved salts. Ocean water is salty primarily because rivers and winds carry sediment and salts from the continents into the ocean. Evaporation then re- moves pure water, leaving a higher concentration of salts. If we were able to evaporate all the water from the oceans, the world’s ocean basins would be covered with a layer of dried salt 63 m (207 ft) thick.

Surface waters of the oceans are warmer than subsurface waters because the sun heats them and because warmer wa- ter is less dense. Deep below the surface, water is dense and sluggish, unaffected by winds and storms, sunlight, and daily temperature fluctuations. Ocean water travels in currents, vast riverlike flows that move in the upper 400 m (1,300 ft) of water, horizontally and for great distances (FIGURE 12.9). Wind, solar heating and cooling, gravity, density differences, and the Coriolis effect (pp. 281–282) drive the global system of ocean currents.

Surface winds and heating also create vertical currents in seawater. Upwelling is the rising of cold, dense water toward the surface. Because this water is rich in nutrients from the bottom, upwellings often support high primary productivity (p. 32) and lucrative fisheries. At downwellings, warm sur- face water rich in dissolved gases is displaced downward, pro- viding an influx of oxygen for deep-water life.

Although oceans are depicted on most maps and globes as smooth swaths of blue, parts of the ocean floor are rugged and complex (FIGURE 12.10). Underwater volcanoes shoot forth enough magma to build islands above sea level, such as the Hawaiian Islands (see Figure 11.7, p. 232). Steep canyons similar in scale to Arizona’s Grand Canyon lie just offshore of some continents. The deepest spot in the oceans—the Mariana Trench in the South Pacific— is deeper than Mount Everest is high, by over a mile. Our planet’s longest mountain range is under water—the Mid-Atlantic Ridge (pp. 228–229) runs the length of the Atlantic Ocean.

Some ocean regions support more life than others. The uppermost 10 m (33 ft) of ocean water absorbs 80% of the solar energy that reaches its surface. For this reason, nearly all of the oceans’ primary productivity occurs in the well-lit top layer, or photic zone. Generally, the warm, shallow waters of continental shelves are most biologically productive and sup- port the greatest species diversity. Habitats and ecosystems occurring between the ocean’s surface and floor are classified as pelagic, whereas those that occur on the ocean floor are classified as benthic.

Currents affect climate The horizontal and vertical movements of ocean water can have far-reaching effects on climate globally and regionally. The thermohaline circulation is a worldwide current system in which warmer, fresher water moves along the surface and colder, saltier water (which is denser) moves deep beneath

forested areas (FIGURE 12.8). Bogs are ponds covered with thick floating mats of vegetation and can represent a stage in aquatic succession. Vernal pools are seasonal wetlands that form in early spring from rain and snowmelt and dry up once weather becomes warmer.

Wetlands are extremely valuable habitat for wildlife. Lou- isiana’s coastal wetlands, for example, provide habitat for ap- proximately 1.8 million migratory waterbirds each year. Wet- lands also provide important ecosystem services by slowing runoff, reducing flooding, recharging aquifers, and filtering pollutants. Cypress swamps in coastal Louisiana are home to many rare species that suffer habitat loss when cypress trees are ground up to produce cypress mulch for landscaping. Environmental groups lobbied major home-improvement retailers to cease selling cypress mulch from Louisiana, and in 2007 Walmart announced it would no longer buy or sell cypress mulch from the state. Other major retailers have yet to join Walmart, however, so Louisiana’s cypress forests remain threatened.

Despite the vital roles played by wetlands, people have drained and filled them extensively for agriculture. Many wetlands are lost when people divert and withdraw water, channelize rivers, and build dams. Southern Canada and the United States, for example, have lost well over half their wet- lands since European colonization.

THE OCEANS The oceans are an important component of Earth’s intercon- nected aquatic systems. The vast majority of rivers empty into oceans (a small number of rivers empty into inland seas), so the oceans receive most of the inputs of water, sediments, pollutants, and organisms carried by freshwater systems. The oceans touch and are touched by virtually every environmen- tal system and every human endeavor. Even if you live in a landlocked region far from the coast, the oceans affect you. They provide fish for people to eat in Iowa, they supply oil for cars in New Mexico, and they influence the weather in Tennessee.

FIGURE 12.8  Freshwater wetlands such as this bald cypress swamp in Louisiana support biologically diverse and productive ecosystems.

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Antarctic Circumpolar

South Equatorial

North Equatorial CC North Equatorial CC

South Equatorial

Br az

M al

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Benguela

Agu tha

Azores Gulf Str

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Caribbean

North Brazil

North Equatorial

North Atlantic

North Equatorial

Kuroshio

Monsoon

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South E quator

ial

Ala ska

California

Lo op

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/C hi

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Antarctic CircumpolarAntarctic Circumpolar

FIGURE 12.9  The upper waters of the oceans flow in surface currents, long-lasting and predictable global patterns of water movement. Warm- and cold-water currents interact with the planet’s climate system, and people have used them for centuries to navigate the oceans. Source: Adapted from Rick Lumpkin (NOAA/AOML).

Sediment

Continental shelf

Continental slope

Continental rise

Oceanic ridge

Trench

Volcanic island arc

Shelf-slope break

FIGURE 12.10  A stylized bathymetric profile shows key geologic features of the submarine environment. Shallow water exists around the edges of continents over the continental shelf, which drops off at the shelf-slope break. The steep continen- tal slope gives way to the more gradual continental rise, all of which are underlain by sediments from the continents. Vast ar- eas of seafloor are flat abyssal plain. Sea- floor spreading occurs at oceanic ridges, and oceanic crust is subducted in trenches (p. 229). Volcanic activity along trenches may give rise to island chains such as the Aleutian Islands. Features on the left side of this diagram are more characteristic of the Atlantic Ocean, and features on the right side of the diagram are more charac- teristic of the Pacific Ocean. Adapted from Thurman, H.V., 1990. Essentials of oceanography,

4th ed. New York: Macmillan.

the surface (FIGURE 12.11). One segment of this worldwide conveyor-belt system is the warm surface water in the Gulf Stream that flows across the Atlantic Ocean to Europe. As this water releases heat to the air, keeping Europe warmer than it would otherwise be, the water cools, becomes saltier through evaporation, becomes denser, and sinks. This creates a region of downwelling known as the North Atlantic Deep Water (NADW).

Scientists hypothesize that interrupting the thermohaline circulation could trigger rapid climate change. If global warm- ing (Chapter 14) causes much of Greenland’s ice sheet to melt, the resulting freshwater runoff into the North Atlantic would make surface waters less dense (because fresh water is less dense than salt water). This could potentially stop the NADW formation and shut down the northward flow of warm wa- ter, causing Europe to cool rapidly. Some data suggest that the

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thermohaline circulation in this region is already slowing, but other researchers maintain that Greenland will not produce enough runoff to cause a shutdown this century.

Another interaction between ocean currents and the atmosphere that influences climate is the El Niño–Southern Oscillation (ENSO), a systematic shift in atmospheric pressure, sea surface temperature, and ocean circulation in the tropical Pacific Ocean. Under normal conditions, prevailing winds blow from east to west along the equator, from a region of high pres- sure in the eastern Pacific to one of low pressure in the western Pacific, forming a large-scale convective loop in the atmosphere (FIGURE 12.12A). The winds push surface waters westward, causing water to “pile up” in the western Pacific. As a result, wa- ter near Indonesia can be 50 cm (20 in.) higher and 8 °C warmer than water near South America. The westward-moving surface waters allow cold water to rise up from the deep in a nutrient- rich upwelling along the coast of Peru and Ecuador.

El Niño conditions are triggered when air pressure decreases in the eastern Pacific and increases in the western Pacific, weakening the equatorial winds and allowing the warm water to flow eastward (FIGURE 12.12B). This sup- presses upwelling along the Pacific coast of the Americas, shutting down the delivery of nutrients that support marine life and fisheries. Coastal industries such as Peru’s anchovy fisheries are devastated by El Niño events, and the 1982–1983 El Niño caused over $8 billion in economic losses worldwide. El Niño events alter weather patterns around the world, cre- ating rainstorms and floods in areas that are generally dry (such as southern California) and causing drought and fire in regions that are typically moist (such as Indonesia).

La Niña events are the opposite of El Niño events; in a La Niña event, cold waters rise to the surface and extend westward in the equatorial Pacific when winds blowing to the west strengthen, and weather patterns are affected in opposite ways. ENSO cycles are periodic but irregular, occurring every 2–8 years. Scientists are exploring whether globally warming air and sea temperatures (Chapter 14) may be increasing the frequency and strength of these cycles.

Solar warming of ocean waters

Equator

Pacific Ocean

Atlantic Ocean

Sea-to-air heat transfer forms NADW

Europe

Greenland

Warm surfa ce

cur ren

Cold de ep current

FIGURE 12.11  As part of the oceans’ thermohaline circulation, warm surface currents carry heat from equatorial waters northward toward Europe, where they warm the atmosphere and then cool and sink, forming the North Atlantic Deep Water (NADW). Scientists de- bate whether rapid melting of Greenland’s ice sheet could interrupt this heat flow and cause Europe to cool dramatically.

Convective loop

WindsEquator

Equator

Indonesia

Movement of water

Increased convection

Movement of water

Peru

(a) Normal conditions

(b) El Niño conditions

Winds

Pacific Ocean

Upwelling of deep, cold water

Peru

Deep, cold water stays below surface

Equator

Indonesia

FIGURE 12.12 ▲ In these diagrams, red and orange colors denote warmer water, and blue and green colors denote colder water. Under normal conditions (a), prevailing winds push warm surface waters toward the western Pacific. Under El Niño conditions (b), winds weaken, and the warm water flows back across the Pacific toward South America, like water sloshing in a bathtub. This shuts down upwelling along the American coast and alters precipitation patterns regionally and globally. Adapted from National Oceanic and Atmospheric Administration, Tropical Atmospheric Ocean Project.

MARINE AND COASTAL SYSTEMS With their variation in topography, temperature, salinity, nutrients, and sunlight, marine and coastal environments fea- ture a variety of ecosystems. These systems may not give us the water we need for drinking and growing crops, but they teem with biodiversity and provide many other necessary resources.

Fresh water meets salt water in estuaries Water bodies where rivers f low into the ocean, mixing fresh water with salt water, are called estuaries (FIGURE 12.13). Estuaries are biologically productive ecosystems that expe- rience f luctuations in salinity with the daily and seasonal

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fish and shellfish species. Salt marshes also filter pollution and stabilize shorelines against storm surges.

Mangrove forests line coasts in the tropics and subtropics In tropical and subtropical latitudes, mangrove forests replace salt marshes along the coasts. Mangroves are salt-tolerant, and they have unique roots that curve upward like snorkels to attain oxygen, or that curve downward like stilts to sup- port the tree in changing water levels (FIGURE 12.15). Fish, shellfish, crabs, snakes, and other organisms thrive among the root networks, and birds feed and nest in the dense foliage of these coastal forests. Mangroves protect shorelines from storm surges, filter pollutants, and capture eroded soils, pro- tecting offshore coral reefs. They also provide materials that people use for food, medicine, tools, and construction. Half the world’s mangrove forests have been destroyed as people have developed coastal areas, often for tourist resorts and shrimp farms.

Intertidal zones undergo constant change Where the ocean meets the land, intertidal, or littoral, eco- systems (FIGURE 12.16) spread between the uppermost reach of the high tide and the lowest limit of the low tide. Tides are the periodic rising and falling of the ocean’s height at a given location, caused by the gravitational pull of the moon and sun. Intertidal organisms spend part of each day submerged in water, part of the day exposed to air and sun, and part of the day being lashed by waves.

Life abounds in the crevices of rocky shorelines, which provide shelter and pools of water (tide pools) during low tides. Sessile (stationary) animals such as anemones, mussels, and barnacles live attached to rocks, filter-feeding on plank- ton in the water that washes over them. Urchins, sea slugs, chitons, and limpets eat intertidal algae or scrape food from

variations in tides and freshwater runoff. The shallow water of estuaries nurtures eelgrass beds and other plant life and provides critical habitat for shorebirds and for many com- mercially important shellfish species.

Estuaries everywhere have been affected by coastal devel- opment, water pollution, habitat alteration, and overfishing. The Chesapeake Bay estuary (profiled in Chapter 2) is one such example. Estuaries and other coastal ecosystems have borne the brunt of human impact because two out of every three peo- ple choose to live within 160 km (100 mi) of the ocean.

Salt marshes line temperate shorelines Along many of the world’s coasts at temperate latitudes, salt marshes occur where the tides wash over gently sloping sandy or silty substrates. Rising and falling tides flow into and out of channels called tidal creeks and at highest tide spill over onto elevated marsh flats, like those in coastal Louisiana (FIGURE 12.14). Marsh flats grow thick with salt-tolerant grasses, as well as rushes, shrubs, and other herbaceous plants.

Salt marshes boast very high primary productivity and provide critical habitat for shorebirds, waterfowl, and many

FIGURE 12.13 ▲ Freshwater rivers mix with salt water to form brackish water in estuaries. Elevated inputs of nutrients from river waters make estuaries one of the most productive ecosystem types on Earth.

FIGURE 12.14  Salt marshes occur in temperate intertidal zones where the substrate is muddy. Tidal waters flow in channels called tidal creeks amid flat areas called benches, sometimes partially submerging the salt-adapted grasses.

FIGURE 12.15  Mangrove forests line tropical and subtropical coastlines. Mangrove trees, with their unique roots, are adapted for growing in salt water and provide habitat for many fish, birds, crabs, and other animals.

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with stinging tentacles. They also derive nourishment from symbiotic algae known as zooxanthellae, which inhabit their bodies and produce food through photosynthesis (and pro- vide the diversity of vibrant colors in reefs). Most corals are colonial, and the colorful surface of a coral reef consists of millions of densely packed individuals. As corals die, their skeletons remain part of the reef while new corals grow atop them, increasing the reef ’s size.

Coral reefs protect shorelines by absorbing wave energy. They also host tremendous biodiversity (FIGURE 12.18A). This is because coral reefs provide complex physical structure (and thus many habitats) in shallow nearshore waters, which are

the rocks. Sea stars (starfish) creep slowly along, preying on the filter-feeders and herbivores. Crabs clamber around the rocks, scavenging detritus. The rocky intertidal zone is so di- verse because environmental conditions such as temperature, salinity, and moisture change dramatically from the high to the low reaches.

Kelp forests harbor many organisms Along many temperate coasts, large brown algae, or kelp, grow from the floor of continental shelves, reaching up toward the sunlit surface. Some kelp reaches 60 m (200 ft) in height and can grow 45 cm (18 in.) per day. Dense stands of kelp form un- derwater “forests” (FIGURE 12.17). Kelp forests supply shelter and food for invertebrates and fish, which in turn provide food for predators such as seals and sharks. (Indeed, kelp forests were the setting for our discussion of sea otters as keystone spe- cies in Chapter 4, pp. 71–73). Kelp forests absorb wave energy and protect shorelines from erosion. People eat some types of kelp, and kelp provides compounds that serve as thickeners in cosmetics, paints, ice cream, and other consumer products.

Coral reefs are treasure troves of biodiversity Shallow subtropical and tropical waters are home to coral reefs. A coral reef is a mass of calcium carbonate composed of the skeletons of tiny invertebrate animals known as cor- als. Corals are related to jellyfish and capture passing food

Supratidal zone (splash zone)

Intertidal zone

Subtidal zone

Level of low tide

Level of high tide

FIGURE 12.16  The rocky intertidal zone stretches along rocky shorelines between the lowest and highest reaches of the tides, providing niches for a diversity of organisms. Areas higher on the shoreline are exposed to the air more frequently and for longer periods, so organisms that tolerate exposure best specialize in the upper intertidal zone. The lower intertidal zone is exposed less frequently and for shorter periods, so organisms less tolerant of exposure thrive in this zone.

FIGURE 12.17  “Forests” of tall brown algae known as kelp grow from the floor of the continental shelf. Numerous fish and other creatures eat kelp or find refuge among its fronds.

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regions of high primary productivity. Besides the staggering diversity of anemones, sponges, tubeworms, and other sessile invertebrates, innumerable molluscs, flatworms, and urchins patrol reefs, while thousands of fish species find food and shelter in the reef structure. The beauty and biodiversity of coral reefs makes them valuable ecotourism destinations that provide economic benefit to coastal communities.

Coral reefs are experiencing alarming declines world- wide, however. Nutrient pollution in coastal waters pro- motes the growth of algae, which smothers reefs. Many reefs have undergone “coral bleaching,” a process that occurs when zooxanthellae die or leave the coral, depriving it of nutrition (FIGURE 12.18B). Coral bleaching is thought to re- sult from increased sea surface temperatures associated with global climate change, from the influx of pollutants, from unknown natural causes, or from combinations of these factors.

(a) Coral reef community

(b) Bleached coral

FIGURE 12.18  Corals reefs provide food and shelter for a tremendous diversity of fish and other creatures (a). Today these reefs face multiple stresses from human impacts. Many corals have died as a result of coral bleaching (b), in which corals lose their zooxanthellae. Bleaching is evident in the whitened portion of this coral.

Coral reefs have been affected by climate change in other ways, too. The oceans have soaked up roughly a third of the excess carbon dioxide (CO2) that people have added to the atmosphere so far, and this has slowed the onset of global climate change. However, this excess CO2 has lowered the pH of seawater, a phenomenon called ocean acidification. This causes a series of chemical reactions that reduce the ocean’s concentration of the carbonate ions that coral and other creatures need to build their shells. These reactions also pro- duce molecules that dissolve the calcium carbonate in coral shells. By dissolving shells at an increased rate and decreasing the rate at which new shells are formed, ocean acidification threatens the persistence of coral reefs. Ocean acidification increases along with atmospheric concentrations of CO2. If atmospheric concentrations of CO2 reach 500 parts per mil- lion (ppm) (up from 391 ppm today), very little ocean area will have carbonate ion concentrations sufficient to support coral reefs.

Open-ocean ecosystems vary in their biodiversity Biological diversity in pelagic regions of the open ocean is highly variable in its distribution. Primary production (p. 32) and animal life near the surface are concentrated in regions of nutrient-rich upwelling. Microscopic phytoplank- ton constitute the base of the marine food chain in the pelagic zone and are the prey of zooplankton (p. 25), which in turn become food for fish, jellyfish, whales, and other free-swim- ming animals (FIGURE 12.19). Predators at higher trophic lev- els include larger fish, sea turtles, and sharks.

In the little-known deep-water ecosystems, animals have adapted to tolerate extreme water pressures and to live in the dark without food from autotrophs (p. 30). Many of these of- ten bizarre-looking creatures scavenge carcasses or organic detritus that falls from above. Others are predators, and still

FIGURE 12.19  The uppermost reaches of ocean water con- tain billions upon billions of phytoplankton—tiny photosynthetic algae, protists, and bacteria that form the base of the marine food chain—as well as zooplankton, small animals and protists that dine on phytoplankton and comprise the next trophic level.

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satisfy demands for water supplies, transportation, and flood control. As we have seen in our Central Case Study, dams and channelization in the Mississippi River basin have led to ad- verse impacts at the river’s mouth. What we do in one part of the interconnected aquatic system affects many others, some- times in significant ways.

Fresh water is unevenly distributed across Earth The availability of fresh water varies widely around the world because different regions possess varying amounts of groundwater, surface water, and precipitation. When we compare human populations with fresh water supplies on a map, we find that people are not distributed across the globe in accordance with the amount of fresh water (FIGURE 12.20). Because of the mismatched distribution of water and popu- lation, human societies have always struggled to transport fresh water from its source to where people need it.

Fresh water is distributed unevenly in time as well as space. India’s monsoon storms can dump half of a region’s annual rain in just a few hours, for example. Rivers have sea- sonal differences in flow. For this reason, dams store water from wetter months that can be used in drier times of the year when river flow is reduced.

As if the existing mismatches between water availabil- ity and human need were not enough, global climate change (Chapter 14) may worsen conditions in many regions by altering precipitation patterns, melting glaciers, causing early season runoff, and intensifying droughts and flooding.

others attain food from mutualistic (p. 68) bacteria. Ecosys- tems also form around hydrothermal vents, where heated water spurts from the seafloor, carrying minerals that precipi- tate to form rocky structures. Tubeworms, shrimp, and other creatures in these recently discovered deepwater systems use symbiotic bacteria to derive their energy from chemicals in the heated water rather than from sunlight.

Aquatic systems are affected by human activities Our tour of aquatic systems has shown the ecological and economic value of freshwater and marine ecosystems. We will now see how people affect these systems when we with- draw water for human use, build dams and levees, and in- troduce pollutants that alter water’s chemical, biological, and physical properties.

HUMAN ACTIVITIES AFFECT WATERWAYS Although water is a limited resource, it is also a renewable resource as long as we manage our use sustainably. Unfortu- nately, people are withdrawing water at unsustainable levels and are depleting many sources of surface water and ground- water. Already, one-third of the world’s people are affected by water shortages.

Additionally, people have intensively engineered fresh- water waterways with dams, levees, and diversion canals to

Less than 1,000

Available fresh water (cubic meters per capita per year)

1,000-2,000

2,000-5,000

5,000-10,000

10,000-20,000

20,000-100,000

More than 100,000

Insufficient data

Major inland waterway

FIGURE 12.20  Nations vary tremendously in the amount of fresh water per capita available to their citizens. For example, Iceland, Papua New Guinea, Gabon, and Guyana (dark blue in this map) each have over 100 times more water per person than do many Middle Eastern and North African countries. Data from Harrison, P., and F. Pearce, 2000. AAAS atlas of population and the environment, edited by the American Association for the Advancement of Science, © 2000 by

the American Association for the Advancement of Science. Used by permission of the publisher, University of California Press.

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provide drinking water, facilitate irrigation, and generate electricity.

Worldwide, we have erected more than 45,000 large dams (greater than 15 m, or 49 ft, high) across rivers in over 140 nations. We have built tens of thousands of smaller dams. Only a few major rivers in the world remain undammed and free-flowing. These run through the tundra and taiga of Canada, Alaska, and Russia and in remote regions of Latin America and Africa.

Dams produce a mix of benefits and costs, as illustrated in FIGURE 12.22 and as mentioned in our Central Case Study. As an example of this complex mix, consider the world’s larg- est dam: the Three Gorges Dam on China’s Yangtze River, at 186 m (610 ft) high and 2.3 km (1.4 mi) wide. This dam was completed in 2008 (FIGURE 12.23A), and its reservoir stretches for 616 km (385 mi; as long as Lake Superior). This dam will help control the large floods that have historically occurred on the banks of the Yangtze. It will also enable boats and barges to travel farther upstream and will generate enough hydroelectric power to replace dozens of large coal or nuclear plants.

However, the Three Gorges Dam cost $39 billion to build, and its reservoir flooded 22 cities and displaced 1.24 million peo- ple (FIGURE 12.23B). The rising water submerged 10,000-year- old archaeological sites, productive farmlands, and wildlife habitat. Tidal marshes at the Yangtze’s mouth are eroding away, deprived of the sediments that now settle in the dam’s reservoir as the river water slows. Many scientists also worry that the Yang- tze’s many pollutants will be trapped in the reservoir, making the water undrinkable.

People who feel that the costs of some dams outweigh their benefits are pushing for such dams to be dismantled. By removing dams and letting rivers flow free, they say, we can restore ecosystems, reestablish economically valuable fisher- ies, and revive river recreation, such as fly-fishing and rafting. Many aging dams are in need of costly repairs or have outlived their economic usefulness, and these are candidates for re- moval. Some 400 dams have been removed in the United States

Water supplies households, industry, and especially agriculture We all use water at home for drinking, cooking, and clean- ing. Most mining, industrial, and manufacturing processes require water. Farmers and ranchers use water to irrigate crops and water livestock. Globally, we allot about 70% of our annual fresh water use to agriculture. Industry accounts for roughly 20%, and residential and municipal uses for only 10%.

When we remove water from an aquifer or surface water body and do not return it, this is called consumptive use. Our primary consumptive use of water is for agricultural irrigation (p. 145–146). In contrast, nonconsumptive use of water does not remove, or only temporarily removes, water from an aqui- fer or surface water body. Using water to generate electricity at hydroelectric dams is an example of nonconsumptive use; water is taken in, passed through dam machinery to turn tur- bines, and released downstream.

We use so much water in agriculture because our rapid pop- ulation growth requires us to feed and clothe more people each year. Overall, we withdraw 70% more water for irrigation today than we did 50 years ago and have doubled the amount of land under irrigation. Irrigation can more than double crop yields; as a result, the 18% of world farmland that we irrigate yields fully 40% of our produce, including 60% of the global grain crop.

Worldwide, roughly 15–35% of water withdrawals for ir- rigation are thought to be unsustainable. In areas where agri- culture is demanding more fresh water than can be sustain- ably supplied, water mining—withdrawing water faster than it can be replenished—is taking place. In these areas, aquifers are being depleted or surface water is being piped in from other regions.

We build dikes and levees to control floods Flooding is a normal, natural process that occurs when snow- melt or heavy rain swells the volume of water in a river so that water spills over the river’s banks. This process naturally spreads nutrient-rich sediments over the floodplain, benefit- ing natural ecosystems and human agriculture.

As cities and towns grew on the floodplains of the Missis- sippi River and other waterways, however, people tired of the property damage caused by flooding. Communities and gov- ernments constructed dikes and levees (long raised mounds of earth) along riverbanks to hold water in main channels. These structures prevent flooding most of the time, but they can some- times worsen flooding because they force water to stay in chan- nels and accumulate, building up enormous energy and lead- ing to occasional catastrophic overflow events (FIGURE 12.21). Many of the levees along the Mississippi River were constructed after the lower Mississippi River flooded catastrophically in 1927 and the public demanded greater protection from flooding.

We have erected thousands of dams A dam is any obstruction placed in a river or stream to block its f low. Dams create reservoirs, artificial lakes that store water for human use. We build dams to prevent floods,

FIGURE 12.21  Unusually high water levels in the Mississippi River in May 2011 caused this levee in East Carroll Parish, Louisi- ana, to collapse, flooding adjacent farmland. Levees upstream in some less-populated areas were intentionally destroyed and the floodplain inundated in order to lower river volumes and protect more-populated areas downriver.

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Excessive water withdrawals are draining rivers and lakes As a result of our diversions and our consumption, in many places we are withdrawing surface water at unsustainable rates

in the past decade, and more will come down in the next 10 years, when the licenses of over 500 dams come up for renewal.

We divert surface water to suit our needs People have long diverted water from rivers and lakes to farm fields, homes, and cities. The world’s largest diversion pro- ject is underway in China. There, government leaders are pushing through an ambitious plan to pipe water from the Yangtze River in southern China (where water is plentiful) to northern China’s Yellow River, which routinely dries up at its mouth because the climate is drier and people withdraw its water. Three sets of massive aqueducts (human-made river channels), totaling 2,500 km (1,550 mi) in length, are being built to move trillions of gallons of water northward. China’s leaders hope the diversions will solve water shortages for northern farms and cities. However, many scientists say the $62 billion project won’t transfer enough water to satisfy northern China’s water demands, yet it will cause extensive environmental impacts and displace hundreds of thousands of people.

Reliable drinking water if watershed lands are protected

Reliable irrigation for farming

Carbon emissions much lower than power from fossil

fuels

New recreational opportunities on reservoir

Sediment settles behind dams, filling reservoir and not nourishing

downstream floodplains

Small risk of catastrophic failure

Habitat alteration (upstream and downstream)

Lost recreational opportunities on river

Fisheries declines from thermal pollution and blockage of migration

40–80 million people displaced by dam projects

in past 50 years

Disruption of flooding that builds topsoil

Flood control by storing seasonal

surges

Generation of renewable electricity

FIGURE 12.22  Damming rivers has diverse consequences for people and the environment. The generation of clean and renewable electricity is one of several major benefits (green boxes) of hydroelectric dams. Habitat alteration is one of several negative impacts (red boxes).

Reaching for Water Controversial diversions of fresh water occur in the United States as well. The rapidly growing Las Vegas metropolitan area is exceeding its allotment of water from the

Colorado River and has proposed a $3.5-billion project that would divert groundwater from 450 km (280 mi) away to meet the growing demand in Nevada’s largest city. Do you think such diversions are ethically justified? If rural communities and wetland ecosystems at the diver- sion site in eastern Nevada are destroyed by this project, is this an acceptable cost given the economic activity generated in Las Vegas? How else might cities like Las Vegas meet their future water needs?

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Uzbekistan and Kazakhstan. Once the fourth-largest lake on Earth, the Aral Sea lost over four-fifths of its volume in just 45 years (FIGURE 12.24), as water was diverted from the two rivers that feed it, to provide irrigation for cotton farming in this arid region. The shrinking of the Aral Sea has caused the loss of 60,000 fishing jobs, but scientists, engineers, and local people are struggling to save the northern portion of the Aral Sea, and may now have begun reversing its decline.

and drastically reducing flows in rivers. Many major rivers—the Colorado River in North America, the Yellow River in China, and the Nile in Africa—regularly run dry before reaching the sea due to excessive water withdrawals. This reduction in flow not only threatens the future of the cities and farms that depend on these rivers, but also drastically alters the ecology of the riv- ers and their deltas, changing plant communities, wiping out populations of fish and invertebrates, and devastating fisheries.

Nowhere are the effects of surface water depletion so evident as at the Aral Sea, on the border of present-day

(a) The Three Gorges Dam in Yichang, China

(b) Displaced people in Sichuan Province, China

FIGURE 12.23  China’s Three Gorges Dam (a), completed in 2008, is the world’s largest dam. Over 1.2 million people were displaced and whole cities were leveled for its construction, as shown here (b) in Sichuan Province.

(a) Satellite image of Aral Sea, 1987

(b) Satellite image of Aral Sea, 2009

Black Sea Iran

Kazakhstan

Uzbekistan

Turkmenistan

Russia

Caspian Sea

FIGURE 12.24  The Aral Sea in central Asia has been shrinking (a, b) because so much water was withdrawn to irrigate cotton crops. Along its former shorelines, ships lie stranded in the sand (c) because the waters receded so far and so quickly. Today restoration efforts are beginning to reverse the decline in the northern portion of the sea, and waters there are slowly rising.

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cross national borders, and transboundary disagreements are common. Water is already a key element in the disagreements among Israel, the Palestinian people, and neighboring nations.

The United States has its share of conflicts over water. The Colorado River’s water allocations have long been a source of conflict between farms and growing cities in the arid West. The states of Georgia, Alabama, and Florida are also currently embroiled in legal disputes over water withdrawals from shared rivers.

Yet on the positive side, many nations have cooperated to resolve water disputes. India has struck agreements to co-man- age transboundary rivers with Pakistan, Bangladesh, Bhutan, and Nepal. In Europe, nations along the Rhine and Danube rivers have signed water-sharing treaties. Such progress gives reason to hope that water wars will be few and far between.

SOLUTIONS TO THE DEPLETION OF FRESH WATER To address the depletion of fresh water, we can aim either to increase supply or to reduce demand. Increasing water sup- plies by constructing large dams was a common solution to water shortages in the past. However, we have already devel- oped the vast majority of suitable sites for large dams, and most of those that remain are in remote locations in develop- ing regions. Building more dams therefore does not appear to be a viable solution in most locations for meeting people’s demands for fresh water.

Another strategy for increasing water supplies is to gen- erate fresh water by desalination, the removal of salt from sea- water or other water of marginal quality. This can be done by evaporating the salt water and condensing the freshwater vapor that is produced. Unfortunately, desalination is an en- ergy-intensive process that requires large inputs of fossil fuels or electricity. This makes it expensive and limits its intensive use to arid, oil-rich nations like those in the Persian Gulf re- gion. Hence, its ability to increase water supplies in most na- tions is limited.

Groundwater depletion affects people and ecosystems Groundwater is more easily depleted than surface water because most aquifers recharge very slowly. If we compare an aquifer to a bank account, we are making more withdraw- als than deposits, and the balance is shrinking. Today we are mining groundwater, extracting 160 km 3 (5.65 trillion ft 3 ) more water each year than returns to the ground. This is a problem because one-third of Earth’s human population— including 99% of the rural population of the United States— relies on groundwater for its needs.

As aquifers are mined, water tables drop deeper under- ground. This deprives freshwater wetlands of groundwater inputs, causing them to dry up. Groundwater also becomes more difficult and expensive to extract, and eventually it may run out. In parts of Mexico, India, China, and multiple Asian and Middle Eastern nations, water tables are falling 1–3 m (3–10 ft) per year.

When groundwater is overpumped in coastal areas, salt water can intrude into aquifers from the ocean, making water undrinkable. This has occurred in Florida, the Middle East, and other locations. Moreover, as aquifers lose water, they become less able to support overlying strata, and the land surface above may sink or collapse. Mexico City, Venice, Bangkok, and Beijing are slowly sinking, causing streets to buckle and undergound pipes to rupture. Once the ground sinks, soil and rock becomes compacted, losing the porosity that enabled it to hold water. Re- charging a depleted aquifer thereafter becomes more difficult.

Bottled water has ecological costs These days, our groundwater is being withdrawn for a new purpose—to be packaged in plastic bottles and sold on super- market shelves. In 2009 the average American drank over 27 gallons of bottled water, and sales topped $10.5 billion in the United States and $60 billion worldwide.

Most people who buy bottled water do so for portabil- ity and convenience, or because they believe it is superior to tap water. However, in blind taste tests people think tap water tastes just as good, and chemical analyses show that bottled water is no safer or healthier than tap water (see THE SCIENCE BEHIND THE STORY , pp. 268–269 ).

Bottled water also exerts substantial ecological impact. A 2009 study calculated the energy costs of bottled water to be 1,000–2,000 times greater than the energy costs of tap water. Most energy was used in manufacturing the bottle and trans- porting the product. Furthermore, since at least three out of four bottles in the United States are thrown away after use, we must dispose of 30–40 billion containers per year.

Will we see a future of water wars? Population growth, expansion of irrigated agriculture, and industrial development doubled our annual fresh water use between 1960 and 2000. Increased withdrawals of fresh water can lead to shortages, and resource scarcity can lead to con- flict. Many predict that water’s role in regional conflicts will increase as human population continues to grow and as cli- mate change alters precipitation patterns. A total of 261 major rivers (whose watersheds cover 45% of the world’s land area)

FAQ

Q: Can’t we just use desalination to fulfill our demand for water? A: Given the seemingly endless supply of water in Earth’s oceans, many people assume that desalination is the answer to our world’s water crises. So why aren’t we eagerly utilizing this technology everywhere?

Simply put, we lack the abundant, clean energy sources needed to make the widespread use of desalination economically viable and environmentally sustainable. For example, the United States withdraws over 700 billion liters (185 billion gallons) of fresh water every day for use in food production, industry, and public supplies. Diverting the energy necessary to supply even a tiny fraction of this quantity from desalination would cause prices for electricity, gasoline, and other fuels to skyrocket. Using fossil fuels as an energy source for desalination would also drastically increase U.S. emissions of air pollutants and

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recycling treated municipal wastewater for irrigation and industrial uses. Finding and patching leaks in pipes has saved some cities and companies large amounts of water— and money. Boston and its suburbs reduced water demand by 31% over 17 years by patching leaks, retrofitting homes with efficient plumbing, auditing industry, and promoting conservation. This program enabled Massachusetts to avoid an unpopular $500 million river diversion scheme.

WATER POLLUTION AND ITS CONTROL We have seen that people affect aquatic systems by with- drawing too much water and by altering the systems’ natural processes by engineering waterways with dams, diversions, and levees. However, people also affect aquatic ecosystems and threaten human health when we introduce toxic sub- stances and disease-causing organisms into surface waters and groundwater.

Developed nations have made admirable advances in cleaning up water pollution over the past few decades. Still, the World Commission on Water recently concluded that over half the world’s major rivers remain “seriously depleted and polluted, degrading and poisoning the surrounding eco- systems, threatening the health and livelihood of people who depend on them.” The largely invisible pollution of ground- water, meanwhile, has been termed a “covert crisis.”

Water pollution comes from point sources and non-point sources Pollution is the release into the environment of matter or energy that causes undesirable impacts on the health or well- being of people or other organisms. Pollution can be physi- cal, chemical, or biological and can affect water, air, or soil. Water pollution comes in many forms and can cause diverse impacts on aquatic ecosystems and human health.

Most forms of water pollution are not conspicuous to the human eye, so scientists and technicians measure water’s chemical properties (such as pH, nutrient concentrations, and dissolved oxygen concentration), physical characteristics (such as temperature and turbidity—the density of suspended particles in a water sample), and biological properties (such as the presence of harmful microorganisms or the species diver- sity in aquatic ecosystems).

Some water pollution is emitted from point sources— discrete locations, such as a factory, sewer pipe, or oil tanker. In contrast, non-point-source pollution is cumulative, arising from multiple inputs over larger areas, such as farms, city streets, and residential neighborhoods (FIGURE 12.25). The U.S. Clean Water Act (pp. 100–101) addressed point-source pollution with some success by targeting industrial discharges. Water quality in the United States today suffers most from non-point-source pollution resulting from countless common activities, such as applying fertilizers and pesticides to lawns, applying salt to roads in winter, and changing automobile oil. To minimize non-point-source pollution of drinking water, governments limit development on watershed land surround- ing reservoirs.

We can decrease our demand for water Because supply-based strategies do not hold great promise for increasing water supplies, people are increasingly turn- ing to demand-based solutions. Strategies for reducing fresh water demand include conservation and efficiency measures. Such strategies require changes in individual behaviors and can therefore be politically difficult, but they offer better economic returns and cause less ecological and social dam- age. Our existing shift from supply-based to demand-based solutions is already paying dividends. The United States, for example, decreased its water consumption by 5% from 1980 to 2005 thanks to conservation measures, even while its population grew 31%. Let’s examine approaches that can conserve water in agriculture, households, industry, and municipalities.

Agriculture Farmers can improve efficiency by adopt- ing more efficient irrigation methods. “Flood and furrow” irrigation, in which fields are liberally f looded with water, accounts for 90% of irrigation worldwide. However, crop plants end up using only 40% of the water applied, and other methods are far more efficient. Low-pressure spray irriga- tion squirts water downward toward plants, and drip irriga- tion systems target individual plants and introduce water directly onto the soil (pp. 145-146). Experts estimate that drip irrigation (in which as little as 10% of water is wasted) could cut water use in half while raising yields by 20–90% and producing $3 billion in extra annual income for farmers of the developing world.

Choosing crops to match the land and climate in which they are farmed can also save huge amounts of water. Elimi- nating the planting of crops that require a great deal of water (such as cotton, rice, and alfalfa) in arid areas with govern- ment-subsidized irrigation could greatly reduce water use. Biotechnology may also play a role by producing crop variet- ies that require less water through selective breeding (p. 48) and genetic modification (pp. 149–152).

Households In our residences, we can reduce water use by installing low-flow faucets, showerheads, washing machines, and toilets. High-efficiency toilets and showerheads provide the biggest savings because these are typically the two largest indoor uses of water. Outdoor water use can be minimized by catching and storing rain runoff from your roof in a bar- rel. Replacing exotic vegetation with native plants adapted to your region’s natural precipitation patterns can also reduce watering demand. Xeriscaping, landscaping using plants adapted to arid conditions, has become a popular way in the U.S. Southwest to reduce outdoor water use.

Industry and municipalities Manufacturers are shift- ing to processes that use less water and in doing so are re- ducing their costs. Las Vegas is one of many cities that are

greenhouse gases. Due to these constraints, it is unlikely that desalination will be widely embraced in the United States unless we are able to find abundant, environmentally- friendly energy sources.

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Issuing and enforcing more stringent regulations of in- dustry can help reduce releases of many toxic chemicals. We can also modify our industrial processes and our purchasing decisions to rely less on these substances.

Pathogens and waterborne diseases Disease-caus- ing organisms (pathogenic viruses, protists, and bacteria) can enter drinking water supplies when they are contami- nated with human waste from inadequately treated sewage or animal waste from feedlots (p. 153). Biological pollution by pathogens causes more human health problems than any other type of water pollution, killing around 5 million people a year. This occurs because many people, particularly in rural areas of Asia and Africa, lack access to safe drinking water and sanitation facilities. Although improvements are occur- ring, the World Health Organization indicates that nearly 1 billon people do not have safe drinking water and that 2.6 billion people—40% of the human population—still lack ad- equate sewer or sanitation facilities.

We reduce the risks posed by waterborne pathogens by disinfecting drinking water (p. 270) and by treating waste-

Water pollution takes many forms Water pollution comes in many forms that can impair water- ways and threaten people and organisms that drink or live in affected waters. Let’s survey the major classes of water pollut- ants affecting waters in the world today.

Toxic chemicals Our waterways and coastal ecosystems have become polluted with toxic organic substances of our own making, including pesticides, petroleum products, and other synthetic chemicals (pp. 146–147, 210). Many of these can poison animals and plants, alter aquatic ecosystems, and cause an array of human health problems, including cancer. In addition, toxic metals such as arsenic, lead, and mercury damage human health and the environment, as do acids from acid precipitation (pp. 291–294) and from acid drainage from mining sites (p. 238). With its massive watershed encompassing rural, urban, and suburban areas, the Mississippi River is one waterway that carries toxic pollutants from sources in agriculture, industry, and homes and businesses.

Point sources of water pollutionPollutantNon-point sources of water pollution

Fertilizers, herbicides, and

pesticides

Nutrients, waste, and bacteria

Sewage treatment plants

Oil tankers

Salt on winter roads; oil, grease, and

chemicals from urban runoff

Industrial waste and toxic chemicals

Eroded soil

Oil spills

Factories and disposal sites

Residential neighborhoods and urban streets

Farms, lawns, and golf courses

Abandoned mines (also point source)

Animal feedlots (also non-point source)

Acid drainage

Construction sites, and

deforested and overgrazed

land

FIGURE 12.25  Point-source pollution (on right) comes from discrete facilities or locations, usually from single outflow pipes. Non-point-source pollution, such as runoff from streets, residential neighborhoods, lawns, and farms (on left), originates from numerous sources spread over large areas.

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because of an increased emphasis on spill prevention and re- sponse (FIGURE 12.26B).

The U.S. Oil Pollution Act of 1990 created a $1 billion prevention and cleanup fund. It also required that by 2015 all oil tankers in U.S. waters be equipped with double hulls as a precaution against puncture. In the wake of the Deepwater Horizon spill, the U.S. government is considering tighter reg- ulations on offshore drilling operations.

Nutrient pollution The Chesapeake Bay’s dead zone shows how nutrient pollution causes eutrophication and hy- poxia in surface waters (Chapter 2, pp. 23–25). When excess nitrogen and/or phosphorus enters a water body, it fertilizes algae and aquatic plants, boosting their growth. Algae then spread and cover the water’s surface, depriving underwater plants of sunlight. As algae die off, bacteria consume them. Because this decomposition requires oxygen, the increased bacterial activity drives down levels of dissolved oxygen. These levels can drop too low to support fish and shellfish, leading to dramatic changes in freshwater and saltwater ecosystems.

A “dead zone” of very low dissolved oxygen levels appears annually in the northern Gulf of Mexico, fueled by nutrients from Midwest farms carried by the Mississippi and Atchafa- laya rivers (FIGURE 12.27). The low oxygen conditions have adversely affected marine life and reduced catches of shrimp and fish to half of what they were in the 1980s.

Excessive nutrient concentrations sometimes give rise to population explosions among several species of marine al- gae that produce powerful toxins. Blooms of these algae are known as harmful algal blooms and are sometimes called red tides because some toxic algal species produce a red pig- ment that discolors the water. Harmful algal blooms can cause illness and death in aquatic animals and people and adversely

water (pp. 270–273). Other measures to lessen health risks include public education to encourage personal hygiene and government enforcement of regulations to ensure the cleanli- ness of food production, processing, and distribution.

Oil pollution Oil pollution in freshwater and marine sys- tems comes from spills of all sizes. Large spills occur infre- quently, but their impacts can be staggering near the spill site. The danger of oil spills to fisheries, economies, and ecosys- tems became clear in 2010 when British Petroleum’s Deep- water Horizon offshore drilling platform exploded, killing 11 workers and sinking into the ocean off the Louisiana coast (pp. 326–327). Oil gushed from the platform’s underwater well at rates of 1,800 gallons per minute, was spread widely by ocean currents, and washed up on coastal areas across the northern Gulf of Mexico (FIGURE 12.26A). The economic and ecological impacts of the spill were visited on hundreds of miles of water, sediments, and shoreline along the coasts of Louisiana, Mississippi, Alabama, and Florida.

Yet despite the severity of events like the Deepwater Ho- rizon oil spill, nearly half (47%) of the nearly 1.3 million met- ric tons of petroleum entering the world’s oceans each year originates from natural seeps in the ocean bottom. Another 38% accumulates in waters from innumerable, widely spread, small non-point sources. Shipping vessels and recreational boats can leak oil as they ply the waters of rivers, lakes, estuar- ies, and oceans. Motor oil from vehicles on roads and parking lots is also washed into streams by rains and carried to rivers, coastal areas, and oceans. Spills during petroleum transport account for 12% of oil pollution, and 3% comes from leak- age that occurs during the extraction of oil in offshore loca- tions. Although the Deepwater Horizon spill was catastrophic, the good news is that the amount of oil spilled from tankers worldwide has decreased over the past three decades, in part

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(a) Oil from Deepwater Horizon spill in a Louisiana salt marsh (b) Quantity of petroleum spilled from tankers, 1970–2010

FIGURE 12.26  Pollution was severe along Louisiana’s coastline (a) after British Petroleum’s Deepwater Horizon oil drilling platform exploded and disgorged millions of gallons of crude oil into the Gulf of Mexico in 2010. But less oil is being spilled into ocean waters today in large tanker spills (b), thanks in part to regulations on the oil shipping industry and improved spill response techniques. The bar chart shows cumulative quantities of oil spilled worldwide from nonmilitary spills over 7 metric tons. Data for (b) from International Tanker Owners Pollution Federation Ltd.

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THE SCIENCE BEHIND THE STORY

“Is It Better in a Bottle?”

Which is safer and healthier for you to drink, tap water or bottled water?It’s hard to know the answer, because companies are not required to tell us anything about the quality of the water in their bottles, or even where the water comes from.

Municipalities that provide tap water to their residents need to submit regular reports to the Environmental Protection Agency describing their sources, treatment methods, and con- taminants. In contrast, bottled water is regulated much more lightly as a “food” by the Food and Drug Admin- istration (FDA). Bottling companies do not have to inform the public or the government where their water comes from or how it is treated, and they are not required to test samples with certi- fied laboratories or notify the FDA of contamination problems.

So, to find out what’s in bottled water, scientists have had to do some detective work. In 2008, re- search scientists at the Environmen- tal Working Group (EWG), based in Washington, D.C., sent samples of 10 major brands of bottled water to

the University of Iowa’s Hygienic Laboratory for analysis. The lab’s chemists ran a battery of tests and detected 38 chemical pollutants, in- cluding traces of heavy metals, radio- active isotopes, caffeine and pharma- ceuticals from wastewater pollution, nitrate and ammonia from fertilizer, and various industrial compounds such as solvents and plasticizers (first figure).

Each brand contained eight contaminants on average, and two brands had levels of chemicals that exceeded legal limits in California and industry safety guidelines. Two brands showed the chemical composition of standard municipal water treatment. This is not surprising: An estimated 25–44% of bottled water is simply tap water, bottled and sold at elevated prices.

In 2009, researchers Martin Wagner and Jörg Oehlmann of Johann Wolfgang Goethe Univer- sity in Frankfurt, Germany, tested 20 brands of bottled water for the presence of hormone-disrupting chemicals that mimic estrogen (pp. 206–207). The researchers compared nine brands packaged in glass bot- tles, nine brands packaged in plastic bottles (PET, or polyethylene tereph- thalate, the “#1” type plastic), and two brands packaged in “Tetra Pak” paperboard boxes with an inner plastic coating. They placed samples (as well as tap-water samples as con- trols) in a “yeast estrogen screen,” a standard test-tube screening proce- dure that uses yeast cells engineered with genes to change color when exposed to estrogen-mimicking compounds.

Each year, 30–40 billion plastic water bottles are thrown away in the United States.

affect the economies of communities that rely on beach tour- ism and fishing.

Eutrophication is a natural process, but nutrient input from wastewater and fertilizer runoff from farms, golf courses, lawns, and sewage can dramatically increase the rate at which

it occurs. We can reduce nutrient pollution by specially treat- ing municipal wastewater to remove nutrients, reducing fertil- izer applications, using phosphate-free detergents, and planting vegetation and protecting natural areas around streams and riv- ers to reduce nutrient inputs into waterways.

3 3

5 5

7 5

4 1

2 2

7.0

Dissolved oxygen (mg/L)

Atchafalaya RiverLake Calcasieu

Sabine lake Mississippi River

Gulf of Mexico 50 km

FIGURE 12.27  Dissolved oxygen concentrations were mapped in the Gulf of Mexico off the Louisiana coast in 2010. Areas in red indicate the lowest oxygen levels. Regions considered hypoxic (< 2 mg/L) are encircled with a black line. Data from N. Rabalais, Louisiana Universities Marine Consortium (LUMCON), and R. E. Turner, Louisiana State University.

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The researchers detected estro- genic contamination in 60% of the samples (second figure). Both Tetra Pak brands and seven of nine plastic brands contained hormone-mimicking substances that apparently leached from the packaging. So did three of the glass-bottled brands, presumably from contamination at the bottling plant.

Wagner and Oehlmann then tested whether the chemicals in the water would affect a living animal, a type of snail that is known to increase its reproduction when exposed to an

estrogenic substance. They raised some snails in PET plastic containers and others in glass containers. After 56 days, the snails in the PET containers had produced 39–122% more embryos than snails in control condi- tions, whereas snails in glass containers showed no difference. This suggested that estrogenic compounds were leaching from the plastic. Their results were published in the journal Environ- mental Science and Pollution Research.

Research on the quality and safety of bottled water is just getting started,

but already these studies and others like them have indicated that bottled water can contain a range of contami- nants, some of which may pose health risks. Tap water may also contain plenty of pollutants, but municipalities are required to test and report on their water quality. Based on these findings, more and more people are reassessing their assumptions about the safety of bottled water.

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Control Glass PET Tetra Pak 0

Estrogenic activity (as determined by a yeast cell culture test) was high- est in bottled water from Tetra Pak containers, followed by PET plastic containers, and then glass containers. A negative control showed no appre- ciable estrogenic potency. With kind permission from Springer Science and Busi- ness Media and the author, from Wagner, M., and J. Oehlmann, 2009. Endocrine disruptors in bottled mineral water: Total es- trogenic burden and migration from plastic bottles. Environmental Science and Pollution Research 16: 278–286, Fig 3a.

Biodegradable Wastes Introducing large quantities of biodegradable materials into waters decreases dissolved oxy- gen levels, too. When human wastes, animal manure, paper pulp from paper mills, or yard wastes (grass clippings and leaves) enter waterways, bacterial decomposition escalates as organic material is metabolized. This lowers dissolved oxygen levels in the water, just as in waters receiving elevated inputs of plant nutrients. Wastewater is water affected by human activities and is a source of biodegradable wastes. It includes water from toilets, showers, sinks, dishwashers, and wash- ing machines; water used in manufacturing or industrial cleaning processes; and stormwater runoff. The widespread practice of treating wastewater to remove organic matter has greatly reduced impacts from biodegradable wastes in riv- ers in developed nations. Oxygen depletion remains a major problem in some developing nations, however, where waste- water treatment is less common.

Sediment As we saw in the Central Case Study, eroded soils are carried to rivers by runoff and transported long distances by river currents. Clear-cutting, mining, clearing land for development, and cultivating farm fields all expose

soil to wind and water erosion (pp. 140–144) and increase the amount of soil entering waterways. High sediment concen- trations impair aquatic ecosystems by interfering with the respiration of fish and invertebrates and smothering benthic organisms. Sediment also clouds waters, blocking the sun- light needed by rooted aquatic plants. We can reduce sedi- ment pollution by adopting sustainable soil practices, avoid- ing large-scale disturbance of vegetation, and maintaining riparian vegetation to trap sediments at the water’s edge.

Thermal pollution Water’s ability to hold dissolved oxy- gen decreases as temperature rises, so some aquatic organ- isms may not survive when human activities raise water tem- peratures. When we withdraw water from a river and use it to cool an industrial facility, we transfer heat from the facility back into the river where the water is returned. People also raise water temperatures by removing streamside vegetation that shades water.

Too little heat can also cause problems. On the Missis- sippi River and its tributaries, as in many other dammed riv- ers, water at the bottoms of reservoirs is colder than water at the surface. When dam operators release water from the

Number of brands with contamination

Industrial chemicals

Radioactive contamination

Fertilizer pollution

Bacterial contamination

Disinfection products from water treatment

Pharmaceuticals

Boron

Arsenic

Fluoride

0 2 4 6 8 10

Of 10 leading brands of bottled water tested, most contained industrial chemicals, radioactive isotopes, and fertilizer pollution, as well as other contaminants. Source: Naidenko, O., et al., 2008. Bottled water quality investi- gation: 10 major brands, 38 pollutants. Environmental Working Group, Washington, D.C.

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without a permit. Thanks to such legislation, point-source pollution in the United States was reduced, and rivers and lakes became noticeably cleaner.

The Great Lakes of Canada and the United States repre- sent an encouraging success story in fighting water pollution. In the 1970s these lakes were badly polluted with wastewater, fertilizers, and toxic chemicals, and Lake Erie was even declared “dead.” Today, efforts of the Canadian and U.S. governments have paid off. According to Environment Canada, releases of seven toxic chemicals into the lakes are down by 71%, municipal phosphorus has decreased by 80%, and chlorinated pollutants from paper mills are down by 82%. Levels of PCBs and DDE are down by 78% and 91%, respectively. Bird populations are rebounding, and Lake Erie is now home to the world’s largest walleye fishery. The Great Lakes’ troubles are by no means over, however—sediment pollution is still heavy, PCBs and mercury still settle from the air, and fish are not always safe to eat. Even so, the progress made thus far shows how conditions can im- prove when citizens push their governments to take action.

Such successes require effective enforcement of envi- ronmental regulations. A 2009 investigation by the New York Times revealed that violations of the Clean Water Act rose in the decade preceding the report and that underfunded and understaffed state and federal regulatory agencies acted on only a tiny percentage of these violations. As a result, 1 in 10 Americans have been exposed to unsafe drinking wa- ter. Soon after the report’s release, Environmental Protec- tion Agency (EPA) Administrator Lisa Jackson promised to strengthen enforcement to help remedy these issues.

We treat our drinking water The treatment of drinking water is a widespread practice in de- veloped nations today. Before being sent to your tap, water from a reservoir or aquifer is treated with chemicals to remove par- ticulate matter; passed through filters of sand, gravel, and char- coal; and/or disinfected with small amounts of an agent such as chlorine or ozone. The EPA sets standards for over 90 drinking water contaminants, which local governments and private wa- ter suppliers are obligated to meet. The treatment of drinking water treatment has greatly reduced mortality from the water- borne diseases that once claimed many lives, and it is one of the most significant technological advances of modern times.

We treat our wastewater Prior to the passage of the Clean Water Act, cities and towns regularly released untreated sewage into waterways. This led to oxygen depletion and pathogen contamination in many waters. Wastewater treatment is now a mainstream practice. In rural areas, septic systems are the most popular method of treating wastewater. In a septic system, wastewater runs from the house to an underground septic tank, inside which solids and oils separate from water. The clarified water proceeds downhill to a drain field of perforated pipes laid horizontally in gravel-filled trenches underground. Microbes decompose pollutants in the wastewater these pipes emit. Periodically, solid waste from the septic tank is pumped out and taken to a landfill.

In more densely populated areas, municipal sewer sys- tems carry wastewater from homes and businesses to central-

depths of a reservoir, downstream water temperatures drop suddenly and affect wildlife.

Nets and plastic debris Discarded fishing nets, plastic bags and bottles, fishing line, buckets, floats, and other trash can harm aquatic organisms. Aquatic mammals, seabirds, fish, and sea turtles may mistake floating plastic debris for food and can die as a result of ingesting material they cannot digest or expel.

In recent years scientists have learned that plastic trash is accumulating in certain regions of the oceans where currents converge. One such area is the “Great Pacific Garbage Patch” in the northern Pacific. The site is often estimated as being twice the size of Texas, and one study documented 3.3 plastic bits per square meter in its waters (see ENVISIONIT, p. 271). In 2006, the U.S. Congress responded to such ocean pollution by passing the Marine Debris Research, Prevention, and Reduc- tion Act. However, more is needed. We can all help by reduc- ing our use of unnecessary plastic, reusing the plastic items we do use, and recycling the plastic we discard.

Water pollutants can contaminate groundwater Many of these pollutants affect groundwater as well as surface waters. Groundwater pollution is more difficult to detect and more problematic to address than surface wa- ter pollution. Groundwater f lows slowly and may harbor pollutants for long periods. Decomposition is also slower in groundwater than surface water because groundwater is not exposed to sunlight, contains fewer microbes and min- erals, and holds less dissolved oxygen and organic matter.

Some chemicals that are toxic at high concentrations, including aluminum, fluoride, nitrates, and sulfates, occur naturally in groundwater. However, groundwater pollution resulting from human activity is widespread. Industrial, ag- ricultural, and urban wastes—from heavy metals to petro- leum products to solvents to pesticides—can leach through soil and seep into aquifers. For example, nitrate from fertil- izers has leached into aquifers in Canada and in 49 U.S. states. Nitrate in drinking water has been linked to cancers, miscar- riages, and “blue-baby” syndrome, which reduces the oxygen- carrying capacity of infants’ blood. Other pollutants can en- ter groundwater through leaky underground tanks, through improperly designed wells, and from the pumping of liquid hazardous waste below ground (pp. 394–395).

Legislative and regulatory efforts have helped to reduce water pollution As numerous as our freshwater pollution problems may seem, it is important to remember that many were worse a few de- cades ago (pp. 98–99). Citizen activism and government response during the 1960s and 1970s in the United States resulted in legislation such as the Federal Water Pollution Control Act of 1972 (later amended and renamed the Clean Water Act in 1977). These acts set standards for industrial wastewater, set standards for contaminant levels in surface waters, funded construction of sewage treatment plants, and made it illegal to discharge pollution from a point source

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➤

Dead albatross with stomach full of plastic

Charles Moore holds water from the “Great Paci�c Garbage Patch”

Booms trap trash headed to the ocean in the Los Angeles River

Roadside litter and dumped garbage end up in streams, then rivers, then the ocean.

Countless trillions of pieces of plastic trash are accumulating in the oceans, concentrated by currents.

Marine debris kills animals that get entangled in nets or swallow plastic.

You Can Make a Difference

Clean a beach with the International Coastal Cleanup.

Pick up litter wherever you are; trash in a stream today could be in the ocean tomorrow.

Reduce consumption, and reuse and recycle items. You will generate far less waste.

Endangered Hawaiian Monk Seal caught in �shing tackle

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lowed to sit in settling tanks. Wastewater then proceeds to secondary treatment, in which water is stirred and aerated so that aerobic bacteria consume most of the small particles of organic matter that remain in the wastewater. Roughly 90% of suspended solids may be removed after secondary treat-

ized treatment locations, called wastewater treatment plants (FIGURE 12.28). At the plant, incoming wastewater is first passed through screens to capture large objects. It is then sent to primary treatment, where about 60% of the suspended solids in the wastewater settle out when the wastewater is al-

Screens and grit tank Solid objects and grit removed

Raw sewage enters treatment facility

Effluent discharged into waterways

Oils and greases float to the top

Solids disposed at landfill

Gases chemically treated to reduce

odor

Sludge sent to anaerobic digester

Gas to generate electricity

Biosolids for cropland

Primary clarifier Oils, greases, and solids removed

Aeration basin Microbes consume organic matter

Secondary clarifier Remaining oils, greases, and solids removed

Filtering and disinfection Water filtered with coal and sand, and/or disinfected with chlorine or UV light

Solids sink to the bottom

Some solids returned to seed aeration basin with new microbes

FIGURE 12.28  Shown here is a generalized process from a modern, environmentally sensi- tive wastewater treatment facility.

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ment. In some cases, water is subject to additional treatments to remove pollutants of particular concern. Some wastewater treatments plants along the Mississippi River, for example, subject water to additional treatments that remove nitrogen and phosphorus inputs into the northern Gulf of Mexico.

Finally, the clarified water is treated with chlorine, and sometimes ultraviolet light, to kill bacteria. The treated water, called wastewater effluent, is then typically released into a river or the ocean. In some situations, effluent is “reclaimed” and used for lawns and golf courses, for irrigation, or for in- dustrial purposes.

The solids collected during the treatment process, called sludge, are sent to digesting vats, where microorganisms de- compose much of the matter. The resultant “biosolids” are then dried and either landfilled, incinerated, or used as fertil- izer on cropland.

Wetlands can aid wastewater treatment Artificial wetlands have been used to aid wastewater treat- ment. Most of these sites use microbes, algae, and aquatic plants to “polish” the water released from wastewater treat- ment plants. Water cleansed in the wetland can then be released into waterways or allowed to percolate underground. One of the first constructed wetlands was established in the 1980s in Arcata, a town on northern California’s scenic Redwood Coast. There are currently around 500 artificially constructed or re- stored wetlands performing this service in the United States.

The release of wastewater effluent even shows promise for preserving coastal wetlands along the Gulf Coast. At a study site in Louisiana, wastewater effluent was released into coastal wetlands, where it elevated growth in marsh grasses due to the effluent’s elevated nutrient concentrations. The increased plant growth led to increased deposition of plant organic mat- ter on marsh sediments, offsetting the depth increases caused by natural soil compaction and sustaining the ecosystem.

EMPTYING THE OCEANS We affect the oceans and their biological resources by en- gineering waterways with dams and levees and by introduc- ing water pollutants directly into the ocean or into rivers that empty into the sea. In addition, people are putting pressure on oceans by overharvesting marine fish species and threatening the balance and functioning of marine and coastal ecosystems.

Over half the world’s marine fish populations are fully ex- ploited, meaning that we cannot harvest them more intensively without depleting them, according to the U.N. Food and Agri- culture Organization (FAO). An additional 28% of marine fish populations are overexploited and already being driven toward extinction. If current trends continue, a comprehensive 2006 study in the journal Science predicted, populations of all ocean species that we fish for today will collapse by the year 2048.

If fisheries collapse as predicted, we will lose the ecosys- tem services they provide. Productivity will be reduced, eco- systems will become more sensitive to disturbance, and the filtering of water by vegetation and organisms (such as oys- ters) will decline, making harmful algal blooms, dead zones, fish kills, and beach closures more common. Aquaculture (raising fish in tanks or pens) is booming and is helping to relieve pressure on wild stocks, but fish farming comes with its own set of environmental dilemmas (pp. 153–154). All this makes it vital, many scientists and fisheries managers say, that we turn immediately to more sustainable fishing practices.

Fishing has industrialized Total global fisheries catch, after decades of increases, leveled off after about 1988 (FIGURE 12.29) and has remained fairly constant since then. This seeming stability in catch can be explained by several factors that conceal population declines: Fishing fleets are exploiting increasingly remote fishing ar- eas, are engaging in more intensive fishing, are capturing smaller fish than before, and are targeting less desirable fish species they formerly overlooked.

Today’s industrialized commercial fishing fleets employ fossil fuels, huge vessels, and powerful new technologies to capture fish in large numbers using several methods. Some vessels set out long driftnets that span large expanses of water (FIGURE 12.30A). These chains of transparent nylon mesh nets are arrayed to drift with currents so as to capture passing fish, and they are held vertical by floats at the top and weights at the bottom. Longline fishing (FIGURE 12.30B) involves setting out extremely long lines (up to 80 km [50 mi] long) with up to sev- eral thousand baited hooks spaced along their lengths. Trawl- ing entails dragging immense cone-shaped nets through the water, with weights at the bottom and floats at the top. Trawl- ing in open water captures pelagic fish, whereas bottom-trawl- ing (FIGURE 12.30C) involves dragging weighted nets across the floor of the continental shelf to catch benthic organisms.

Year

China World, excluding China

0 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

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100 FIGURE 12.29  The total global fisheries catch has stalled for the past 20 years, and many fear that a global decline is imminent if conservation measures are not taken. The figure shows trends with and without China’s data, because research suggests that China’s data may be somewhat inflated. Data from the Food and Agriculture Organiza-

tion of the United Nations, 2010. The state of

world fisheries and aquaculture 2010. Fig 3. By

permission.

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Unfortunately, these fishing practices catch more than just the species they target. Bycatch, the accidental capture of nontarget animals, accounts for the deaths of millions of animals each year. Driftnetting captures dolphins, seals, and sea turtles, as well as countless nontarget fish. Longline fishing kills turtles, sharks, and seabirds. Bottom-trawling is often lik- ened to clear-cutting (p. 194) and strip-mining (pp. 237–238), and it is especially destructive to structurally complex areas, such as reefs, that provide shelter and habitat for animals.

Industrial fishing fleets have depleted many fisheries Throughout the world’s oceans, today’s industrialized fishing fleets are depleting marine populations. In a 2003 study, fish- eries biologists Ransom Myers and Boris Worm analyzed fish- eries data and concluded that the oceans today contain only one-tenth of the large-bodied fish and sharks they once did.

Many fisheries have collapsed in recent years. Ground- fish (species that live in benthic habitats, such as Atlantic cod, haddock, halibut, and flounder) powered the economies of New England and Maritime Canada for close to 400 years. Yet fishing pressure became so intense that most stocks collapsed, bringing fishing economies down with them.

With Canada’s cod stocks down by 99% and showing no sign of recovery, the Canadian government in 1992 ordered a complete ban on cod fishing in the Grand Banks region off Newfoundland and Labrador. On the U.S. side of the bor- der, bans are helping to restore depleted fisheries. When the groundfish fisheries of Georges Bank in the Gulf of Maine collapsed in the mid-1990s, the National Marine Fisheries Service (NMFS) closed three prime fishing areas to fishing. The closures worked. Spawning stocks of haddock and yel- lowtail flounder have risen, and biomass of sea scallops in- creased 14-fold. Fishers began having better luck, especially just outside the closed regions. Unfortunately, cod have not recovered. Research suggests that once mature cod were eliminated, the species they preyed upon proliferated, and now those species compete with and prey on young cod, pre- venting the population from rebuilding.

Red snapper stocks have been similarly depleted by over- fishing in the Gulf of Mexico. Red snapper were identified as severely overfished in 1989, and current populations are at a mere 3% of their historical abundance. Gulf populations of the species are affected by harvesting and by mortality experienced when they are taken as bycatch on shrimping vessels. The re- covery plan for red snapper approved by the NMFS in 2005 was criticized by environmental groups as simply maintaining the “status quo” management approach that had proven un- successful for over 15 years. These groups sued in federal court, and the court ruled in their favor in 2007 and ordered NMFS to develop a more comprehensive plan with greater probability of success in restoring populations of red snapper.

Our purchasing choices can influence fishing practices By exercising careful choice when we buy seafood, we as con- sumers can encourage more sustainable fisheries practices. Several marine conservation organizations have devised

(a) Driftnetting

(b) Longlining

FIGURE 12.30  Commercial fishing fleets use several methods of capture. In drift netting (a), long transparent nylon nets are set out to drift through open water to capture schools of fish. In longlining (b), lines with numerous baited hooks are set out in open water. In bottom-trawling (c), weighted nets are dragged along the floor of the continental shelf. All methods result in large amounts of bycatch, the capture of nontarget animals. The illustrations above are simplified for clarity and do not por- tray the immense scale that these technologies can attain; for instance, industrial trawling nets can be large enough to engulf several Boeing 747 jumbo jets.

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concise guides to help consumers differentiate fish and shellfish that are overfished or whose capture is ecologically damaging from those that are harvested more sustainably. For instance, the Monterey Bay Aquarium provides a wealth of this information on its website.

Marine reserves protect ecosystems Fisheries managers conduct surveys, study fish population biology, and monitor catches to determine the number of fish of a given species that can be harvested without reducing fu- ture catches—a concept called maximum sustainable yield (p. 192). Despite the use of this technique, a number of fish and shellfish stocks have plummeted. Thus, many scientists and managers feel it is time to shift the focus away from individual species and toward viewing marine resources as elements of larger ecological systems. One key aspect of such ecosystem- based management (p. 192) is to set aside areas of ocean where systems can function without human interference.

Hundreds of marine protected areas (MPAs) have been established, most of them along the coastlines of developed countries. However, despite their name, nearly all MPAs al- low fishing or other extractive activities and so are not fully protected from impacts from people.

Because of the lack of true refuges from fishing pressure, many scientists want to establish areas where fishing is pro- hibited. Such “no-take” areas have come to be called marine reserves. Designed to preserve ecosystems intact, marine re- serves are also intended to improve fisheries. Scientists argue that marine reserves can act as production factories for fish for surrounding areas, because fish larvae produced inside re- serves will disperse outside and stock other parts of the ocean. By serving both purposes, proponents maintain, marine re-

serves are a win-win proposition for environmentalists and fishers alike. However, many commercial and recreational fishers dislike the idea of no-take reserves and have opposed nearly every marine reserve that has been established.

Reserves can work for both fish and fishers Over the past two decades, data from marine reserves around the world have been indicating that reserves can work as win- win solutions that benefit ecosystems, fish populations, and fishing economies. A comprehensive review of data from ma- rine reserves as of 2001 revealed that just one to two years after their establishment, marine reserves:

▶ Increased densities of organisms on average by 91%. ▶ Increased biomass of organisms on average by 192%. ▶ Increased average size of organisms by 31%. ▶ Increased species diversity by 23%.

If marine reserves work in principle, the question becomes how large reserves need to be, how many there need to be, and where they need to be placed to take best advantage of ocean currents. Of several dozen studies that have estimated how much area of the ocean should be protected in no-take reserves, estimates range from 10% to 65%, with the majority falling between 20% and 50%. Most scientists feel that involv- ing fishers directly in the planning process is crucial for com- ing up with answers to all these questions. If marine reserves can be made to work and to be accepted, then they may well seed the seas and help lead us toward solutions to one of our most pressing environmental problems.

➤ CONCLUSION Our planet’s diverse aquatic systems comprise an intercon- nected web of ecosystems that exchange water. The introduc- tion of pollutants and the engineering of waterways therefore cause impacts that cascade through the system. Our expand- ing population and increasing water use are straining water supplies and affecting surface waters and groundwater around the world. Water pollutants threaten human health and eco- system stability, and overharvesting of marine fish populations threatens the oceans’ biodiversity.

There is plenty of reason for optimism, however. Im- provements in water use efficiency show promise for reduc- ing demand for water, even with increasing human popula- tions. Water quality in many freshwater bodies has improved in recent decades, thanks to legislative action from policy- makers and the efforts of millions of concerned citizens. In the oceans, marine reserves give hope that we can restore ecosystems and fisheries at the same time.

concerns have China’s Three Gorges Dam and its reser- voir raised?

5. Why do the Colorado, Rio Grande, Nile, and Yellow rivers now slow to a trickle or run dry before reaching their deltas?

6. Name three major types of water pollutants, and provide an example of each. Explain which classes of pollutants you think are most important in your local area.

7. Define groundwater and list some anthropogenic ( human) sources of groundwater pollution. Why do

1. Explain why the distribution of water on Earth makes it difficult for many people to access adequate fresh water.

2. Pick one of the aquatic systems profiled in this chapter, and provide three examples of ways it interacts with other aquatic systems.

3. Why are coral reefs biologically valuable? How are they being degraded by human impact? What is causing the disappearance of mangrove forests and salt marshes?

4. Describe three benefits and three costs of damming riv- ers. What particular environmental, health, and social

T E S T I N G Y O U R C O M P R E H E N S I O N

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S E E K I N G S O L U T I O N S

1. How can we lessen agricultural demand for water? Describe some ways we can reduce household water use. How can industrial uses of water be reduced?

2. Describe three ways in which your own actions contrib- ute to water pollution. Now describe three ways in which you could diminish these impacts.

3. Describe the trends in global fish capture over the past 55 years and over the past 20 years, and explain several factors that account for these trends.

4. THINK IT THROUGH Your state’s governor has put you in charge of water policy for the state. The aquifer beneath your state has been overpumped, and many wells have run dry. Agricultural production last year decreased for the first time in a generation, and farm- ers are clamoring for you to do something. Meanwhile,

the state’s largest city is growing so fast that more wa- ter is needed for its burgeoning urban population. What policies would you consider to restore your state’s water supply? Would you try to take steps to increase supply, decrease demand, or both? Explain your choices.

5. THINK IT THROUGH You are mayor of a coastal town where some residents are employed as commercial fishers and others make a living serving ecotourists who come to snorkel and scuba dive at the nearby coral reef. In recent years, several fish stocks have crashed, and ecotourism is dropping off as fish disappear from the increasingly degraded reef. Scientists are urging you to help establish a marine reserve around portions of the reef, but most commercial and recreational fishers are opposed to this idea. What steps would you take to restore your commu- nity’s economy and environment?

C A L C U L A T I N G E C O L O G I C A L F O O T P R I N T S

One of the single greatest personal uses of water is for show- ering. Old-style showerheads that were standard in homes and apartments built before 1992 dispense at least 5 gallons of water per minute, but low-flow showerheads produced after that year dispense just 2.5 gallons per minute. Given an average daily shower time of 8 minutes, calculate the amounts of water used and saved over the course of a year with old standard versus low-flow showerheads, and record your results in the table.

many scientists consider groundwater pollution a great- er problem than surface water pollution?

8. Describe and explain the major steps in the process of wastewater treatment. How can artificially constructed wetlands aid such treatment?

9. Name three industrial fishing practices, and explain how they create by-catch and harm marine life.

10. How does a marine reserve differ from a marine protect- ed area? Why do many fishers oppose marine reserves? Explain why many scientists say no-take reserves will be good for fishers.

Annual water use with standard showerheads (gallons)

Annual water use with low-flow showerheads (gallons)

Annual water savings with low-flow showerheads (gallons)

You Your class Your state United States

1. In 2010, under its WaterSense program, the EPA began promoting showerheads that produce still-lower flows of 2 gallons per minute (gpm). Some cities are already re- quiring these, and some models today go even lower. How much water would you save per year by using a 2-gpm showerhead instead of a 2.5-gpm showerhead?

2. How much water would you be able to save annually by shortening your average shower time from 8

Go to www.masteringenvironmentalscience.com for homework assignments, practice quizzes, Pearson eText, and more.

minutes to 6 minutes? Assume you use a 2.5-gpm showerhead.

3. Compare your answers to Questions 1 and 2. Do you save more water by showering 8 minutes with a 2-gpm showerhead or 6 minutes with a 2.5-gpm showerhead?

4. Can you think of any factors that are not being consid- ered in this scenario of water savings? Explain.

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A bad air day in Tehran, Iran

13 Atmospheric Science and Air Pollution Upon completing this chapter, you will be able to:

➤ Describe the composition, structure, and function of Earth’s atmosphere ➤ Relate weather and climate to atmospheric conditions ➤ Identify major outdoor air pollutants, outline the scope of outdoor air pollution, and assess potential

solutions ➤ Explain stratospheric ozone depletion and identify steps taken to address it ➤ Define acid deposition and illustrate its consequences ➤ Identify major indoor air pollutants, characterize the scope of indoor air pollution, and assess potential

solutions

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UNITED STATES

NORTH AMERICA

SOUTH AMERICA

ATLANTIC OCEAN

INDIAN OCEAN

ASIA

AFRICA

IRAN

Los Angeles, California

Tehran

PACIFIC OCEAN

EUROPE

The Sister Cities pro- gram was devised to en- courage world peace and understanding by foster- ing relationships between people of U.S. cities and others around the world. Hundreds of American cit- ies share government visits and cultural ties with their sister cities. One of L.A.’s sister cities is Tehran, the capital of Iran. As it hap- pens, one thing these two cities share is smog.

Tehran’s parks and tree-lined streets make it a gorgeous city, but the quality of Tehran’s air is so bad that residents commonly wear face masks when they step outside to go to the bank, the laundromat, or the grocery store. On especially bad days the government closes schools and advises people to stay indoors.

Mehdi Gharakhanlou, a 38-year-old business- man, says he returns home from the central city each winter day “fatigued, nauseated, with itchy eyes, a sore throat, a bad headache or even with all of these symptoms. . . . I wear a mask when I am out, but even that doesn’t help much.”

“People suffer from eye allergies, nose irrita- tion, . . . headaches, burn- ing eyes,” adds Tehran journalist Alborz Maleki. “Children and elderly peo- ple are facing many [health] problems.”

Indeed, each day people die in Iran’s capi- tal from outdoor air pol- lution. Health authorities blame several thousand premature deaths per year in Tehran on respiratory diseases resulting from air pollution. In 2006, 3,600

people succumbed in just a month. As in Los Angeles, automobile traffic generates

most of the pollution in Tehran. An estimated 80% of Tehran’s pollutants come from the exhaust of its 3 million vehicles, many of which are over 20 years old and lack basic pollutant-filtering technology such as catalytic converters. Tehran’s limited public transpor- tation system forces most people to rely on cars, mo- torcycles, and taxis to get around. Moreover, Iran’s government spends billions of dollars in subsidies to give its citizens some of the cheapest gasoline in the

CENTRAL CASE STUDY

L.A. and its Sister City, Tehran, Struggle for a Breath of Clean Air

“Poisonous gases emitted from vehicles and industries continue to wreak havoc on Tehran’s air.” —Yousef Rashidi, Director of Tehran’s Air Quality Control Company

“I left L.A. in 1970, and one of the reasons I left was the horrible smog. And then they cleaned it up. That was one of the greatest things the government has ever done for me. You have beautiful days now.

It’s a much, much nicer place to live.” —Actor and comedian Steve Martin, speaking to Los Angeles Magazine

L os Angeles has long symbolized air pollution in Americans’ minds. Smog blanketed

the city from the 1960s through the 1990s, fed by the exhaust of millions of automo-

biles. But in recent years, policy efforts and technological advances have improved air

quality in Los Angeles. Today L.A. still suffers the nation’s worst smog, but its skies are clearer

than skies in some of its “sister cities” elsewhere in the world.

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world. With 40-cent-per-gallon gas, people have lit- tle financial incentive to conserve, and Iranian vehicles guzzle gas at four times the rate of European vehicles.

In both Los Angeles and Tehran, topography wors- ens air quality problems. Each city lies in a valley, sur- rounded by mountains that trap pollution. In Tehran, wintertime is the worst, when thermal inversions con- fine pollutants over the city and there is little wind to blow them away. And as with Los Angeles in recent decades, people are streaming into Tehran from else- where, so efforts to rein in pollution are being over- whelmed by population growth.

All in all, modern-day Tehran is dangerously pol- luted for the same reasons that Los Angeles was a few decades ago. L.A. and Tehran today typify a difference between cities of developed nations and cities of devel- oping nations. Nations that are industrializing as they try to build wealth for their citizens are running into the same urban and industrial air pollution problems that plagued the United States a generation and more ago.

Los Angeles has 24 other sister cities. Many of them also struggle with severe air pollution: Athens, Greece; Jakarta, Indonesia; Mumbai, India; Guangzhou, China; Taipei, Taiwan; and San Salvador, El Salvador all expe- rience poor air quality on a regular basis. Best known among L.A.’s sister cities is Mexico City, Mexico, which may have the most polluted air in the world.

Most of these cities are taking steps to improve their air quality, just as Los Angeles and other Ameri- can cities have done. We will examine the solutions sought in Los Angeles, Tehran, and L.A.’s other sister cities as we learn about Earth’s atmosphere and how to reduce the pollutants we release into it. ■

THE ATMOSPHERE Every breath we take reaffirms our connection to the atmo- sphere, the layer of gases that surrounds Earth. The atmo- sphere provides oxygen, absorbs hazardous solar radiation, burns up incoming meteors, transports and recycles water and nutrients, and moderates climate.

Earth’s atmosphere consists of 78% nitrogen (N2) and 21% oxygen (O2). The remaining 1% is composed of argon (Ar) and minute concentrations of water vapor and several other gases (FIGURE 13.1). Over our planet’s long history, the atmosphere’s composition has changed. Oxygen began to build up about 2.7 billion years ago, with the emergence of microbes that emitted oxygen by photosynthesis (p. 30). Today, human activity is alter- ing the concentrations of some atmospheric gases, such as carbon dioxide (CO2), methane (CH4), and ozone (O3).

The atmosphere is layered The atmosphere that seems to stretch so high above us is ac- tually just 1/100th of Earth’s diameter, like the fuzzy skin of

Nitrogen (N2) (78.08%)

Argon (Ar): 0.93%

Other permanent gases Neon (Ne): 0.0018% Helium (He): 0.0005% Hydrogen (H2): trace Xenon (Xe): trace

Variable gases Water vapor (H2O): 0–4% Carbon dioxide (CO2): 0.038% Methane (CH4): 0.00017% Nitrous oxide (N2O): trace Ozone (O3): trace Chlorofluorocarbons (CFCs): trace

Oxygen (O2) (20.95%)

FIGURE 13.1  Earth’s atmosphere consists of nitrogen, oxygen, argon, and several gases at dilute concentrations. Permanent gases are fixed in concentration. Variable gases vary in concentration as a result either of natural processes or of human activities. Data from Ahrens, C.D., 2007. Meteorology today, 8th ed. Belmont, CA: Brooks/Cole.

a peach. It consists of four layers that differ in temperature, density, and composition (FIGURE 13.2).

Movement of air within the bottommost layer, the tropo- sphere, is largely responsible for the planet’s weather. Although it is thin (averaging 11 km [7 mi] high) relative to the atmo- sphere’s other layers, the troposphere contains three-quarters of the atmosphere’s mass, because gravity pulls mass downward, making air denser near Earth’s surface. Tropospheric air gets colder with altitude, dropping to roughly −52 ºC (−62 ºF) at the top of the troposphere. At this point, temperatures cease to de- cline with altitude, marking a boundary called the tropopause. The tropopause acts as a cap, limiting mixing between the tro- posphere and the atmospheric layer above it, the stratosphere.

The stratosphere extends 11–50 km (7–31 mi) above sea level. Similar in composition to the troposphere, the stratosphere is 1,000 times drier and less dense. Its gases experience little vertical mixing, so once substances (including pollutants) enter it, they tend to remain for a long time. The stratosphere warms with altitude because its ozone and oxygen absorb and scatter the sun’s ultraviolet (UV) radiation (p. 30). Most of the atmosphere’s ozone concentrates in a portion of the stratosphere roughly 17–30 km (10–19 mi) above sea level, a re- gion called the ozone layer. The ozone layer greatly reduces the amount of UV radiation that reaches Earth’s surface. Because UV light can damage living tissue and induce genetic mutations, the ozone layer’s protective effects are vital for life on Earth.

Above the stratosphere lies the mesosphere, where air pressure is extremely low and temperatures decrease with alti- tude. The thermosphere, our atmosphere’s top layer, extends to an altitude of 500 km (300 mi).

The sun and the atmosphere drive weather and climate An enormous amount of energy from the sun continuously bombards the upper atmosphere—over 1,000 watts/m2, thou- sands of times more than all the electricity generated by hu- man society. Of that solar energy, about 70% is absorbed by

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A lti

tu de

(k m

)

120

110

100

0 5 15 20

Ozone concentration (milli-Pascals)

10 25 30 35

-80 -60 -40

Temperature (ºC)

-20 0

Thermosphere (up to 500 km)

Mesosphere

Stratosphere

Troposphere

Ozone concentration

Tropopause

Temperature profile

FIGURE 13.2  Temperature (red line) drops with altitude in the troposphere, rises with altitude in the stratosphere, drops in the mesosphere, then rises again in the thermosphere. The tropopause separates the troposphere from the stratosphere. Ozone (blue shaded area) reaches a peak in a portion of the stratosphere, giv- ing rise to the term ozone layer. Adapted from Jacobson, M.Z., 2002. Atmospheric pollution: History, science, and regulation. Cambridge: Cam-

bridge University Press; and Parson, E.A., 2003. Protecting the ozone layer:

Science and strategy. Oxford: Oxford University Press.

mate is what we expect; weather is what we get.” For example, Los Angeles has a climate characterized by warm dry summers and mild rainy winters, yet on occasional autumn days, dry Santa Ana winds blow in from the desert and bring extremely hot weather.

Under most conditions, air in the troposphere decreases in temperature as altitude increases. Because warm air rises, verti- cal mixing results (FIGURE 13.3A). Occasionally, however, a layer of cool air may form beneath a layer of warmer air. This depar- ture from the normal temperature profile is known as a tem- perature inversion, or thermal inversion (FIGURE 13.3B). The band of air in which temperature rises with altitude is called an inversion layer (because the normal direction of temperature change is inverted). Thermal inversions can occur in different ways, sometimes involving cool air at ground level and some- times producing an inversion layer higher above the ground. One common type of inversion (shown in Figure 13.3B) occurs in mountain valleys where slopes block morning sunlight, keep- ing ground-level air within the valley shaded and cool.

Because the cooler air at the bottom of an inversion layer is denser than the warmer air above it, it resists vertical mixing and remains stable. Vertical mixing allows air pollution to be carried

(b) Thermal inversion

(a) Normal conditions

Low Warm

Vertical mixing

Pollution trapped

Sun warms surface Heat radiates

up from surface

High Cool

Te m

p er

at u

A lti

tu de

Low

High

A lti

tu de

Warm

Te m

p er

at u

Cool

FIGURE 13.3 ▲ Under normal conditions (a), air becomes cooler with altitude, and air of different altitudes mixes, dispersing pol- lutants upward and outward. During a thermal inversion (b), dense cool air remains near the ground, and air warms with altitude within the inversion layer. Little mixing occurs, and pollutants are trapped near the surface.

the atmosphere and planetary surface, and the rest is reflected back into space (see Figure 14.1, p. 301).

Land and surface water absorb solar energy and then ra- diate heat, causing some water to evaporate. Air near Earth’s surface therefore tends to be warmer and moister than air at higher altitudes. These differences set into motion a process of convective circulation. Warm air, being less dense, rises and creates vertical currents. As air rises into regions of lesser at- mospheric pressure, it expands and cools. Once the air cools, it becomes denser and descends, replacing warm air that is rising. The descending air picks up heat and moisture near ground level and prepares to rise again, continuing the process. Convective circulation influences both weather and climate.

Weather and climate both involve physical properties of the troposphere, such as temperature, pressure, humidity, cloudiness, and wind. Weather specifies atmospheric conditions within small geographic areas over minutes, hours, or days. In contrast, climate describes patterns of atmospheric conditions found across large geographic regions over seasons, years, or mil- lennia. Mark Twain once noted the distinction by saying, “Cli-

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rise to tropical rainforests near the equator. After releasing much of its moisture, this air diverges and moves in currents heading north and south. The air in these currents cools and descends back to Earth at about 30 degrees latitude north and south. Because the descending air is now dry, the re- gions around 30 degrees latitude are quite arid, giving rise to deserts. Two further pairs of convective cells, called Ferrel cells and polar cells, lift air and create precipitation around 60 degrees latitude north and south and cause drier air to de- scend at around 30 degrees latitude and in the polar regions.

These three pairs of convective cells create wet climates near the equator, arid climates near 30 degrees latitude, moist regions near 60 degrees latitude, and dry conditions near the poles. These patterns, combined with temperature variation, help explain why biomes tend to be arrayed in latitudinal bands (Figure 4.15, p. 78).

The Hadley, Ferrel, and polar cells also interact with Earth’s rotation to produce global wind patterns (FIGURE 13.4B). As Earth rotates on its axis, locations on the equator spin faster than locations near the poles. This means that as air currents of the convective cells flow north to south, some regions of the planet’s surface move west to east beneath them more quick- ly than others. As a result, from the perspective of an Earth- bound observer, these air currents appear to be deflected from

upward and diluted, but thermal inversions trap pollutants near the ground. An inversion persisting for several days sparked a “killer smog” crisis in London, England, in 1952. The inversion trapped pollutants from factories and coal-burning stoves, cre- ating foul conditions that killed 4,000 people—and by some es- timates up to 12,000. Both Los Angeles and Tehran suffer their worst pollution when thermal inversions prevent pollutants from being dispersed. Both cities are encircled by mountains, which trap pollutants by promoting inversion layers and by interrupt- ing air flow. Tehran experiences thermal inversions on more than 250 days each year. Inversions regularly concentrate pol- lution over large metropolitan areas in valleys ringed by moun- tains, from Mexico City to Seoul, Korea, to São Paulo, Brazil.

Large-scale circulation systems produce global climate patterns At large geographic scales, convective air currents contrib- ute to long-term climate patterns (FIGURE 13.4A). Near the equator, solar radiation sets in motion a pair of convective cells known as Hadley cells. Here, where sunlight is most in- tense, surface air warms, rises, and expands. As it does so, it releases moisture, producing the heavy rainfall that gives

60º N (moist)

30º N (arid)

0º (Equator) (wet, tropical)

Ferrel cell

(a) Convection currents

30º S

Arid climate

Wet, tropical climate

Hadley cell Hadley cell

30º N Equator

Polar cell

60º S (moist)Polar cell

30º S (arid)

Westerlies

60º N

30º N

30º S

60º S

Equator

Westerlies

(b) Global wind patterns

NE trade winds

SE trade winds

(Doldrums)

Hadley cells

FIGURE 13.4  A series of large-scale convective cells (a) helps determine global patterns of humidity and aridity. Warm air near the equator rises, expands, and cools; and moisture condenses, giving rise to a wet climate in tropical regions. Air travels toward the poles and descends around 30 degrees latitude. This air, which lost its moisture in the tropics, causes regions around 30 degrees latitude to be arid. This convective circulation, a Hadley cell, occurs on both sides of the equator. Between roughly 30 and 60 degrees latitude north and south, Ferrel cells occur; and between 60 and 90 degrees latitude, polar cells occur. Air rises around 60 degrees latitude, creating a moist climate, and falls around 90 degrees, creating a dry climate. Global wind currents (b) show latitudinal patterns as well. Trade winds between the equator and 30 degrees latitude blow westward, whereas westerlies between 30 and 60 degrees latitude blow eastward.

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Tornadoes (FIGURE 13.5B) can form when a mass of warm air meets a mass of cold air and the warm air rises quickly, set- ting a powerful convective current in motion. If high-altitude winds are blowing faster and in a different direction from low- altitude winds, the rising column of air may begin to rotate. Eventually the spinning funnel of rising air may lift up soil and objects in its path, with winds up to 500 km per hour (310 mph). In North America, tornadoes are most apt to form in the Great Plains and the Southeast, where cold air from Can- ada and warm air from the Gulf of Mexico frequently meet.

Understanding how the atmosphere functions can help us predict violent storms and warn people of their ap- proach. Such knowledge can also help us comprehend how our pollution of the atmosphere affects climate, ecological systems, economies, and human health.

OUTDOOR AIR POLLUTION Throughout human history, we have made the atmosphere a dumping ground for our airborne wastes. Whether from simple wood fires or modern coal-burning power plants, people have generated air pollutants, gases and particulate material added to the atmosphere that can affect climate or harm people or other organisms. Air pollution refers to the emission or release of air pollutants into the atmosphere.

In recent decades, government policy and improved tech- nologies have helped us reduce most types of outdoor air pol- lution (often called ambient air pollution) in industrialized nations. However, outdoor air pollution remains a problem, particularly in developing nations and in urban areas. The great- est air pollution problem today may be our emission of green- house gases (p. 300), which contribute to global climate change. (We discuss this issue separately and in depth in Chapter 14.)

Natural sources can pollute When we think of outdoor air pollution, we tend to envision smokestacks belching smoke from industrial plants. How- ever, natural processes produce a great deal of air pollution (FIGURE 13.6). Fires (p. 196) from burning vegetation generate soot and gases, and over 60 million ha (150 million acres) of forest and grassland burn in a typical year. Volcanic eruptions (pp. 231–233) release large quantities of particulate matter and sulfur dioxide into the troposphere, and major eruptions may blow matter into the stratosphere. Winds sweeping over arid terrain can send huge amounts of dust aloft—sometimes even from one continent to another. In July 2009, the bustling city of Tehran came to a standstill when windstorms blew sand and dust from drought-stricken Iraq into Iran, enveloping half of the country. Businesses, schools, and government offices were closed for several days, airplane flights were cancelled, and people were warned to stay indoors to safeguard their health.

Some natural impacts are made worse by human activ- ity and land use policies. Farming and grazing practices that strip vegetation from the soil promote wind erosion and dust storms (p. 141). Suppression of fire allows fuel to build up and eventually leads to more-destructive fires (pp. 196–197). And in the tropics, many farmers set fires to clear forest for agri- culture (pp. 139–140).

a straight path. This deflection is called the Coriolis effect, and it results in the curving global wind patterns displayed in Figure 13.4B. For centuries, people used these global wind patterns to facilitate ocean travel by wind-powered sailing ships.

Storms pose hazards Atmospheric conditions can sometimes create storms that threaten life and property. Hurricanes (FIGURE 13.5A) can form when winds rush into areas of low pressure where warm moisture-laden air over tropical oceans is rising. In the North- ern Hemisphere, these winds turn counterclockwise because of the Coriolis effect. In other regions, such cyclonic storms are called cyclones or typhoons. The powerful convective cur- rents of these storms draw up immense amounts of water va- por. As the warm moist air rises and cools, water condenses (because cool air cannot hold as much water vapor as warm air) and falls heavily as rain. In North America, the Gulf Coast and Atlantic Coast are most susceptible to hurricanes.

(a) Satellite image of a hurricane

(b) Photograph of a tornado

FIGURE 13.5  Hurricanes (a) and tornadoes (b) are two types of cyclonic storms that pose hazards to our life and property.282

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We create outdoor air pollution Human activity introduces many sources of air pollution. As with water pollution, air pollution can emanate from point sources or non-point sources (pp. 265–266). A point source de- scribes a specific location from which large quantities of pol- lutants are discharged, such as a coal-fired power plant. Non- point sources are more diffuse, consisting of many small, widely spread sources (such as thousands of automobiles).

Primary pollutants, such as soot and carbon monoxide, are pollutants emitted into the troposphere in a form that can cause harm or can react to form harmful substances. Harmful substances produced once primary pollutants react with con- stituents of the atmosphere are called secondary pollutants.

Pollutants differ in the amount of time they spend in the atmosphere—called their residence time—because sub- stances differ in how readily they react in air and in how quickly they settle to the ground. Pollutants with brief resi- dence times exert localized impacts over short time periods. Most particulate matter and most pollutants from automobile exhaust stay aloft only hours or days, which is why air quality in a city like Tehran or Los Angeles can change from day to day. In contrast, pollutants with long residence times can ex- ert impacts regionally or globally for long periods, even centu- ries. The pollutants that drive global climate change and those that deplete Earth’s ozone layer (two separate phenomena!— see FAQ, p. 290) are each able to cause these global and long- lasting impacts because they persist in the atmosphere for so long. FIGURE 13.7 shows this relationship, with examples.

Clean Air Act legislation addresses pollution in the United States To address air pollution in the United States, Congress has passed a series of laws, notably the Clean Air Act, first

enacted in 1963 and amended multiple times since, particu- larly in 1970 and 1990. This body of legislation funds research on pollution control, sets standards for air quality, imposes limits on emissions from new sources, and enables citizens to sue parties violating the standards. It also introduced an emissions trading program (p. 108) for sulfur dioxide. Begin- ning in 1995, businesses and utilities were allocated permits for emitting this pollutant and could buy, sell, or trade these allowances. Each year the overall amount of allowed pollu- tion was lowered. This market-based incentive program has helped reduce sulfur dioxide emissions nationally (see Figure 5.18, p. 109). The Los Angeles region adopted its own cap- and-trade program in 1994. The Regional Clean Air Incen- tives Market (RECLAIM) helped the L.A. basin decrease sul- fur dioxide emissions by 47% and nitrogen oxide emissions by 61% by 2003, and further cuts are being achieved as the program continues.

Under the Clean Air Act, the U.S. Environmental Pro- tection Agency (EPA) sets nationwide standards for emis- sions and for concentrations of pollutants in ambient air throughout the nation. It is largely up to the states to monitor air quality and develop, implement, and enforce regulations within their borders. States submit plans to the EPA for approval, and if a state’s plans are not adequate, the EPA can take over enforcement. When a region fails to clean up its air, the EPA can prevent it from receiving federal money for transportation projects.

The EPA sets standards for “criteria pollutants” The EPA and the states focus on six criteria pollutants, pollut- ants judged to pose especially great threats to human health— carbon monoxide (CO), sulfur dioxide (SO2), nitrogen diox- ide (NO2), tropospheric ozone (O3), particulate matter, and

(a) Natural fire in California (b) Mount Saint Helens eruption, 1980

FIGURE 13.6  Fires (a) in forests and grasslands are one source of natural air pollution. Volcanoes are another, as shown by Mount Saint Helens (b), which erupted in the state of Washington in 1980. Dust storms are a third source. Trade winds blowing soil across the Atlantic Ocean from Africa to the Americas (c) carry fungal and bacterial spores linked to die-offs in Caribbean coral reef systems, although they also bring nutrients to the Amazon rainforest.

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lead (Pb). For these, the EPA has established maximum con- centrations allowable in outdoor air.

Carbon monoxide Carbon monoxide is a colorless, odor- less gas produced primarily by the incomplete combustion of fuel. Vehicles and engines account for 78% of CO emissions in the United States. Other sources include industrial processes, fires, waste combustion, and residential wood burning. Car- bon monoxide can deprive us of oxygen because it can bind to hemoglobin in red blood cells, preventing the hemoglobin from binding with oxygen.

Sulfur dioxide Sulfur dioxide is a colorless gas with a pungent odor. Most SO2 pollution results from the combus- tion of coal for electricity generation and industry. During combustion, elemental sulfur (S) in coal reacts with oxygen (O2) to form SO2. Once in the atmosphere, SO2 may react to form sulfur trioxide (SO3) and sulfuric acid (H2SO4), which may return to Earth in acid deposition (pp. 291–294).

Nitrogen dioxide Nitrogen dioxide is a foul-smelling red- dish-brown gas that contributes to smog and acid deposition. Along with nitric oxide (NO), NO2 belongs to a family of com- pounds called nitrogen oxides (NOx). Nitrogen oxides result when atmospheric nitrogen and oxygen react at the high tem- peratures created by combustion engines. Most U.S. NOx emis- sions result from combustion in vehicle engines. Electrical utili- ty and industrial combustion account for most of the remainder.

Tropospheric ozone Although ozone in the stratosphere protects us by filtering UV radiation, ozone from human ac- tivity accumulates low in the troposphere. Here, this colorless gas is a secondary pollutant, created by the interaction of sun- light, heat, nitrogen oxides, and volatile carbon-containing chemicals. A major component of smog, O3 poses health risks as a result of its instability as a molecule; this triplet of oxygen atoms will readily split into a molecule of oxygen gas and a

free oxygen atom. The oxygen atom may then participate in reactions that can damage living tissues and cause respirato- ry problems. Tropospheric ozone is the pollutant that most frequently exceeds its EPA standard.

Particulate matter Particulate matter is composed of solid or liquid particles small enough to be suspended in the atmosphere and able to damage respiratory tissues when in- haled. Particulate matter includes primary pollutants such as dust and soot, as well as secondary pollutants such as sulfates and nitrates. The EPA classifies particulate pollution by the size of the particles. PM10 pollution consists of particles less than 10 microns in diameter (one-seventh the width of a hu- man hair), whereas PM2.5 pollution consists of still-finer par- ticles less than 2.5 microns in diameter. Most PM10 pollution is from road dust, whereas most PM2.5 pollution results from combustion processes.

Lead Lead is a heavy metal that enters the atmosphere as a particulate pollutant. The lead-containing compounds tetra- ethyl lead and tetramethyl lead, when added to gasoline, improve engine performance. However, exhaust from the combustion of leaded gasoline emits airborne lead, which can be inhaled or can settle on land and water. Lead can enter the food chain, accumu- late in body tissues, and cause central nervous system malfunc- tion, developmental problems in children, and other ailments (p. 209). Since the 1980s, leaded gasoline has been phased out in most industrialized nations (p. 5), and lead pollution has plum- meted. However, auto exhaust still generates lead pollution in developing nations where leaded gasoline has not been banned.

EPA monitoring finds that many Americans live where concentrations of criteria pollutants regularly reach unhealthy levels. Residents of Los Angeles County, for instance, breathe air that violates safety standards for four of the six criteria pol- lutants. All together, as of 2008, 127 million Americans lived in counties that violated the national ambient air quality stan- dards for at least one of the six criteria pollutants.

Ho urs Da

ys We

eks

Mo nth

s Yea

Ce ntu

rie s

Local

Regional

Hemispheric

M ax

im um

s ca

le o

f im

pa ct

Residence time in the atmosphere

Global

SO2

CO2 CH4 SF6 HFCs PFCs HCFCs CFCs

N2O

SO4 2– NO3

–

NH4 + PM2.5

NO2 NH3 PM10

Tropospheric ozone

FIGURE 13.7  Substances with short residence times in the atmo- sphere affect air quality only locally, whereas those with long residence times affect air quality regionally or globally. Data from United Nations Environment Programme, 2007. Global

environmental outlook (GEO-4), Nairobi,

Kenya.

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(pp. 108–109) and clean coal technologies (p. 336) have re- duced SO2 emissions. Technologies such as baghouse filters, electrostatic precipitators, and scrubbers (FIGURE 13.10) that chemically convert or physically remove airborne pollutants before they are emitted from smokestacks have allowed facto- ries, power plants, and refineries to reduce emissions of sev- eral pollutants. And the phaseout of leaded gasoline caused U.S. lead emissions to plummet by 93% in the 1980s alone.

The reduction of outdoor air pollution since 1970 rep- resents one of the greatest accomplishments achieved by the United States in safeguarding human health and environmen- tal quality. The EPA estimates that between 1970 and 1990, clean air regulations saved the lives of 200,000 Americans.

Plenty of room for improvement remains, however, be- cause concerns over new pollutants are emerging and because greenhouse gas emissions (p. 302) continue to rise. U.S. car- bon dioxide emissions rose 44% from 1970 to 2008. The U.S. Supreme Court in 2007 ruled that the EPA has the legal au- thority to regulate carbon dioxide as a pollutant. The EPA took the first steps toward doing so in 2011, but faces logistical challenges and formidable political opposition. Yet because we

Agencies monitor emissions Besides measuring concentrations of the six criteria pollut- ants in ambient air, state and local agencies also monitor and report to the EPA emissions of pollutants that affect ambient concentrations of the criteria pollutants. Emissions are moni- tored for the four criteria pollutants that are primary pollut- ants (carbon monoxide, sulfur dioxide, particulate matter, and lead), as well as for all nitrogen oxides (because NO re- acts in the atmosphere to form NO2, which is both a primary and secondary pollutant). Tropospheric ozone is a secondary pollutant only; we do not emit it. Instead, agencies monitor emissions of volatile organic compounds (VOCs), carbon- containing chemicals (such as hydrocarbons; p. 28) that can react to produce ozone and other secondary pollutants. The largest sources of anthropogenic VOC emissions include in- dustrial use of solvents and vehicle emissions. In the United States in 2008, human activity polluted the air with 123 mil- lion tons of the six monitored pollutants (FIGURE 13.8).

We have reduced U.S. air pollution Since the Clean Air Act of 1970, we have reduced emissions of each of the six monitored pollutants (FIGURE 13.9A), and total emissions of the six together have declined by 60%. These dramatic reductions in emissions have occurred de- spite substantial increases in the nation’s population, energy consumption, miles traveled by vehicle, and gross domestic product (FIGURE 13.9B).

We have achieved this success as a result of policy steps and technological developments, each motivated by grass- roots social demand for cleaner air. Cleaner-burning motor vehicle engines and automotive technologies such as catalytic converters have played a large part. In a catalytic converter, engine exhaust reacts with several metals that convert hy- drocarbons, CO2, and NOx into carbon dioxide, water vapor, and nitrogen gas. The sulfur dioxide permit-trading program

Carbon monoxide

(63%)

Nitrogen oxides (13%)

Sulfur dioxide (9%)

Particulate matter (2%)

Lead (0.002%)

Volatile organic

compounds (13%)

FIGURE 13.8  In 2008, the United States emitted 123 million tons of the six major pollutants whose emissions are monitored by the EPA and state and local agencies. Carbon monoxide accounted for most of these emissions, by mass. Data from U.S. EPA.

(a) Declines in six major pollutants

(b) Trends in major indicators

Emissions Population Energy consumption

Vehicle miles

traveled

GDP

NOx VOCs

Air pollutant

SO2 PM Pb 0

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Th ou

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(–62%) Year 1970 2008

(% change)

(–39%)(–54%) (–63%)

(–99%)

(–71%)

FIGURE 13.9 The EPA tracks emissions of several major pollut- ants into U.S. air. Each of these pollutants has shown substantial declines since 1970 (a), and emissions from all six together have declined by 60%. We have achieved these reductions in emissions despite increases (b) in U.S. population, energy consumption, vehicle miles traveled, and gross domestic product (GDP). Data for particulate matter is for PM10 since 1985. Data from U.S. EPA.

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were able to reduce emissions of several major pollutants by 60% while expanding our economy, we can hope that similar success might soon be achieved with greenhouse gas emissions.

Toxic pollutants pose health risks We are also reducing emissions of toxic air pollutants, substances known to cause cancer, reproductive defects, or neurological, developmental, immune system, or respiratory problems. Under the 1990 Clean Air Act, the EPA regulates 188 toxic air pollutants, ranging from mercury (from coal- burning power plant emissions and other sources) to VOCs such as benzene (a component of gasoline) and methylene chloride (found in paint stripper).

Based on monitoring at 300 sites across the United States, experts estimate that toxic air pollutants cause cancer in 1 out of every 28,000 Americans (36 cancer cases per 1 million people). Although residents of areas such as Los Angeles still experience high health risks, the EPA estimates that Clean Air Act regu- lations have helped to reduce emissions of toxic air pollutants since 1990 by more than 35%.

Industrializing nations are suffering increasing air pollution Although the United States and other industrialized nations have improved their air quality, outdoor air pollution is grow- ing worse in many industrializing countries. In these societies,

proliferating factories and power plants are emitting more pol- lutants as governments encourage economic growth. Addition- ally, more citizens own and drive automobiles (FIGURE 13.11). At the same time, most people continue to burn traditional sources of fuel such as wood, charcoal, and coal for cooking and home heating. Iran is typical: Studies find that each resident of Tehran inhales 7–9 kg (15–20 lb) of dust per year and that lev- els of CO, SO2, particulate matter, and other pollutants are well above international safety standards.

The people of China suffer some of the world’s worst air pollution. China has fueled its rapid industrial development with its abundant reserves of coal, the most-polluting fossil fuel (pp. 329–330). Power plants and factories have sprung up across the nation, often using outdated, inefficient, heav- ily polluting technology because it is cheaper and quicker to build. Car ownership is skyrocketing; in the capital of Beijing alone, 1,500 new cars hit the roads each day. In many cities the haze is often too thick for people to see the sun. Reports by Chinese scientists, the World Bank, and the World Health Or- ganization all estimate that outdoor air pollution causes over 300,000 premature deaths in China each year. Some of China’s pollution even reaches North America, blown by winds across the Pacific Ocean to western U.S. cities such as Los Angeles.

China’s government is now striving to reduce pollution. The government is closing down some heavily polluting fac- tories and mines, phasing out some subsidies for polluting in- dustries, and installing pollution controls in power plants. It

Dirty flue gas enters

Excess mist condenses on screen

Mist eliminator

Clean air

Spray nozzles

Mist of chemically treated water

Purified flue gas exits to stack5

Water is reused in spray nozzles

Settling tank

Piping

Water with pollutants

Gas rises through shower of mist

Mist captures pollutants and brings them to bottom

Dirty water is drained to tank and cleansed Dirty water is drained to tank and cleansed

Sludge is disposed of as hazardous waste

Polluted air

Pollutants removed from water

FIGURE 13.10  In this spray-tower wet scrubber, polluted air � rises through a chamber while arrays of nozzles spray a mist of water mixed with lime or other active chemicals �. The falling mist captures pollut- ants and carries them to the bottom of the chamber �, essentially washing them out of the air. Excess mist is captured on a screen �, and air emitted from the scrubber has largely been cleansed �. Periodical- ly, the dirty water is drained from the chamber �, cleansed in a settling tank, and recirculated � through the spray nozzles. The resulting sludge must be disposed of � as hazardous waste (pp. 392–395). Scrubbers and other pollution control devices come in many designs; the type shown here typically removes at least 90% of particulate matter and gases such as sulfur dioxide.

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is subsidizing people to buy efficient electric heaters for their homes, mandating cleaner formulations for gasoline and die- sel, raising standards for fuel efficiency, and tightening regula- tions on automotive emissions. In Beijing, mass transit is being expanded, many buses run on natural gas, and heavily pollut-

FIGURE 13.11  Automobile traffic in Tehran illustrates a major cause of air pollution in today’s industrializing nations. Officials estimate that Tehran’s vehicles emit 5,000 tons of pollutants each day as traffic creeps along at an average of 18 kph (11 mph).

ing vehicles are restricted in the central city. China is also ag- gressively developing cleaner wind, solar, and nuclear power to substitute for power produced by burning coal.

Smog is our most common air quality problem Let’s now take a closer look at our most widespread type of air pollution: smog. Smog is an unhealthy mixture of air pollutants that often forms over urban areas as a result of fossil fuel combustion. Since the onset of the industrial revo- lution, cities have suffered a type of smog known as industrial smog. When coal or oil is burned, some portion is completely combusted, forming CO2; some partially combusts, produc- ing CO; and some remains unburned and is released as soot (particles of carbon). Moreover, coal contains contaminants such as mercury and sulfur. Sulfur reacts with oxygen to form sulfur dioxide, which can undergo a series of reactions to form sulfuric acid and ammonium sulfate (FIGURE 13.12A). These chemicals and others produced by further reactions, along with soot, are the main components of industrial smog.

In the wake of London’s 1952 “killer smog” and other fa- tal pollution episodes, governments of developed nations be- gan regulating industrial emissions and have greatly reduced

Burning

Sulfur dioxide (SO2)

Oxygen (O2)

Carbon monoxide (CO) and carbon dioxide (CO2)

Coal and oil

Burning

Oxygen (O2)

Sulfur trioxide (SO3)

Oxygen (O2)

Ammonium sulfate ((NH4)2SO4)

Sulfur (S) in coal and oil Carbon (C)

Water vapor (H2O)

Ammonia (NH3)

Sulfuric acid (H2SO4)

(a) Formation of industrial smog

(b) Donora, Pennsylvania, at midday in the 1948 smog event

FIGURE 13.12  Emissions from the combustion of coal and oil in manufacturing plants and utilities without pollution control technologies can create industrial smog. Industrial smog consists primarily of sulfur dioxide and particulate matter, as well as carbon monoxide and carbon dioxide from the carbon component of fossil fuels. When fossil fuels are combusted, sulfur contaminants give rise to sulfur dioxide, which in the presence of other chemicals in the atmosphere can produce several other sulfur compounds (a). Under certain weather conditions, industrial smog can blanket whole towns or regions, as it did in Donora, Pennsylvania, shown here (b) in the daytime during its deadly 1948 smog episode.

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Nitrogen (N2)

Burning

Nitric oxide (NO)

Oxygen (O2)

and and

Ozone (O3)

Various pollutants

(a) Formation of photochemical smog

(b) Photochemical smog over Mexico City

Acid rain

Volatile organic compounds (VOCs)

Nitrogen dioxide (NO2)

Nitric oxide (NO)

Oxygen atom (O)

Nitric oxide (NO) Peroxyacyl nitrates (PANs)

HydrocarbonsWater vapor (H2O)

UV radiation

Nitric acid (HNO3)

Oxygen (O2) FIGURE 13.13  Nitric oxide, a key element of photo- chemical smog, can start a chemical chain reaction (a) that results in the production of other compounds, includ- ing nitrogen dioxide, nitric acid, ozone, and peroxyacyl nitrates (PANs). PANs can induce further reactions that damage living tissues in animals and plants. Nitric acid contributes to acid deposition as well as photochemical smog. Photochemical smog is common today over many urban areas, especially those with hilly topography or frequent inversion layers. Mexico City (b) is one city that frequently experiences photochemical smog.

industrial smog. However, in industrializing regions such as China, India, and eastern Europe, coal burning and lax pol- lution controls result in industrial smog that poses significant health risks.

As we’ve seen, weather and topography play roles in smog formation. Four years before London’s killer smog, a similar event occurred in Pennsylvania in the small town of Donora (FIGURE 13.12B). Air near the ground cooled in the night, and because Donora is in hilly terrain, morning sun did not reach the valley floor to warm and disperse the air. The resulting thermal inversion trapped smog from a steel and wire factory. Twenty-one people died, and over 6,000 people—nearly half the town—became ill.

Photochemical smog results from a series of reactions In most cities today, including Los Angeles and Tehran, pol- lution from automobile exhaust generates a different type of smog: photochemical smog. Photochemical smog forms when sunlight drives chemical reactions between primary pollutants and normal atmospheric compounds, produc- ing a mix of over 100 different chemicals, with tropospheric ozone often the most abundant (FIGURE 13.13A). Because it

also includes NO2, photochemical smog generally appears as a brownish haze (FIGURE 13.13B).

Hot, sunny, windless days in urban areas provide perfect conditions for the formation of photochemical smog. Exhaust from morning traffic releases NO and VOCs into a city’s air, and sunlight then promotes the production of ozone and other pollutants. Photochemical smog in urban areas typically peaks in midafternoon. Cities like Los Angeles and Tehran are prone to photochemical smog because they have sunny climates and because nearby mountains trap air and pro- mote thermal inversions. L.A.’s sister city of Mexico City, also ringed by mountains and with a sunny climate, suffers some of the world’s worst photochemical smog. In 1996, a 5-day cri- sis there killed at least 300 people and sent 400,000 to hospitals with eye, nose, throat, and respiratory problems.

We can take steps to reduce smog Los Angeles’s struggle with air pollution began in 1943, when the city’s first major smog episode cut visibility to three blocks. Since then, L.A. residents have dealt with headaches, eye irritation, asthma, lung damage, and related illnesses. However, Los Angeles confronted its problem and has made great progress in clearing the air since the 1970s.

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keep streaming into the city, and residents buy cars even fast- er. Together these trends have overtaken the government’s ef- forts, and pollution worsened again after 2006. In response, officials lowered gasoline subsidies, rationed fuel, and began expanding the subway system.

Of all the world’s cities, Mexico City is gaining attention today for its success in reducing its smog problem—once the world’s worst—even as its population, cars, and economic ac- tivity have grown. Regulations now require cars to have cata- lytic converters, get emissions tests, and stay off the roads one day per week. Some industrial facilities cleaned up their pro- cesses, and others were forced out. The national oil company Pemex removed lead from gasoline, improved its refineries, imported cleaner gasoline, and removed pollutants from the fuel that city residents use for cooking and heating. The sub- way system and a fleet of low-emission buses are being ex- panded, and a bike-hiring system has been introduced. As a result, smog is down, along with other pollutants such as lead and sulfur dioxide.

Current California health standard

0.20

0.25

0.30

0.35

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0.15

0.10

Pe ak

8 -h

ou r

oz on

e le

ve l (

pp m

)

0.05

0.00 1970 1980

Year

1990 2000 2010

FIGURE 13.14  Peak levels of tropospheric ozone in the Los An- geles region have been reduced since the 1970s, thanks to govern- ment policy and improved automotive technology. Ozone pollution still violates the state health standard, however. Data from Environ- ment California, 2010. Clean cars in California: Four decades of progress in the

unfinished battle to clean up our air.

California took the lead among U.S. states in adopting pol- lution control technology, setting emissions standards for vehicles, and pushing the federal government to do the same. California’s demands helped lead the auto industry to develop less-polluting cars. A 2010 study by the nonprofit group Environment California concluded that a new car to- day generates just 1% of the smog-forming emissions of a 1960s-era car. Because today’s cars are 99% cleaner, the air is cleaner, even with more drivers on the road. In Los An- geles, peak smog levels have decreased 60–70% since 1980 (FIGURE 13.14).

Today in California and 33 other U.S. states, drivers are required to have their vehicle exhaust inspected periodically. Vehicle inspection programs have cut emissions leading to photochemical smog by 30% in these states.

Despite its progress, Los Angeles still suffers the worst tropospheric ozone pollution of any U.S. metropolitan area, according to a 2010 study by the American Lung Association. L.A. residents breathe air exceeding California’s health stan- dard for ozone on more than 130 days per year. A 2008 study calculated that air pollution in the L.A. basin and the nearby San Joaquin Valley each year caused nearly 3,900 premature deaths and cost society $28 billion (due to hospital admis- sions, lost work days, etc.).

Tehran is estimated to lose $130 million for each “smog holiday” its government declares, when pollution forces it to close schools and offices and advise people to stay home. City officials there have taken steps to combat the problem. Vehicle inspections are now required in Tehran, traffic into the city center is restricted, and people are paid to turn in old cars for newer, cleaner ones. Sulfur was reduced in diesel fuel, lead was removed from gasoline, and buses began running on (cleaner-burning) natural gas. Citywide pollution maps were made available online, and 22 electronic indicator boards were installed around the city, displaying current pollutant levels. All these efforts helped reduce pollution. Yet people

Your Region’s Air Quality What outdoor air pollution challenges exist in your region? Explore one of the EPA websites that let you browse information on the air you breathe: www.

airnow.gov, www.epa.gov/aircompare, or www.epa.gov/ air/emissions/where.htm. How does your region’s air quality compare to the rest of the nation? What factors do you think influence the quality of your region’s air? Propose three steps for reducing air pollution in your region. What benefits might your region enjoy after taking such steps?

Synthetic chemicals deplete stratospheric ozone Although ozone in the troposphere is a pollutant in photo- chemical smog, ozone in the stratosphere protects life on Earth by absorbing the sun’s ultraviolet radiation, which can damage tissues and DNA. One generation ago, scientists discovered that our planet’s stratospheric ozone was being depleted, posing a threat to human health and the environ- ment. Years of research by hundreds of scientists (see THE SCIENCE BEHIND THE STORY, pp. 292–293) revealed that cer- tain airborne chemicals destroy ozone, and that most of these ozone-depleting substances are human-made.

In particular, researchers pinpointed halocarbons— human-made compounds derived from simple hydrocarbons (p. 28) in which hydrogen atoms are replaced by halogen atoms such as chlorine, bromine, or fluorine. Industry was mass-producing one class of halocarbon, chlorofluorocar- bons (CFCs), at 1 million tons per year in the 1970s. CFCs were useful as refrigerants, as fire extinguishers, as propel- lants for aerosol spray cans, as cleaners for electronics, and for making polystyrene foam. Because CFCs rarely reacted with other chemicals, scientists surmised that they would be harmless. Alas, because they are nonreactive, CFCs reach the

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FIGURE 13.16  The “ozone hole” consists of a region of thinned ozone density in the stratosphere over Antarctica and the southernmost ocean regions. It has reappeared seasonally each September in recent decades. This colorized satellite imagery of Earth’s Southern Hemisphere from September 24, 2006, shows the ozone hole (purple and blue colors) at its maximal recorded extent to date.

Chlorofluorocarbons

Refrigerators, aerosol spray cans, air conditioners

(sources of chlorofluorocarbons)

Chlorine

Stratosphere

Oxygen Oxygen atom O

Chlorine monoxide

Oxygen

Ozone

UV radiation

CFC

FIGURE 13.15  A chlorine atom released from a CFC molecule in the presence of UV radiation reacts with an ozone molecule, form- ing one molecule of oxygen gas and one chlorine monoxide (ClO) molecule. The oxygen atom of the ClO molecule will then bind with a stray oxygen atom to form oxygen gas, leaving the chlorine atom to begin the destructive cycle anew. In this way, a chlorine atom can destroy up to 100,000 ozone molecules.

FAQ

Q: Is the ozone hole related to global warming?

A: This is a common misconception held by the public. Some people believe that the depletion of stratospheric ozone helps to prevent global warming by letting heat or greenhouse gases out of the atmosphere. Other people believe that ozone depletion worsens warming by letting heat into the atmosphere. Neither is true. Ozone depletion lets in excess ultraviolet radiation from the sun, but this does not appreciably warm or cool the atmosphere. However, research published in 2011 suggested that the ozone hole does apparently affect atmospheric circulation and rainfall in the Southern Hemisphere. We still have more to learn, and perhaps researchers will discover further connections between aspects of climate change and ozone depletion.

stratosphere unchanged and can linger there for a century or more. In the stratosphere, intense solar radiation breaks bonds in CFC molecules, releasing their constituent chlorine atoms. In a two-step chemical reaction ( FIGURE 13.15 ), each newly freed chlorine atom can split an ozone molecule and then ready itself to split another one. During its long resi- dence time in the stratosphere, each free chlorine atom can destroy as many as 100,000 ozone molecules!

In 1985, researchers shocked the world by announcing that stratospheric ozone levels over Antarctica in springtime had declined by nearly half in just the previous decade, leaving a thinned ozone concentration that was soon dubbed the ozone hole ( FIGURE 13.16 ). During each Southern Hemisphere spring since then, ozone concentrations over this immense region have dipped to roughly half their historic levels.

The Montreal Protocol addressed ozone depletion International policy efforts to restrict production of CFCs bore fruit in 1987 with the Montreal Protocol . In this treaty, signatory nations (eventually numbering 196) agreed to cut CFC production in half by 1998. Five follow-up agreements deepened the cuts, advanced timetables for compliance, and addressed additional ozone-depleting substances. The sub- stances covered by these agreements have now been mostly phased out, and industry has been able to shift to safer alterna- tive chemicals.

As a result, we have evidently stopped the Antarctic ozone hole from growing worse ( FIGURE 13.17 ). However, the ozone layer is not expected to recover completely until 2060 –2075. Much of the 5 billion kg (11 billion lb) of CFCs emitted into the troposphere has yet to diffuse up into the stratosphere, so concentrations may not peak there until 2020. Because of this and the long residence times of many halocarbons, we can ex- pect a long time lag between implementation of policy and the desired environmental effect.

Because of its success in addressing ozone depletion, the Montreal Protocol is widely viewed as a model for interna- tional cooperation on other global problems, from biodiver- sity loss (p. 177 ) to persistent organic pollutants (p. 223 ) to climate change (p. 319 ).

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(pp. 27–28), primarily sulfuric acid and nitric acid. Suspended in the troposphere, droplets of these acids may travel days or weeks for hundreds of kilometers (FIGURE 13.18).

Acid deposition has many impacts Acid deposition has wide-ranging detrimental effects on ecosystems and on our infrastructure (TABLE 13.1). Acids leach nutrients such as calcium, magnesium, and potassium ions out of the topsoil, altering soil chemistry and harm- ing plants and soil organisms. Acid precipitation also “mo- bilizes” toxic metal ions such as aluminum, zinc, mercury, and copper by chemically converting them from insoluble forms to soluble forms. Elevated soil concentrations of

Acid deposition is another transboundary pollution problem Just as stratospheric ozone depletion crosses political bound- aries, so does acid deposition, the deposition of acidic (p. 27) or acid-forming pollutants from the atmosphere onto Earth’s surface. This can take place by precipitation (commonly re- ferred to as acid rain, but also including acid snow, sleet, and hail), by fog, by gases, or by the deposition of dry particles. Acid deposition is one type of atmospheric deposition, which refers more broadly to the wet or dry deposition of a variety of pollutants, including mercury, nitrates, organo- chlorines, and others.

Acid deposition originates primarily with the emission of sulfur dioxide and nitrogen oxides, largely through fossil fuel combustion by automobiles, electric utilities, and industrial facilities. Once airborne, these pollutants can react with wa- ter, oxygen, and oxidants to produce compounds of low pH

Year

Si ze

o f

oz on

e ho

le (m

ill io

ns o

f sq

k m

) Montreal Protocol

1979 1985 1990 1995 2000 2005 2010

FIGURE 13.17  The Antarctic ozone hole grew in size quickly after its appearance in 1979, but phaseouts of ozone-depleting substances beginning in 1987 have apparently halted its growth. Data from NASA, reflecting averages from 7 Sept.–13 Oct. each year.

Acid precipitation

Primary pollutants Secondary pollutants

Sulfuric acid (H2SO4)

Nitric acid (HNO3)

Water (H2O) Oxygen (O2) and oxidants

Sulfur dioxide (SO2)

Nitric oxide (NO) FIGURE 13.18  Acid deposi- tion can have consequences far downwind from its source. Sulfur dioxide and nitric oxide emitted by industries and utilities are transformed into sulfuric acid and nitric acid through chemical reac- tions in the atmosphere. These acidic compounds descend to Earth’s surface in rain, snow, fog, and dry deposition.

TABLE 13.1 Effects of Acidic Deposition

Acidic deposition in northeastern U.S. forests has ▶ Accelerated leaching of base cations (ions such as Ca2+,

Mg2+, Na+, and K+, which counteract acid deposition) from soil

▶ Allowed sulfur and nitrogen to accumulate in soil (Excess N may overfertilize native plants and encourage weeds.)

▶ Increased dissolved inorganic aluminum in soil, hindering plant uptake of water and nutrients

▶ Leached calcium from needles of red spruce, leading to tree mortality from wintertime freezing

▶ Increased mortality of sugar maples due to leaching of base cations from soil and leaves

▶ Acidi�ed 41% of Adirondack, New York, lakes and 15% of New England lakes

▶ Lowered lakes’ capacity to neutralize further acids ▶ Elevated aluminum levels in surface waters ▶ Reduced species diversity and abundance of aquatic life,

a ecting entire food webs

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THE SCIENCE BEHIND THE STORY

Discovering Ozone Depletion and the Substances Behind It

In discovering and coming to understand the depletion of stratospheric ozone, scientists have used historical records, field observations, laboratory experiments, computer models, and satellite technology.The story starts back in 1924, when British scientist G.M.B. Dobson built an instrument that measured atmospheric ozone concentrations by sampling sun- light at ground level and comparing the intensities of wavelengths that ozone does and does not absorb. By the 1970s, the Dobson ozone spectrophotometer was being used to monitor ozone by a global network of observation stations.

Drs. F. Sherwood Rowland (left), Mario Molina (center), and Paul Crutzen (right) at a press conference after receiving the Nobel Prize

metal ions such as aluminum weaken plants by damaging root tissue, hindering their uptake of water and nutrients. In some areas, acid fog with a pH of 2.3 (equivalent to vinegar, and over 1,000 times more acidic than normal rainwater) has enveloped forests and killed trees.

When acidic water runs off from land, it affects streams, rivers, and lakes. Thousands of lakes in Canada, Scandinavia, the United States, and elsewhere have lost their fish because acid precipitation leaches aluminum ions from soil and rock into waterways, where they damage the gills of fish and disrupt their salt balance, water balance, breathing, and circulation. Terrestrial animals are affected, too; populations of snails and

other invertebrates typically decline, and this reduces the food supply for birds.

Acid deposition damages crops, erodes stone buildings, corrodes cars, and erases the writing on tombstones. Ancient cathedrals in Europe, temples in Asia, and monuments in Washington, D.C., are experiencing billions of dollars of dam- age as their features dissolve away.

Because the pollutants leading to acid deposition can travel long distances, their effects may be felt far from their sources. Regions of greatest acidification, shown in FIGURE 13.19, tend to be downwind from heavily industrialized source areas of pollution.

Meanwhile, atmospheric chemists were learning how stratospheric ozone is created and destroyed. Ozone and oxygen exist in a natural balance, with one occasionally reacting to form the other, and oxygen being far more abun- dant. Researchers found that certain chemicals naturally present in the atmo- sphere, such as nitric oxide (NO), de- stroy ozone. Dutch meteorologist Paul Crutzen reported in 1970 that nitrous oxide (N2O) produced by soil bacteria can make its way to the stratosphere and produce NO. And some human activities, such as fertilizer application, were increasing emissions of N2O.

Following Crutzen’s report, American scientists Richard Stolarski and Ralph Cicerone showed in 1973 that chlorine atoms can destroy ozone even more effectively than N2O can. And two years earlier, British scientist

James Lovelock had developed an instrument to measure trace amounts of atmospheric gases and found that virtually all the chlorofluorocarbons (CFCs) humanity had produced in the past four decades were still aloft, accumulating in the stratosphere.

This set the stage for the key in- sight. In 1974, American chemist F. Sher- wood Rowland and his Mexican post- doctoral associate, Mario Molina, took note of all the preceding research and realized that CFCs were rising into the stratosphere, being broken down by UV radiation, and releasing chlorine atoms that ravaged the ozone layer (see Figure 13.15, p. 290). Molina and Rowland’s analysis, published in the journal Nature, eventually earned them the Nobel Prize in chemistry jointly with Crutzen.

The paper also sparked discussion about setting limits on CFC emissions.

Industry leaders attacked the research; DuPont’s chairman of the board report- edly called it “a science fiction tale . . . a load of rubbish . . . utter nonsense.” But measurements by numerous research- ers soon confirmed that CFCs and other halocarbons were indeed deplet- ing ozone. As a result, the United States and several other nations banned the use of CFCs in aerosol spray cans in 1978. Other uses continued, however, and by the early 1980s global produc- tion of CFCs was on the rise.

Then, a shocking new finding spurred the international community to take action. In 1985, Joseph Farman and colleagues analyzed data from a British research station in Antarctica that had been recording ozone concen- trations since the 1950s. Farman’s team reported in Nature that springtime Antarctic ozone concentrations had plummeted by 40–60% just since the 1970s (see figure, part (a)).

Farman’s team had beaten a group of NASA scientists to the punch. The NASA scientists were sitting on reams of data from satellites showing a global drop in ozone levels (see figure, part (b)), but they had not yet submitted their analysis for publication.

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Year

(a) Monthly mean stratospheric ozone levels at Halley, Antarctica

1955

350 September

October300

250 O

zo ne

le ve

ls (D

ob so

n un

its )

200

150

100 1965 1975 1985 1995 2005 2010

Year

(b) Global stratospheric ozone readings from 3 satellites

1980

310

300

290

O zo

ne (D

ob so

n un

its )

280

1985 1990 1995 2000

Nimbus 7 TOMS

Farman et al. paper

Earth probe TOMS

Meteor 3 TOMS

Data from Halley, Antarctica (a), show a decrease in strat- ospheric ozone concentrations from the 1960s to 1990. Once ozone-depleting substances began to be phased out under the 1987 Montreal Protocol, ozone concentrations stopped declining. The paper by Joseph Farman et al. describing the ozone hole and using these data was published in 1985. Ozone decline and stabilization are also evident globally, as seen in these data (b) from three NASA satellites (averages of all regions between 65°N and 65°S latitude). Data in (a) from British Antarctic Survey. Data in (b) from NASA.

We have begun to address acid deposition Policy has helped us address acid deposition. The emissions trading program for sulfur dioxide established by the Clean Air Act of 1990 has reduced SO2 emissions (see Figure 5.18, p. 109) by 64% so far. A 2005 study calculated that the pro- gram’s economic benefits outweighed its costs by 40 to 1.

The economic incentives created by this cap-and-trade program encourage polluters to invest in technologies such as scrubbers (p. 286) and to devise other ways to become cleaner and more efficient. As a result of declining SO2 emissions, av-

erage sulfate precipitation in 2007–2009 was 43% lower than in 1989–1991 in the eastern United States. Emissions of NOx also have been lowered by EPA regulation and by technologi- cal advances, and wet nitrogen deposition declined between these periods as well. As with ozone depletion, however, it will take time for ecosystems to recover—and scientists advise that further pollution reductions are needed.

Meanwhile, in the industrializing world, acid depo- sition is becoming worse. Coal-dependent China emits the most sulfur dioxide of any nation and has the world’s worst acid rain problem. Overall, data on acid deposition show that although we have made advances in the control

But why should ozone loss be localized over Antarctica in the south- ern spring? To answer this question, atmospheric chemists Susan Solomon, James Anderson, Crutzen, and others mounted expeditions in 1986 and

1987 to analyze atmospheric gases us- ing ground stations and high-altitude balloons and aircraft. They learned that in the frigid Antarctic winter, polar stratospheric clouds form at high-alti- tudes. These icy clouds contain nitric

acid, which splits chlorine atoms off from CFCs. The freed chlorine atoms accumulate in the clouds, trapped over Antarctica by wind currents that swirl around in a circular polar vor- tex. In the Antarctic spring (starting in September), sunshine returns and speeds the chlorine atoms’ destruc- tion of ozone. The ozone hole lingers over Antarctica until December, when warmth shuts down the polar vortex, allowing air to mix with air from else- where. The ozone hole vanishes until the following spring.

By 1987, the mass of scientific evidence helped convince the world’s nations to agree on the Montreal Proto- col. Within two years, further scientific evidence and computer modeling showed that more drastic measures were needed. In 1990, the Montreal Protocol was strengthened to include a complete phaseout of CFCs by 2000, in the first of several follow-up agree- ments. Today, amounts of ozone-de- pleting substances in the stratosphere are beginning to level off.

As the ozone layer begins a long- term recovery, scientists continue their research. In 2009, a team led by A.R. Ravishankara of the National Oce- anic and Atmospheric Administration determined that nitrous oxide (N2O) had now become the leading cause of ozone depletion. Its emissions are not regulated, so its impacts now sur- pass those currently exerted by the remaining halocarbons. Ravi shankara’s team points out that regulating nitrous oxide, which is also a potent greenhouse gas, would help mitigate climate change as well as speed ozone recovery.

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less popular in developed nations in recent years, it is still es- timated to cause over 160,000 lung cancer deaths per year in the United States alone.

Radon gas is the second-leading cause of lung cancer in the developed world, responsible for an estimated 21,000 deaths per year in the United States and for 15% of lung can- cer cases worldwide. Radon (p. 209) is a radioactive gas result- ing from the natural decay of uranium in soil, rock, or wa- ter. It seeps up from the ground and can infiltrate buildings. Colorless and odorless, radon’s presence can be impossible to predict without knowing an area’s underlying geology. The only way to determine whether radon is entering a building is to sample air with a test kit. The EPA estimates that 6% of U.S. homes exceed its safety standard for radon. Since the 1980s, millions of U.S. homes have been tested for radon, close to a

of outdoor air pollution, more can be done. The same can be said for indoor air pollution, a source of human health threats that is less familiar to most of us, but statistically more dangerous.

INDOOR AIR POLLUTION Indoor air generally contains higher concentrations of pol- lutants than does outdoor air. As a result, the health ef- fects from indoor air pollution in workplaces, schools, and homes outweigh those from outdoor air pollution. The World Health Organization (WHO) and other international agencies attribute the majority of the world’s 2–3 million an- nual deaths from air pollution to indoor air pollution. This means that indoor air pollution takes several thousand lives each day.

Risks differ in developing and developed nations Indoor air pollution exerts most impact in the developing world, where poverty forces millions of people to burn wood, charcoal, animal dung, or crop waste inside their homes for cooking and heating with little or no ventilation (FIGURE 13.20). In the process, people inhale dangerous amounts of soot and carbon monoxide. The WHO estimates that indoor air pollution from fuelwood burning kills 1.6 million people each year, comprising over 5% of all deaths in some developing nations and 2.7% of the entire global disease burden.

In developed nations, by contrast, the primary indoor air health risks are cigarette smoke and radon. Smoking ciga- rettes irritates the eyes, nose, and throat; worsens asthma and other respiratory ailments; and greatly increases the risk of lung cancer. Inhaling secondhand smoke causes many of the same problems. Tobacco smoke is a brew of over 4,000 chemical compounds, some of which are known or suspected to be toxic or carcinogenic. Although smoking has become

Lab pH

5.3 5.2 – 5.3

5.1 – 5.2

5.0 – 5.1

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FIGURE 13.19  This U.S. map shows pH values for precipitation. Precipitation is most acidic in the Northeast and Midwest, near and downwind from (roughly east of) areas of heavy industry. Data are for 2009, from the National Atmospheric

Deposition Program.

FIGURE 13.20  In the developing world, many people build fires inside their homes for cooking and heating, as seen here in a Maasai home in Kenya. Indoor fires expose family members to severe particulate matter and carbon monoxide pollution.

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million have undergone radon mitigation, and new homes are being built with radon-resistant features.

Many substances pollute indoor air In our daily lives at home, we are exposed to many indoor air pollutants (FIGURE 13.21). The most diverse are volatile organic compounds. These airborne carbon-containing com- pounds are released by plastics, oils, perfumes, paints, clean- ing fluids, adhesives, and pesticides. VOCs evaporate from furnishings, building materials, color film, carpets, laser printers, fax machines, and sheets of paper. Some products, such as chemically treated furniture, release large amounts of VOCs when new and progressively less as they age. Oth- er items, such as photocopying machines, emit VOCs each

time they are used. Formaldehyde—a VOC widely used in pressed wood, insulation, and other products—irritates mu- cous membranes, induces skin allergies, and causes other ailments. The “new car smell” that fills the interiors of new automobiles comes from a complex mix of dozens of VOCs as they outgas from the newly manufactured plastic, metal, and leather components of the car. Some scientific studies warn of health risks from this brew and recommend that you keep a new car well-ventilated.

VOCs are often held responsible for sick-building syndrome, an illness resulting from indoor pollution in which the specific cause is not identified. Microorganisms such as bacteria, fungi, and mold can also induce allergic responses and cause building-related illness. Heating and cooling sys- tems in buildings make ideal breeding grounds for microbes,

Heating and cooling ducts Pollutants: Mold and bacteria Health risks: Allergies, asthma, respiratory problems

Hot showers with chlorine-treated water Pollutant: Chloroform Health risks: Nervous system damage

Furniture; carpets; foam insulation; pressed wood Pollutant: Formaldehyde Health risks: Respiratory irritation, cancer

Old paint Pollutant: Lead Health risks: Nervous system and organ damage

Leaky or unvented gas and wood stoves and furnaces; car left running in garage Pollutant: Carbon monoxide Health risks: Neural impairment, fatal at high dosesFireplaces;

wood stoves Pollutant: Particulate matter Health risks: Respiratory problems, lung cancer

Gasoline Pollutant: VOCs Health risks: Cancer

Pipe insulation; floor and ceiling tiles Pollutant: Asbetos Health risks: Asbestosis

Unvented stoves and heaters Pollutant: Nitrogen oxides Health risks: Respiratory problems

Tobacco smoke Pollutants: Many toxic or carcinogenic compounds Health risks: Lung cancer, respiratory problems

Pets Pollutant: Animal dander Health risks: Allergies

Computers and office equipment Pollutant: VOCs Health risks: Irritation, neural or organ damage, cancer

Pesticides; paints; cleaning fluids Pollutants: VOCs and others Health risks: Neural or organ damage, cancer

Rocks and soil beneath house Pollutant: Radon Health risks: Lung cancer

FIGURE 13.21  The typical U.S. home contains many sources of indoor air pollution. Shown are common sources, the major pollutants they emit, and some of the health risks they pose.

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How Safe Is Your Indoor Environ- ment? Think about the amount of time you spend indoors. Name some potential indoor air quality hazards in your home, work, or school en-

vironment. Are these spaces well ventilated? What could you do to improve the safety of the indoor spaces you use?

We can reduce indoor air pollution Using low-toxicity materials, monitoring air quality, keeping rooms clean, and providing adequate ventilation are the keys to alleviating indoor air pollution in most situations. In the developed world, we can limit our use of plastics and treated wood when possible and limit our exposure to pesticides, cleaning fluids, and other toxic substances by keeping them in a garage or outdoor shed. The EPA recommends that we test our homes and offices for radon and mold and install de- tectors for carbon monoxide. Keeping rooms and air ducts clean and free of mildew and other biological pollutants will reduce potential irritants and allergens. Most of all, keeping our indoor spaces well ventilated will minimize concentra- tions of pollutants.

providing moisture, dust, and foam insulation as substrates, as well as air currents that blow the organisms through the air. The U.S. Occupational Safety and Health Administration (OSHA) has estimated that 30–70 million Americans have suffered ailments related to the environment of the building in which they live.

➤ CONCLUSION Indoor air pollution poses potentially serious health hazards, but by keeping informed of the latest scientific findings and taking appropriate precautions we can minimize our risk. Outdoor air pollution has been addressed more effectively by government legislation and regulation, together with pol- lution-control technologies. In fact, reductions in outdoor air pollution in the United States and other developed nations represent some of the greatest strides made in environmental

protection to date. Room for improvement remains, however, particularly in reducing acid deposition and photochemical smog. In the developing world, indoor and outdoor air pollut- ant levels are higher and take a heavy toll on people’s health. Reducing pollution from automobile exhaust, coal combustion in outmoded facilities, indoor fuelwood burning, and other sources pose challenges as the world’s less-wealthy nations industrialize.

T E S T I N G Y O U R C O M P R E H E N S I O N 1. About how thick is Earth’s atmosphere? Name one char-

acteristic of the troposphere and one characteristic of the stratosphere.

2. Where is the “ozone layer” located? How and why is stratospheric ozone beneficial for people, whereas tropo- spheric ozone is harmful?

3. How does solar energy influence weather and climate? How do Hadley, Ferrel, and polar cells help to determine long-term climatic patterns and the location of biomes?

4. Describe a thermal inversion. How do inversions con- tribute to severe smog episodes like the ones in London and in Donora, Pennsylvania?

5. How does a primary pollutant differ from a secondary pollutant? Give an example of each.

6. What has happened with concentrations of “criteria pol- lutants” in U.S. ambient air in recent decades? What has

happened with our emissions of major pollutants? Name one health risk from toxic air pollutants.

7. How does photochemical smog differ from industrial smog? How do the weather and topography influence smog formation?

8. How do chlorofluorocarbons (CFCs) deplete stratospher- ic ozone? Why is this depletion considered a long-term international problem? What has been done to address this problem?

9. Why are the effects of acid deposition often felt in areas far from where the primary pollutants are produced? List three impacts of acid deposition.

10. Name three sources of indoor pollution and their associ- ated health risks. For each pollution source, describe one way to reduce exposure to the source.

S E E K I N G S O L U T I O N S 1. Consider responses to the photochemical smog that has

plagued Los Angeles, Tehran, Mexico City, and other metropolitan areas. Describe several ways in which ma- jor cities have tried to improve their air quality.

2. Describe how and why emissions of major pollutants have been reduced by over 50% in the United States since 1970, despite increases in population, energy use, and economic activity.

3. International regulatory action has produced reductions in CFCs, but other transboundary pollution issues, in- cluding acid deposition, have not yet been addressed as effectively. What types of actions do you feel are appro- priate for pollutants that cross political boundaries?

4. THINK IT THROUGH You have become the head of your county health department, and the EPA informs you that your county has failed to meet the national

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emissions are you responsible for creating? How many pounds would you prevent if you were to reduce by half the vehicle miles you travel? What percentage of your to- tal NOx emissions would that be?

3. How might you reduce your vehicle miles traveled by 50%? What other steps could you take to reduce the NOx emissions for which you are responsible?

ambient air quality standards for ozone, sulfur dioxide, and nitrogen dioxide. Your county is partly rural but is home to a city of 200,000 people and 10 sprawling sub- urbs. There are several large and aging coal-fired power plants, a number of factories with advanced pollution control technology, and no public transportation system. What steps would you urge the county government to take to meet the air quality standards? Explain how you would prioritize these steps.

5. THINK IT THROUGH You have been elected mayor of the largest city in your state. Your city’s residents are complaining about photochemical smog and traffic

congestion. Traffic engineers and city planners project that population and traffic will grow by 20% in the next decade. Some experts are urging you to restrict traffic into the city, allowing only cars with odd-numbered license plates on odd-numbered days, and those with even-numbered plates on even-numbered days. How- ever, business-owners fear losing money if shoppers are discouraged from visiting. Consider the particulars of your city, and then decide whether you will pursue an odd-day/even-day driving program, and explain why or why not. What other steps would you take to address your city’s smog problem?

C A L C U L A T I N G E C O L O G I C A L F O O T P R I N T S “While only some motorists contribute to traffic fatalities, all motorists contribute to air pollution fatalities.” So stated a writer for the Earth Policy Institute, in pointing out that air pollution kills far more people than vehicle accidents. According to EPA data, emissions of nitrogen oxides in the United States in 2008 totaled 16.3 million tons. Nitro- gen oxides come from fuel combustion in motor vehicles,

power plants, and other industrial, commercial, and resi- dential sources, but fully 9.5 million tons of the 2008 total came from vehicles. The U.S. Census Bureau estimates the nation’s population to have been 304.1 million in 2008 and projects that it will reach 334.1 million in 2020. Considering these data, calculate the missing values for 2008 in the table below (1 ton = 2,000 lb).

1. By what percentage is the U.S. population projected to increase between 2008 and 2020? Do you think that NOx emissions will increase, decrease, or remain the same over that period of time? Why? (You may want to refer to Figure 13.9.)

2. Assume you are an average American driver. Using the 2008 emissions totals, how many pounds of NOx

Total NOx emissions (lb) NOx emissions from vehicles (lb) You Your class Your state United States

Data from U.S. EPA.

Go to www.masteringenvironmentalscience.com for homework assignments, practice quizzes, Pearson eText, and more.

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Global Climate Change

Upon completing this chapter, you will be able to:

� Describe Earth’s climate system and explain factors influencing global climate � Characterize human influences on the atmosphere and on climate � Summarize how researchers study climate � Outline current trends and impacts of global climate change � Describe predicted future trends and impacts of global climate change � Suggest and assess ways we may respond to climate change

The Maldives’ underwater cabinet meeting

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CENTRAL CASE STUDY

Rising Seas May Flood the Maldives “Global warming and climate change can effectively kill us off, make us refugees. . . .”

—Ismail Shafeeu, Minister of Environment, Maldives

“If we can’t save the Maldives today, we can’t save London, New York, or Hong Kong tomorrow.” —Mohamed Nasheed, President, Maldives

O n October 17, 2009, President Mohamed Nasheed of the Maldives, a nation of low-

lying islands in the Indian Ocean, donned scuba gear and dove into the blue waters

of Girifushi Island lagoon. He was followed by his entire cabinet.

These officials pro- ceeded to hold a cabinet meeting underwater—no doubt the world’s first such meeting ever. Sitting at a table beneath the waves, they signed a declaration reading:

SOS from the front line: Cli- mate change is happening and it threatens the rights and security of everyone on Earth. With less than one degree of global warming, the glaciers are melting, the ice sheets collapsing, and low-lying areas are in danger of being swamped. We must unite in a global effort to halt further tem- perature rises, by slashing carbon dioxide emissions to a safe level of 350 parts per million.

Known for its spectacular tropical setting, color- ful coral reefs, and sun-drenched beaches, the Mal- dives seems a paradise to its many visiting tourists— while for 370,000 Maldives residents, the islands are home. But residents and tourists alike now fear that the Maldives could be submerged by rising seas brought by global climate change.

Nearly 80% of the Maldives’ land area lies less than 1 m (39 in.) above sea level. In a nation of 1,200 islands whose highest point is just 2.4 m (8 ft) above sea level, rising seas are a matter of life or death. The world’s oceans rose 10–20 cm (4–8 in.) during the 20th century as warming temperatures expanded ocean water and as melting polar ice discharged wa- ter into the ocean. According to current projections,

sea level will rise another 18–59 cm (7–23 in.) by the year 2100.

Higher seas would flood large areas of the Maldives and cause salt wa- ter to contaminate drinking water supplies. Storms in- tensified by warmer water temperatures are expected to erode beaches and dam- age the coral reefs that are so vital to the tourism and fishing industries that drive the nation’s economy. The

Maldives government has evacuated residents from several of the lowest-lying islands, and residents of other islands are considering moving.

President Nasheed is making sure the world knows of his country’s situation. He points out that small island nations like his are not responsible for the carbon emissions driving global climate change, yet they are the ones bearing the brunt of the con- sequences. “If things go business as usual,” he has said, “we will not live; we will die. Our country will not exist.”

The underwater cabinet meeting kicked off a global campaign to draw attention to the impacts of climate change, sponsored by the nonprofit group 350.org. This campaign culminated in an International Day of Climate Action on October 24, 2009, when 5,200 events took place in 181 nations.

Residents of the Maldives are not alone in their predicament. Other island nations, from the Galápagos to Fiji to the Seychelles, also fear encroaching seawater.

Maldives

Indian Ocean

EUROPE ASIA

INDIA

AFRICA

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how our planet’s climate functions. Thus, we first will sur- vey the fundamentals of Earth’s climate system—a complex and finely tuned system that has nurtured life for billions of years.

Three factors influence climate Three natural factors exert the most influence on Earth’s cli- mate. The first is the sun. Without it, Earth would be dark and frozen. The second is the atmosphere. Without it, Earth would be on average 33 °C (59 °F) colder, and temperature differences between night and day would be far greater than they are. The third is the oceans, which store and transport heat and moisture.

The sun supplies most of our planet’s energy. Earth’s at- mosphere, clouds, land, ice, and water together absorb about 70% of incoming solar radiation, and reflect the remaining 30% back into space ( FIGURE 14.1 ).

Greenhouse gases warm the lower atmosphere As Earth’s surface absorbs solar radiation, the surface in- creases in temperature and emits infrared radiation (p. 30 ), radiation with wavelengths longer than those of visible light. Atmospheric gases with three or more atoms in their molecules tend to absorb infrared radiation very effective- ly. These include water vapor, ozone (O 3 ), carbon dioxide (CO 2 ), nitrous oxide (N 2 O), and methane (CH 4 ), as well as halocarbons, a diverse group of mostly human-made gases that includes chlorofluorocarbons (CFCs; p. 289 ). Such gases are known as greenhouse gases . After absorbing radiation emitted from the surface, greenhouse gases subsequently re-emit infrared radiation in all directions. Some of this re- emitted energy is lost to space, but some travels back down- ward, warming the atmosphere (specifically the troposphere ; p. 279 ) and the planet’s surface in a phenomenon known as the greenhouse effect .

These island nations have organized to make their pos- ition on climate change known to the world through AOSIS, the Alliance of Small Island States.

Mainland coastal areas of the world, from the hurricane-battered coasts of Florida, Louisiana, Texas, the Carolinas, and other states, to coastal cities such as New York and San Francisco, will face similar challeng- es from sea level rise—and this is just one of the many consequences of global climate change. In one way or another, climate change will affect each and every one of us for the remainder of our lifetimes. �

OUR DYNAMIC CLIMATE Climate influences virtually everything around us, from the day’s weather to major storms, from crop success to human health, and from national security to the ecosystems that support our economies. If you are a student in your teens or twenties, the accelerating changes in our climate today may well be the major event of your lifetime and the phenomenon that most shapes your future.

Climate change is also the fastest-developing area of en- vironmental science. New scientific studies that refine our understanding of climate are published every week, and poli- cymakers and businesspeople make decisions and announce- ments just as quickly. By the time you read this chapter, some of its information will already be out of date. We urge you to explore further, with your instructor and on your own, the most recent information on climate change and the impacts it will have on your future.

What is climate change? Climate describes an area’s long-term atmospheric condi- tions, including temperature, precipitation, wind, humidity, barometric pressure, solar radiation, and other character- istics. Climate differs from weather (p. 281 ) in that weather specifies conditions at localized sites over hours or days, whereas climate describes conditions across broader regions over seasons, years, or centuries. Global climate change de- scribes modifications in aspects of Earth’s climate, such as temperature, precipitation, and storm frequency and inten- sity. People often use the term global warming synonymously in casual conversation, but global warming refers specifi- cally to an increase in Earth’s average surface temperature. Global warming is only one aspect of global climate change, although warming does in turn drive other components of climate change.

Over the long term, our planet’s climate varies natu- rally through time. However, today’s climatic changes are unfolding at an exceedingly rapid rate and are creating con- ditions humanity has never before experienced. Scientists agree that human activities, notably fossil fuel combustion and deforestation, are largely responsible. Understanding why today’s climate is changing requires understanding

FAQ

Q: The greenhouse effect works just like a greenhouse, right? A: Actually, not quite. A greenhouse helps plants grow because its glass walls trap heat. In contrast, greenhouse gas molecules in our atmosphere absorb particular wavelengths of light reflected up from the surface, then re-emit radiation at different wavelengths. Some of this radiation travels back toward the surface, keeping the surface and lower atmosphere warmer than they would otherwise be. This phenomenon differs from what happens in a greenhouse, but it was called the “greenhouse effect” in the past, and the name has stuck.

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Radiation emitted by

surface 390

Evapo- transpiration

78 Thermals

Reflected by surface

Absorbed by atmosphere

Reflected by clouds, aerosols, and atmosphere

Emitted by atmosphere and clouds

195

Outgoing longwave radiation

235

Reflected solar radiation

107

Incoming solar radiation

342

Emitted by surface and passing through atmosphere

Greenhouse gases in atmosphere

Absorbed by surface

168

Back radiation

324

Absorbed by surface

324

Units are in watts per square meter

Shorter- wavelength UV and visible light passes through atmosphere Longer-wavelength

infrared radiation is absorbed and re-emitted by atmosphere, creating the greenhouse effect

FIGURE 14.1  Our planet absorbs nearly 70% of the solar radiation it receives, and it reflects the rest back into space (yellow arrows). Most visible and ultraviolet radiation from the sun readily passes through the atmosphere and reaches the surface, and this radiation is absorbed and then re-emitted (orange arrows) as infrared radiation, which has longer wavelengths. Greenhouse gases absorb some of this long-wavelength radiation and then re-emit it, sending some downward to warm the atmosphere and surface by the greenhouse effect. This illustration of major pathways of energy flow shows that our planet naturally emits and reflects 342 watts per square meter, the same amount it receives from the sun. Arrow thicknesses are proportional to flows of energy in each pathway. Data from Kiehl, J.T., and K.E. Trenberth, 1997. Earth’s annual global mean energy budget. Bulletin of the American Meteorological Society

78: 197–208.

Greenhouse gases differ in their ability to warm the troposphere and surface. Global warming potential refers to the relative ability of one molecule of a given greenhouse gas to contribute to warming. TABLE 14.1 shows global warm- ing potentials for several greenhouse gases. Values are ex- pressed in relation to carbon dioxide, which is assigned a global warming potential of 1. Thus, a molecule of methane is 25 times as potent as a molecule of carbon dioxide, and a molecule of nitrous oxide is 298 times as potent as a CO2 molecule.

Although carbon dioxide is less potent on a per-molecule basis than methane or nitrous oxide, it is far more abundant in the atmosphere, so it contributes more to the greenhouse effect. Moreover, greenhouse gas emissions from human ac- tivity consist mostly of carbon dioxide. According to the latest data, CO2 is exerting nearly six times more impact than meth- ane, nitrous oxide, and halocarbons combined.

TABLE 14.1 Global Warming Potentials of Four Greenhouse Gases

Greenhouse gas Relative heat-trapping ability (in CO2 equivalents)

Carbon dioxide 1

Methane 25

Nitrous oxide 298

Hydrochlorofluorocarbon HFC-23

14,800

Data are for a 100-year time horizon, from Intergovernmental Panel on Climate Change, 2007. Fourth assessment report. Climate change 2007: The physical science basis.

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of carbon dioxide between the atmosphere and reservoirs on Earth’s surface.

Methane concentrations are also rising—2.5-fold since 1750 (see Figure 14.2)—and today’s atmospheric concentra- tion is the highest by far in over 800,000 years. We release methane by tapping into fossil fuel deposits, raising livestock that emit methane as a metabolic waste product, disposing of organic matter in landfills, and growing certain crops such as rice.

Human activities have also enhanced atmospheric con- centrations of nitrous oxide. This greenhouse gas, a by-prod- uct of feedlots, chemical manufacturing plants, auto emis- sions, and synthetic nitrogen fertilizers, has risen by nearly 20% since 1750 (see Figure 14.2).

Among other greenhouse gases, ozone concentrations in the troposphere have risen roughly 36% since 1750 because of photochemical smog (p. 288). The contribution of halocar- bon gases to global warming has begun to slow because of the Montreal Protocol and subsequent controls on their produc- tion and use (pp. 290–291).

Water vapor is the most abundant greenhouse gas in our atmosphere and contributes most to the natural greenhouse

Greenhouse gas concentrations are rising fast The greenhouse effect is a natural phenomenon, and green- house gases have been present in our atmosphere for all of Earth’s history. That’s a good thing, because without the nat- ural greenhouse effect, our planet would be too cold to sup- port life as we know it. Thus, it is not the natural greenhouse effect that concerns scientists today, but the anthropogenic (human-generated) intensification of the greenhouse effect. By adding novel greenhouse gases (certain halocarbons) to the atmosphere, and by increasing the concentrations of several natural greenhouse gases over the past 250 years (FIGURE 14.2), we are intensifying our planet’s greenhouse effect beyond what our species has ever experienced. For ex- ample, we have boosted Earth’s atmospheric concentration of carbon dioxide from 280 parts per million (ppm) in the late 1700s to over 392 ppm in 2011 (see Figure 14.2). Today the atmospheric CO2 concentration is at its highest level by far in over 800,000 years, and likely the highest in the last 20 million years.

Why have atmospheric carbon dioxide levels risen so rap- idly? Most carbon is stored for long periods in the upper lay- ers of the lithosphere (pp. 38–39, 329). The deposition, partial decay, and compression of organic matter (mostly plants) in wetland or marine areas hundreds of millions of years ago led to the formation of coal, oil, and natural gas in buried sedi- ments. Over the past two centuries we have extracted these fossil fuels and burned them in our homes, factories, and au- tomobiles, transferring large amounts of carbon from one res- ervoir (the underground deposits that stored the carbon for millions of years) to another (the atmosphere). This sudden flux of carbon is the main reason atmospheric CO2 concentra- tions have increased so dramatically.

At the same time, people have cleared and burned forests to make room for crops, pastures, villages, and cities. Forests serve as a reservoir for carbon as plants conduct photosyn- thesis and then store carbon in their tissues. Thus, when we clear forests it reduces the biosphere’s ability to remove car- bon dioxide from the atmosphere. In this way, deforestation (pp. 188–191) has contributed to rising atmospheric CO2 concentrations. FIGURE 14.3 summarizes scientists’ current understanding of the fluxes (both natural and anthropogenic)

300

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2 (p

pm ),

N 2O

(p pb

)

C H

4 (p

pb )

Carbon Dioxide (CO2)

Methane (CH4) Nitrous Oxide (N2O)

Year

FIGURE 14.2  Since the start of the industrial revolution, global atmospheric concentrations of carbon dioxide, methane, and nitrous oxide have increased markedly. Data from Intergovernmental Panel on Climate Change, 2007. Fourth assessment report; NOAA; and Car-

bon Dioxide Information Analysis Center.

Atmosphere

Land Ocean

Natural fluxes Anthropogenic fluxes

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440

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In cr

ea se

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ta ke

b y

pl an

260

260 70

~15

N et

a cc

um ul

at io

FIGURE 14.3  Human activities since the industrial revolution have sent more carbon dioxide from the Earth to its atmosphere than is moving back from the atmosphere to the Earth. Shown here are all current fluxes of carbon dioxide, with arrows sized according to their mass of CO2. Green arrows indicate natural fluxes, and red arrows indicate anthropogenic fluxes. Adapted from Intergovernmental Panel on Climate

Change, 2007. Fourth assessment report.

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these cycles trigger long-term climate variation. This includes periodic episodes of glaciation during which global surface temperatures drop and ice sheets advance from the poles to- ward the midlatitudes.

Solar output The sun varies in the amount of radiation it emits. For example, at each peak of its 11–year sunspot cycle the sun may emit solar flares, bursts of energy strong enough to disrupt satellite communications. However, scientists are concluding that the variation in solar energy reaching our planet in recent centuries has simply not been great enough to drive significant temperature change on Earth’s surface. Estimates place the radiative forcing of natural changes in so- lar output at only about 0.12 watts/m2—less than any of the anthropogenic causes shown in Figure 14.4.

Ocean absorption The oceans hold 50 times more car- bon than the atmosphere holds. They absorb carbon diox- ide from the atmosphere when this gas dissolves directly in water and when marine phytoplankton use it for photosyn- thesis. However, the oceans are absorbing less CO2 than we are adding to the atmosphere (see Figure 2.20, p. 39). Thus, carbon absorption by the oceans is slowing global warming but is not preventing it. Moreover, recent evidence indicates

effect. Its concentrations vary locally, but its global concentra- tion has not changed over recent centuries, so it is not viewed as a driver of industrial-age climate change.

Other factors warm or cool the surface Whereas greenhouse gases exert a warming effect on the atmosphere, aerosols, microscopic droplets and particles, can have either a warming or cooling effect. Soot parti- cles, or black carbon aerosols, generally cause warming by absorbing solar energy, but most other tropospheric aero- sols cool the atmosphere by ref lecting the sun’s rays. Sul- fate aerosols produced by fossil fuel combustion may slow global warming, at least in the short term. When sulfur dioxide enters the atmosphere, it undergoes various reac- tions, some of which lead to acid precipitation (pp. 291– 294). These reactions can form a sulfur-rich aerosol haze in the upper atmosphere that reduces the sunlight reaching Earth’s surface. Aerosols released by major volcanic erup- tions can cool Earth’s climate for up to several years. This occurred in 1991 with the eruption of Mount Pinatubo in the Philippines.

To measure the degree of impact that any given fac- tor exerts on Earth’s temperature, scientists calculate its radiative forcing, the amount of change in thermal energy that a given factor causes. Positive forcing warms the sur- face, whereas negative forcing cools it. FIGURE 14.4 shows researchers’ best calculations of the radiative forcing that our planet is experiencing today from aerosols, greenhouse gases, and other factors. When scientists sum up the effects of all factors, they find that Earth today is experiencing overall radiative forcing of about 1.6 watts/m2. This means that compared with the pre-industrial Earth of 1750, to- day’s planet is receiving and retaining 1.6 watts/m2 more thermal energy than it is emitting into space. Look back at Figure 14.1 and note that Earth is estimated naturally to receive and give off 342 watts/m2 of energy. Although 1.6 may seem like a small proportion of 342, it is enough to alter climate significantly.

Climate varies naturally for several reasons Besides atmospheric composition, our climate is influenced by cyclic changes in Earth’s rotation and orbit, variation in energy released by the sun, absorption of carbon dioxide by the oceans, and ocean circulation patterns.

Milankovitch cycles In the 1920s, Serbian mathemati- cian Milutin Milankovitch described three types of periodic changes in Earth’s rotation and orbit around the sun. Over thousands of years, our planet wobbles on its axis, varies in the tilt of the axis, and experiences change in the shape of its orbit, all in regular long-term cycles of different lengths. These variations, now known as Milankovitch cycles, alter the way solar radiation is distributed over Earth’s surface (FIGURE 14.5). By modifying patterns of atmospheric heating,

–2

–1Ra di

at iv

e fo

rc in

g (w

at ts

/m 2 )

C ar

bo n

di ox

id e

C H 4

+ N

2 O

al oc

ar bo

O zo

ne Su

rf ac

e al

be do

A er

os ol

s C

lo ud

a lb

ed o

Tropospheric

Stratospheric

Soot on snow

Land use

FIGURE 14.4  For each emitted gas or other human impact on the atmosphere since the industrial revolution, we can estimate the warming or cooling effect this has had on Earth’s climate. We express this as radiative forcing, which in this graph is shown as the amount of influence on climate today relative to 1750, in watts per square meter. Red bars indicate positive forcing (warming), and blue bars indicate negative forcing (cooling). Albedo (p. 309) refers to the reflectivity of a surface. A number of more minor influences are not shown. In total, scientists estimate that human impacts on the atmosphere exert a cumulative radiative forcing of 1.6 watts/m2. Data from Intergovernmental Panel on Climate Change, 2007. Fourth assessment report.

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STUDYING CLIMATE CHANGE To comprehend any phenomenon that is changing, we must study its past, present, and future. Scientists monitor present- day climate, but they also have devised clever means of infer- ring past change and sophisticated methods to predict future conditions.

Proxy indicators tell us about the past To understand past climate, scientists have developed tech- niques to decipher clues from thousands or millions of years ago. Proxy indicators are types of indirect evidence that serve as proxies, or substitutes, for direct measurement and that shed light on past climate.

For example, Earth’s ice caps, ice sheets, and glaciers hold clues to climate history. In frigid areas near the poles and atop high mountains, snow falling year after year for millennia com- presses into ice. Over the ages, this ice accumulates to great depths, preserving within its layers tiny bubbles of the ancient atmosphere ( FIGURE 14.6 ). Scientists examine the trapped air bubbles by drilling into the ice and extracting long columns, or cores. The layered ice, accumulating season after season over thousands of years, provides a timescale. By studying the chemistry of the ice and the bubbles in each layer, scientists can determine atmospheric composition, greenhouse gas concen- trations, temperature, snowfall, solar activity, and frequency of forest fires and volcanic eruptions during each time period.

Recently, researchers drilled and analyzed the deepest ice core ever. At a remote and pristine site in Antarctica, they drilled down 3,270 m (10,728 ft) to bedrock and pulled out more than 800,000 years’ worth of ice! This core chronicles Earth’s history across eight glacial cycles. By analyzing air bubbles trapped in the ice, researchers discovered that over the past 800,000 years, atmospheric concentrations of carbon dioxide, methane, and nitrous oxide have never been as high

that as ocean water warms, it absorbs less CO 2 because gases are less soluble in warmer water—a positive feedback effect (pp. 22–23 ) that accelerates warming of the atmosphere.

Ocean circulation Ocean water exchanges heat with the atmosphere, and ocean currents (pp. 254–255 ) move energy from place to place. For example, the oceans’ ther- mohaline circulation system (pp. 254–256 ) moves warm tropical water northward toward Europe, providing that continent a far milder climate than it would otherwise have. Scientists are studying whether freshwater input from Greenland’s melting ice sheet might shut down this warm- water f low—an occurrence that could have devastating im- pacts on European society.

Multiyear climate variability results from the El Niño– Southern Oscillation (p. 256 ), which involves systematic shifts in atmospheric pressure, sea surface temperature, and ocean circulation in the tropical Pacific Ocean. These shifts overlie longer-term variability from a phenomenon known as the Pacific Decadal Oscillation. El Niño and La Niña events alter weather patterns from region to region in diverse ways, often leading to rainstorms and floods in dry areas and drought and fire in moist areas. This leads to impacts on wildlife, agricul- ture, and fisheries.

FAQ

Q: The climate changes naturally, so why worry about climate change? A: Earth’s climate does indeed change naturally across very long periods of time. However, no known natural factors can account for the change we are experiencing today, and our civilization has never before experienced the degree of change predicted for this century. One challenge is that today’s climate is changing unusually fast. Another is the sheer amount of change: The quantity by which the world’s temperature is forecast to rise is greater than the amount of cooling needed to bring on an ice age. Greenhouse gas concentrations are already higher than they’ve been in over 800,000 years, and are still rising. Our entire civilization arose only in the last few thousand years during an exceptionally stable period in Earth’s climate history. Unless we reduce our emissions, we will soon be challenged by a climate that the human species has never lived through before.

22°

25°

Orbital plane

Equator

(b) Variation of tilt

(a) Axial wobble

Earth

Sun

FIGURE 14.5  There are three types of Milankovitch cycles. The first is an axial wobble (a) that occurs on a 19,000- to 23,000-year cycle. The second is a 3-degree shift in the tilt of Earth’s axis (b) that occurs on a 41,000-year cycle. The third is a variation in Earth’s orbit from almost circular to more elliptical (c) , which repeats itself every 100,000 years. These variations affect the intensity of solar radiation that reaches portions of Earth at different times, contributing to long-term changes in global climate.

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as they are today (FIGURE 14.7). These data demonstrate that by emitting greenhouse gases since the industrial revolution, we have brought ourselves deep into uncharted territory.

The ice core results also confirm that temperature swings in the past were tightly correlated with greenhouse gas con- centrations (compare the top two datasets in Figure 14.7 with the temperature dataset at bottom). This finding bolsters the scientific consensus that greenhouse gas emissions are caus- ing our planet to warm today.

Researchers also drill cores into beds of sediment beneath bodies of water. Sediments often preserve pollen grains and other remnants from plants that grew in the past, as we saw with the study of Easter Island (pp. 6–7). Because climate influences the types of plants that grow in an area, knowing what plants oc- curred can tell us a great deal about the climate at that place and time. Other types of proxy indicators include tree rings (which reveal year-by-year precipitation history and fire occurrence), pack-rat middens (which preserve plant parts for centuries), and coral reefs (which reveal aspects of ocean chemistry).

Direct measurements tell us about the present Today we measure temperature with thermometers, rainfall with rain gauges, wind speed with anemometers, and air pressure with barometers, using computer programs to in- tegrate and analyze this information in real time. With these technologies, we document the fluctuations in weather day by day and hour by hour across the globe. As a result, we have gained an understanding of present-day climate in every region of our planet.

We also measure the chemistry of the atmosphere and the oceans. Direct measurements of carbon dioxide concentrations in the atmosphere reach back to 1958, when scientist Charles

(a) Ice core

(b) Micrograph of ice core

FIGURE 14.6  In Greenland and Antarctica, scientists have drilled deep into ancient ice sheets and removed cores of ice like this one (a), held by Dr. Gerald Holdsworth of the University of Calgary, to extract information about past climates. Bubbles (black shapes) trapped in the ice (b) contain small samples of the ancient atmosphere.

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pe ra

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ar bo

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ox id

e (p

pm v)

M ethane (ppbv)

400,000800,000 600,000 200,000 0

Years before present

CO2 today (392)

CH4 today (1,820)

FIGURE 14.7  Data from the world’s longest ice core reveal changes in surface temperature (black line), atmospheric methane concentration (green line), and atmospheric carbon dioxide concentration (red line) across 800,000 years. High peaks in temperature indicate warm interglacial periods, and low troughs indicate cold glacial periods. Atmospheric concentrations of carbon dioxide and methane rise and fall in tight correlation with temperature. Today’s current values are included at the far top right of the graph, for comparison. "Ppmv" = parts per million by volume, and "ppbv"

= parts per billion by volume. Adapted with

permission from Brook, E., 2008. Windows on the

greenhouse. Nature 453: 291–292, summarizing

data from multiple research papers.

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CURRENT AND FUTURE TRENDS AND IMPACTS It seems that virtually everyone is noticing changes in the climate these days. Maldives fishermen note the seas encroaching on their home island. Texas ranchers suffer a multiyear drought. Florida homeowners find it difficult to obtain insurance against hurricanes and storm surges. New Yorkers, Bostonians, Chicagoans, and Los Angelenos face one unprecedented weather event after another.

We cannot blame any single unusual heat wave, flood, or blizzard on climate change, but extreme weather events are indeed part of a real pattern backed by a tremendous volume of scientific evidence. Climate change has already had nu- merous impacts on the physical properties of our planet, on organisms and ecosystems, and on human well-being. If we continue to emit greenhouse gases into the atmosphere, the consequences of climate change will grow more severe.

The IPCC summarizes evidence and predicts impacts The immense amount of scientific information on climate change is reviewed and summarized in periodic reports issued by the Intergovernmental Panel on Climate Change (IPCC). This international body consists of many hundreds of scien- tists and governmental representatives. Established in 1988 by the United Nations Environment Programme (UNEP) and the World Meteorological Organization (WMO), the IPCC was awarded the Nobel Peace Prize in 2007 for its work in inform- ing the world of the trends and impacts of climate change.

Keeling began analyzing hourly air samples from a monitoring station at the Mauna Loa Observatory in Hawaii. These data show that atmospheric CO2 concentrations have increased from 315 ppm in 1958 to 392 ppm in 2011.

Models help us predict the future To understand how climate systems function and to predict future climate change, scientists simulate climate processes with sophisticated computer programs. Climate models are programs that combine what is known about atmospheric circulation, ocean circulation, atmosphere-ocean interac- tions, and feedback cycles to simulate climate processes (FIGURE 14.8). This requires manipulating vast amounts of data with complex mathematical equations—a task not pos- sible until the advent of modern computers.

Climate modelers essentially provide starting informa- tion to the model, set up rules for the simulation, and then let it run. Researchers test the efficacy of a model by entering past climate data and running the model toward the present. If a model accurately reconstructs current climate, based on well- established data from the past, then we have reason to believe that it simulates climate mechanisms realistically and that it may accurately predict future climate.

Plenty of challenges remain for climate modelers because Earth’s climate system is so complex and because many un- certainties remain in our understanding of feedback processes (pp. 22–23). Yet as scientific knowledge of climate processes improves, as computing power intensifies, and as we glean enhanced data from proxy indicators, climate models are be- coming better and better at predicting climate change region by region across the world.

Outgoing heat

Evaporative and heat exchanges

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temperature, and salinity

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FIGURE 14.8  Modern climate models incorporate many factors, including processes involving the atmosphere, land, oceans, ice, and biosphere. Such factors are shown graphically here, but the actual models deal with them as mathematical equations in computer simulations.

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impacts on society are conservative, because its scientific con- clusions had to be approved by representatives of the world’s national governments, some of which are reluctant to move away from a fossil-fuel-based economy.

Temperatures continue to rise The IPCC’s 2007 report concluded that average surface tem- peratures on Earth rose by an estimated 0.74 °C (1.33 °F) in the century from 1906 to 2005, with most of this increase oc- curring in the last few decades (FIGURE 14.10). According to the WMO, the 17 warmest years on record since global meas- urements began 150 years ago have all been since 1990. The decade from 2001–2010 was the hottest ever, and since the 1960s each decade has been warmer than the last.

In that year the IPCC released its Fourth Assessment Report. This report summarized many thousands of scientific studies, and it documented observed trends in surface tempera- ture, precipitation patterns, snow and ice cover, sea levels, storm intensity, and other factors. It also predicted future changes in these phenomena after considering a range of potential scenar- ios for future greenhouse gas emissions. The report addressed impacts of current and future climate change on wildlife, eco- systems, and society. Finally, it discussed strategies we might pursue in response to climate change. FIGURE 14.9 summarizes some of the IPCC report’s major observed and predicted trends and impacts.

Like all science, the IPCC report deals in uncertainties. Its authors therefore took great care to assign statistical probabil- ities to its conclusions and predictions. Its estimates regarding

Global physical indicators

Regional physical indicators

Major Observed Impacts of Climate Change, from IPCC Fourth Assessment Report, 2007

Social indicators

Biological indicators

Earth’s average surface temperature increased 0.74 °C (1.33 °F) in the past 100 years, and will rise 1.8–4.0 °C (3.2–7.2 °F) in the 21st century.

Eleven of the years from 1995 to 2006 were among the 12 warmest on record. Oceans absorbed >80% of heat added to the climate system, and warmed to depths of at least 3,000 m (9,800 ft).

Glaciers, snow cover, ice caps, ice sheets, and sea ice will continue melting, contributing to sea-level rise.

Sea level rose by an average of 17 cm (7 in.) in the 20th century, and will rise 18–59 cm (7–23 in.) in the 21st century.

Ocean water became more acidic by about 0.1 pH unit, and will decrease in pH by 0.14–0.35 units more by century’s end.

Storm surges increased, and will increase further.1

Carbon uptake by terrestrial ecosystems will peak by mid-21st century and then weaken or reverse, amplifying climate change.2

Farmers and foresters have had to adapt to altered growing seasons and disturbance regimes.

Temperate-zone crop yields will rise until temperature warms beyond 3 °C (5.4 °F), but in the dry tropics and subtropics, crop productivity will fall and lead to hunger.5

Impacts on biodiversity will cause losses of food, water, and other ecosystem goods and services.2

Sea level rise will displace people from islands and coasts.3

Melting of mountain glaciers will reduce water supplies to millions of people.2

Economic costs will outweigh benefits as climate change worsens;2 costs could average 1–5% of GDP globally for 4 °C (7.2 °F) of warming.

Poorer nations and communities suffer more from climate change, because they rely more on climate-sensitive resources and have less capacity to adapt.2

Human health will suffer as increased warm-weather health hazards outweigh decreased cold-weather health hazards.2

Arctic areas warmed fastest. Future warming will be greatest in the Arctic and greater over land than over water.

Summer Arctic sea ice thinned by 7.4% per decade since 1978.

Precipitation will increase at high latitudes and decrease at subtropical latitudes, making wet areas wetter and dry ones drier.1

Droughts became longer, more intense, and more widespread since the 1970s, especially in the tropics and subtropics.1

Droughts and flooding will increase, leading to agricultural losses.2

Hurricanes intensified in the North Atlantic since 19701, and will continue to intensify.1

The thermohaline circulation will slow, but will not shut down and chill Europe in the 21st century.3

Species ranges are shifting toward the poles and upward in elevation, and will continue to shift.

The timing of seasonal phenomena (such as migration and breeding) is shifting, and will continue to shift.

About 20–30% of species studed so far will face extinction risk if temperature rises more than 1.5–2.5 °C (2.7–4.5 °F).5

Species interactions and ecosystem structure and function could change greatly, resulting in biodiversity loss.

Corals will experience further mortality from bleaching and ocean acidification.5,4

FIGURE 14.9  Listed here are some of the main observed and predicted trends and impacts described in the Intergovernmental Panel on Climate Change’s Fourth Assessment Report. Observed phenomena are in plain text, whereas predicted future phenomena are in italicized text. For simplicity, this table expresses mean estimates only. The IPCC report provides ranges of estimates as well. Certainty levels are as follows: 166–90% probability of being correct; 2~80% probability of being correct; 390–99% probability of being correct; 4>99% probability of being correct; 5~50% probability of

being correct. Data from the Intergovernmental Panel on Climate Change, 2007. Fourth assessment report.

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In the next 20 years, we can expect average surface tem- peratures on Earth to rise roughly 0.4 °C (0.7 °F), according to IPCC analysis. At the end of the 21st century, the IPCC predicts global temperatures will be 1.8–4.0 °C (3.2–7.2 °F) higher than today’s, depending on the emission scenario. Temperature changes are predicted to vary from region to re- gion in ways that intensify regional differences already appar- ent (FIGURE 14.11). For example, polar regions will continue to experience the most intense warming.

Sea surface temperatures are also increasing as the oceans absorb heat from the atmosphere. The record number of hurri- canes and tropical storms in 2005—Hurricane Katrina and 27 others—left many people wondering whether global warming was to blame. Recent scientific analyses suggest that warmer seas may not be increasing the number of tropical storms but may be increasing their power, and possibly their duration.

Precipitation is changing, too A warmer atmosphere holds more water vapor, but changes in precipitation patterns have been complex, with some re- gions of the world receiving more rain and snow than usual and others receiving less. In regions such as the southwestern United States, droughts have become more frequent and se- vere, harming agriculture, worsening soil erosion, reducing water supplies, and triggering wildfire. Meanwhile, in dry and humid regions alike, heavy rain events have increased, contributing to flooding, such as the 2008 floods in Iowa and other parts of the Midwest and the 2011 floods along the Mississippi River that killed dozens of people, left thousands homeless, and inflicted billions of dollars in damage.

Future changes in precipitation are predicted to intensify regional changes seen over the past century (FIGURE 14.12). In general, precipitation will increase at high latitudes and decrease at low and middle latitudes, magnifying differences that already exist and worsening water shortages in many de- veloping countries of the arid subtropics.

Melting ice and snow have far-reaching effects As the world warms, mountaintop glaciers are disappear- ing (FIGURE 14.13). Between 1980 and 2009, the world’s gla- ciers on average have each lost mass equivalent to 14 m (46 ft) vertical thickness of water, according to the World Glacier

(a) Mean global temperature measured, 1850-2010

(b) Northern Hemisphere temperature over the past 1,000 years

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FIGURE 14.10  Data from thermometers (a) show changes in Earth’s average surface temperature from 1850 to 2010. Gray shaded area indicates range of uncertainty. In (b), proxy indica- tors (blue line) and thermometer data (red line) together show average temperature changes in the Northern Hemisphere over the past 1,000 years. The gray shaded zone represents the 95% confidence range. Data (a) from the Intergovernmental Panel on Climate Change (IPCC), 2007, Fourth assessment report, and the National Oceanic

and Atmospheric Administration (NOAA); and (b) IPCC 2001, Third assess-

ment report.

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FIGURE 14.11  This map shows projected increases in surface temperature for the decade 2090–2099, relative to temperatures in 1980– 1999. Landmasses are expected to warm more than oceans, and the Arctic will warm the most. The Intergovernmental Panel on Climate Change uses multiple emission scenarios, and this map was generated using an emission scenario that is intermediate in its assumptions, involving an average global temperature rise of 2.8 °C (5.0 °F) by 2100. Data from Intergovernmental Panel on Climate Change, 2007. Fourth assessment report.

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>20% decrease

Percent change in precipitation

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5–10% increase

5% decrease to 5% increase

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(a) Grinnell Glacier in 1938 (b) Grinnell Glacier in 2005

FIGURE 14.13  Glaciers are melting rapidly around the world as global warming pro- ceeds. The Grinnell Glacier in Glacier National Park, Montana, retreated substantially between (a) 1938 and (b) 2005.

Monitoring Service. Many glaciers on tropical mountaintops have disappeared already. In Glacier National Park in Mon- tana, only 25 of 150 glaciers present at the park’s inception remain, and scientists estimate that by 2030 even these will be gone.

Mountains accumulate snow in winter and release melt- water gradually during summer. Over one-sixth of the world’s people live in regions that depend on mountain meltwater. As warming temperatures diminish mountain glaciers, this will reduce summertime water supplies to millions of people, likely forcing whole communities to look elsewhere for water, or to move.

Warming temperatures are also melting vast amounts of ice in the Arctic. Recent research reveals that the immense ice sheet that covers Greenland is melting faster and faster. At the other end of the world, in Antarctica, coastal ice shelves the size of Rhode Island have disintegrated as a result of con- tact with warmer ocean water.

One reason warming is accelerating in the Arctic is that as snow and ice melt, darker, less-reflective surfaces (such as

bare ground and pools of meltwater) are exposed, and Earth’s albedo, or capacity to reflect light, decreases. As a result, more of the sun’s rays are absorbed at the surface, fewer reflect back into space, and the surface warms. In a process of positive feedback, this warming causes more ice and snow to melt, which in turn causes more absorption of radiation and more warming (see Figure 2.1b, p. 23).

Scientists predict that snow cover, ice sheets, and sea ice will continue to diminish near the poles. As Arctic sea ice dis- appears, new shipping lanes are opening up for commerce, and governments and companies are rushing to exploit newly accessible underwater oil and mineral reserves. Russia, Cana- da, the United States, and other nations are jockeying for posi- tion, using new survey data to try to lay claim to regions of the Arctic as the ice melts.

Warmer temperatures in the Arctic are also causing per- mafrost (permanently frozen ground) to thaw. As ice crystals within permafrost melt, the thawing soil settles, destabilizing buildings, pipelines, and other infrastructure. When perma- frost thaws, it also can release methane that it has stored for

FIGURE 14.12  This map shows projected changes in June–August precipitation for the decade 2090–2099, relative to precipita- tion in 1980–1999. Browner shades indicate less precipitation, and bluer shades indicate more precipitation. White indicates areas for which models could not agree. This map was generated using an emission scenario that is intermediate in its assumptions, involving an average global temperature rise of 2.8 °C (5.0 °F) by 2100. Data from Intergovernmental Panel on Climate Change, 2007. Fourth assessment report.

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The Maldives has fared better than many other island na- tions. It saw sea level rise about 2.5 mm per year throughout the 1990s, but most Pacific islands are experiencing greater sea level rise. Regions experience differing amounts of sea lev- el change because land may be rising or subsiding naturally, depending on local geologic conditions.

In the United States, 53% of the population lives in coastal counties. Vulnerability to storm surges became tragi- cally apparent when Hurricane Katrina struck the Gulf Coast. Outside New Orleans today, marshes of the Mississippi River delta are being lost as rising seas eat away at coastal vegeta- tion. These coastal wetlands are also being lost because dams upriver hold back silt that once maintained the delta, because petroleum extraction has caused land to subside, and because salt water is encroaching up channelized waterways, killing freshwater plants. All told, more than 2.5 million ha (1 mil- lion acres) of Louisiana’s coastal wetlands have vanished since 1940. Continued wetland loss will deprive New Orleans of protection against future storm surges.

At the end of the 21st century, the IPCC predicts mean sea level will be 18–59 cm (7–23 in.) higher than today’s, de- pending on our level of emissions. However, these estimates do not take into account recent findings on accelerated ice melting in Greenland (and apparently Antarctica as well), be- cause that research was so new that it had not yet been incor- porated into climate models when the latest IPCC report was being prepared. If polar melting continues to accelerate, then sea levels will rise more quickly.

If sea levels rise as predicted, hundreds of millions of people will be displaced or will need to invest in costly ef- forts to protect against high tides and storm surges. Densely populated regions on low-lying river deltas, such as Bangla- desh, would be most affected. So would storm-prone regions such as Florida, coastal cities such as Houston and Charles- ton, and areas where land is subsiding, such as the U.S. Gulf Coast. Many Pacific islands would need to be evacuated. In the meantime, island nations such as the Maldives are likely to suffer from shortages of fresh water as rising seas bring salt water into aquifers. The contamination of groundwater and soils by seawater also threatens coastal areas such as Tampa, Florida, which depend on small lenses of fresh water that float atop saline groundwater.

thousands of years. Because methane is a potent greenhouse gas, this acts as a positive feedback mechanism (p. 22) that intensifies climate change.

Rising sea levels may affect hundreds of millions of people As glaciers and ice sheets melt, increased runoff into the oceans causes sea levels to rise. Sea levels also are rising be- cause ocean water is warming, and water expands in volume as it warms. Worldwide, average sea levels rose an estimat- ed 17 cm (6.7 in.) during the 20th century (FIGURE 14.14), reaching a rate of 3.2 mm/year since 1993. These numbers represent vertical rises in water level, and on most coastlines a vertical rise of a few inches means many feet of incursion inland.

Higher sea levels lead to beach erosion, coastal flooding, intrusion of salt water into aquifers, and storm surges. A storm surge is a temporary and localized rise in sea level brought on by the high tides and winds associated with storms. The higher that sea level is to begin with, the further inland a storm surge can reach. In 1987, unusually high waves struck the Maldives and triggered a campaign to build a seawall around Malé, the nation’s capital. “The Great Wall of Malé” is intended to pro- tect buildings and roads by dissipating the energy of incoming waves during storm surges.

On December 26, 2004, the Maldives got a taste of what could be in store in the future when a massive tsunami (pp. 233–234) devastated coastal areas throughout the In- dian Ocean. The tsunami killed 100 Maldives residents and left 20,000 homeless. Property damage in the Maldives was estimated at $470 million, an astounding 62% of the nation’s gross domestic product (GDP). Indirect damage from soil erosion, saltwater contamination of aquifers, and other impacts continues to cause further economic losses. The tsunami was caused by an earthquake, not by climate change. Yet as sea levels rise, the damage that natural events can inflict increases considerably.

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FIGURE 14.14  Data from tide gauges (black line) and satellite observations (red line) show that global average sea level has risen over 200 mm (7.9 in.) since 1870. Gray shaded area indicates range of uncertainty. Thermal expansion of water accounts for most sea level rise. Data from Intergovernmental Panel on Climate Change, 2007. Fourth assessment report, and CSIRO.

Environmental Refugees Citizens of the Maldives see an omen of their future in the Pacific island nation of Tuvalu, which has been losing 9 cm (3.5 in.) of elevation per decade to rising

seas. Appeals from Tuvalu’s 11,000 citizens were heard by New Zealand, which began accepting “ environmental refugees” from Tuvalu in 2003. Do you think the rest of the world should grant such environmental refugees international status and assume some responsibility for taking care of them? Do you think a national culture can survive if its entire population is relocated? Think of the tens of thousands of refugees from Hurricane Katrina. How did their lives and culture fare in the wake of that tragedy?

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carbon dioxide can bring both positive and negative results for plant growth (see THE SCIENCE BEHIND THE STORY, pp. 312–313).

In regions where precipitation and stream flow in- crease, erosion and flooding will pollute and alter aquatic systems. In regions where precipitation decreases, lakes, ponds, wetlands, and streams will shrink. The many im- pacts of climate change on ecological systems will diminish the ecosystem goods and services we receive from nature and that our societies depend on, from food to clean air to drinking water.

Climate change affects society Drought, flooding, storm surges, and sea level rise have al- ready taken a toll on the lives and livelihoods of millions of

Climate change threatens coral reefs Around the world, rising seas are eroding the salt marshes and mangrove forests that protect our coasts (p. 257). How- ever, scientists are most concerned about coral reefs (pp. 258–259), which provide habitat for marine species, enhance fisheries, offer snorkeling and scuba diving sites for tourism, and shield coastlines from destructive waves.

Climate change poses two major threats to coral reefs. First, warmer waters contribute to coral bleaching (p. 259), which kills corals. Second, enhanced CO2 concentrations in the atmosphere alter ocean chemistry, leading to ocean acidification (p. 259). As ocean water absorbs atmospheric CO2, it becomes more acidic, and this impairs the ability of coral and other organisms to build exoskeletons of calcium carbonate. The oceans have already decreased by 0.1 pH unit, and are predicted to decline in pH by 0.14–0.35 more units over the next 100 years. This could easily be enough to de- stroy most or all of our planet’s living coral reefs. Such de- struction could be catastrophic for marine biodiversity and fisheries, because so many organisms depend on living coral reefs for food and shelter.

Climate change affects organisms and ecosystems As the coral reef crisis shows, changes in Earth’s physical systems have consequences for living things. Organisms are adapted to their environments, so they are affected when we alter those environments. As global warming proceeds, it is modifying biological phenomena that rely on temperature. In the spring, plants are now leafing out earlier, insects are hatching earlier, birds are migrating earlier, and animals are breeding earlier. These shifts can create mismatches in sea- sonal timing. For example, European birds known as great tits had evolved to time their breeding to raise their young when caterpillars peak in abundance. Now caterpillars are peaking earlier, but the birds have been unable to adjust, and fewer young birds are surviving.

Biologists are also recording spatial shifts in the ranges of organisms, as plants and animals move toward the poles or upward in elevation (i.e., toward cooler regions) as tem- peratures warm (FIGURE 14.15A). Some organisms will not be able to cope, and the IPCC estimates that up to 20–30% of all plant and animal species could be threatened with ex- tinction. Trees may not be able to shift their distributions fast enough. Rare species may be forced out of preserves and into developed areas where they cannot survive. Animals and plants adapted to mountainous environments may be forced uphill until there is nowhere left to go (FIGURE 14.15B; and p. 57).

Effects on plant communities comprise an important component of climate change, because by drawing in CO2 for photosynthesis, plants act as reservoirs for carbon. If higher CO2 concentrations enhance plant growth, then more CO2 might be removed from the air, helping to mitigate carbon emissions. However, if climate change decreases plant growth (through drought, fire, or disease, for instance), then carbon flux to the atmosphere could increase. Today large-scale ex- periments are revealing complex answers, showing that extra

Center of Purple Finch range shifted 700 km north in 40 years

Pikas are disappearing from mountains after being forced upwards

(a) Birds are moving north

(b) Pikas are being forced upslope

FIGURE 14.15  Fully 177 out of 305 North American bird species have shifted their winter ranges significantly northward in the past 40 years, according to a 2009 analysis of Christmas Bird Count data by National Audubon Society researchers. The purple finch (a) has shown the greatest shift. Its center of abundance moved 697 km (433 mi) north, from southeastern Missouri to northern Iowa. Montane animals such as the pika (b), a unique mammal that lives at high elevations in western North America, are being forced upslope (into more limited habitat) as temperatures warm. Many pika populations in the Great Basin have disappeared from mountains already.

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THE SCIENCE BEHIND THE STORY

FACE-ing a High-CO2 Future

Aspen FACE site researcher Dr. Mark Kubiske of the U.S. Forest Service

Plants remove carbon dioxide (CO2) from the atmosphere to use in photosynthesis, and all organisms re- turn CO2 to the atmosphere by cellular respiration (pp. 30–31). As we burn fos- sil fuels and clear forests, we add car- bon dioxide to the atmosphere, which now contains 35% more CO2 than it did just two centuries ago. Will more CO2 mean more plant growth, and will plants be able to absorb and store the extra CO2? Perhaps, but before we rely on forests and phytoplankton to save us from our emissions, we’d better be sure they can do so.

Historically, if a researcher wanted to measure how plants respond to increased carbon dioxide, he or she would adjust gas levels in a small enclosure, such as a lab or a green- house. But can results from such small indoor experiments indicate how entire forests will behave? Many scientists thought not, and eventu- ally some pioneered Free-Air CO2 Enrichment—“FACE” for short. In FACE experiments, researchers pre- cisely control ambient levels of CO2

encompassing areas of forest (or other vegetation) outdoors. With their large scale and open-air conditions, FACE experiments include most factors that influence a plant community in the wild, such as variation in temperature, sunlight, precipitation, herbivorous insects, disease pathogens, and competition among plants. By measur- ing how plants respond to changing gas compositions in such real-world conditions, we can better learn how ecosystems may change in the carbon dioxide–soaked world that awaits us.

Dozens of organizations have sponsored FACE facilities—36 sites in 17 nations so far, including U.S. sites in Arizona, California, Illinois, Minnesota, North Carolina, Tennessee, Nevada, Wisconsin, and Wyoming. The sites cover a variety of ecosystems, from forests to grasslands to rice paddies, and the plots range in size from 1 m to 30 m (from 3 to 98 ft) in diameter.

To understand how a typical FACE study works, let’s visit the Aspen FACE Experiment at the Harshaw Experi- mental Forest (where aspen trees are

common) near Rhinelander, Wisconsin. Here, tall steel and plastic towers and pipes ring 12 circular plots of forest 30 m (98 ft) in diameter (see photo). The pipes release CO2, bathing the plants in an atmosphere 50% richer in CO2 than today’s (equal to what is expected worldwide for the year 2050). Sensors monitor wind conditions, and comput- ers control for the influence of wind by adjusting CO2 releases, keeping ambi- ent concentrations stable within each plot.

The pipes at the Aspen plots also release tropospheric ozone (O3, a major pollutant in urban smog; p. 288), and researchers study how this gas and CO2 affect plant growth, leaf and root conditions, soil carbon content, and much else. Pipes at some plots release normal air, serving as controls for the treatment plots.

Researchers using the Aspen FACE facility have learned a number of things so far, among them:

▶ Elevated CO2 levels increase photosynthesis and tree growth— but moderate levels of ozone offset this increased growth (see graph). Because many modelers have not taken ozone into account when estimating how much carbon trees can sequester, the Aspen FACE data

people. However, climate change will have still more con- sequences. These include impacts on agriculture, forestry, health, and economics.

Agriculture For some crops in the temperate zones, moderate warming may slightly increase production be- cause growing seasons become longer. The availability of additional carbon dioxide to plants for photosynthesis may also increase yields, but as mentioned above, elevated CO2 can have mixed results. Moreover, some research shows that

crops become less nutritious when supplied with more car- bon dioxide. If rainfall shifts in space and time, intensified droughts and f loods will likely cut into agricultural produc- tivity. Considering all factors together, the IPCC predicts global crop yields to increase somewhat, but beyond a rise of 3 °C (5.4 °F), it expects crop yields to decline. In season- ally dry tropical and subtropical regions, growing seasons may be shortened, and harvests may be more susceptible to drought. Thus, scientists predict that crop production will fall in these regions even with minor warming. This

Can fumigating trees with carbon dioxide tell us what to expect from glo-bal climate change? Hundreds of scientists think so, and they are testing plants’ responses to atmospheric change at unique outdoor Free-Air CO2 Enrichment (“FACE”) facilities.

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At the Aspen FACE facility in Wisconsin, tall towers and pipes control the at- mospheric composition around selected patches of trees.

suggest that existing models may overestimate the amount of CO2 that trees will pull out of the air.

▶ As atmospheric levels of ozone and CO2 rise, insects and diseases that attack aspen and birch trees increase.

▶ High CO2 concentrations delay aspen leaf death, which can make aspens vulnerable to frost damage.

Together, such results indicate that rising carbon dioxide levels could have a variety of negative impacts on trees and forests—belying the old expecta- tion that more CO2 makes for happier plants. Indeed, research from other FACE sites is showing that increased

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(1 ,0

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14 Control air CO2 added O3 added CO2 and O3 added

Treatment

Data from clusters of aspens show that trees supplied with carbon dioxide grew more than control trees during the study period, while those supplied with ozone grew less. Trees supplied with both gases did not grow differently from the controls. Shown are data for one typical cluster. Adapted from Isebrands, J.G., et al., 2001. Growth responses of Populus tremuloides clones to interacting elevated carbon dioxide and tropospheric ozone. Environmental Pollution 115: 359–371. Fig. 2, with permission from Elsevier.

would worsen hunger in many of the world’s developing nations.

Forestry In the forests that provide our timber and pa- per products, enriched atmospheric CO2 may spur greater growth in the near term, but other climatic effects such as drought, fire, and disease may eliminate these gains. For- est managers increasingly find themselves battling cata- strophic fires, invasive species, and insect and disease out- breaks. Catastrophic fires are caused in part by decades of

fire suppression (p. 196) but are also promoted by longer, warmer, drier fire seasons. Milder winters and hotter, drier summers are promoting outbreaks of pine beetles, pest insects that are destroying millions of acres of trees (see Figure 9.15, p. 197).

Health As climate change proceeds, we will face more heat waves—and heat stress can cause death, especially among older adults. A 1995 heat wave in Chicago killed at least 485 people, and a 2003 heat wave in Europe killed 35,000

content. This would provide urgently needed data on carbon sequestra- tion, the DOE said, and then millions of dollars could be shifted toward a new and improved generation of FACE experiments.

Many researchers were aghast, however, and argued that the pre- cious and unique long-term sites still had much to teach us. How else can we know how forests and climate will interact after 25 years, or 50, they asked? . . . Except, of course, to wait and let Earth show us—by which time it may be too late to do anything about it.

growth from enhanced CO2 is often temporary and that growth rates later flatten out or decline.

Obtaining solid answers to ques- tions like these takes years or decades, and FACE experiments are designed to monitor plots for the long term as the plants mature. Some FACE sites have been operating for 20 years and are now producing data that cannot be gathered in any other way.

Thus, researchers were shocked in 2008 when the U.S. Department of Energy (DOE), which funds Aspen and other major sites, announced it would cease funding. The DOE ad- vised scientists to cut the trees down and dig up the soil to analyze carbon

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the Stern Review on the Economics of Climate Change headed by economist Nicholas Stern and commissioned by the British government. This exhaustive review concluded that climate change could cost us roughly 5–20% of GDP by the year 2200, but that investing just 1% of GDP starting now could enable us to avoid these future costs.

Impacts will vary regionally The impacts of climate change are subject to regional vari- ation, so the way each of us experiences these impacts will depend on where we live. Temperature changes have been greatest in the Arctic (FIGURE 14.16). Here, ice sheets are melting, sea ice is thinning, storms are increasing, and al- tered conditions are posing challenges for people and wild- life. As sea ice melts earlier, freezes later, and recedes from shore, it becomes harder for Inuit people and for polar bears alike to hunt the seals they each rely on for food. Permafrost is thawing, destabilizing countless buildings. As the strong Arctic warming melts ice caps and ice sheets, this contributes to sea level rise.

For the United States, potential impacts are analyzed by the U.S. Global Change Research Program, which Congress created in 1990 to coordinate federal climate research. In a 2009 report, scientists for this program reviewed current re- search, summarized the effects of climate change on the Unit- ed States, and predicted future impacts (TABLE 14.2). Many impacts will vary by region, yet the report predicted that some would be felt across the nation. Average temperatures

people. A warmer climate also exposes us to other health problems:

▶ Respiratory ailments from air pollution, as hotter temper- atures promote photochemical smog (p. 288)

▶ Expansion of tropical diseases, such as dengue fever, into temperate regions as vectors of infectious disease (such as mosquitoes) move toward the poles

▶ Disease and sanitation problems when floods overcome sewage treatment systems

▶ Injuries and drowning if storms become more frequent or intense

Health hazards from cold weather will decrease, but most re- searchers feel that the increase in warm-weather hazards will more than offset these gains.

Economics People will experience a variety of econom- ic costs and benefits from the impacts of climate change, but on the whole researchers predict that costs will out- weigh benefits. Climate change is also expected to widen the gap between rich and poor, both within and among na- tions. Poorer people have less wealth and technology with which to adapt to climate change, and they rely more on resources (such as local food and water) that are sensitive to climatic conditions.

From a variety of economic studies, the IPCC estimated that climate change will cost 1–5% of GDP on average glo- bally, with poor nations losing proportionally more than rich nations. The highest-profile economic study to date has been

GREENLAND

2000 2002

2010–2030

2040–2060

2070–2090

CANADA

RUSSIA ALASKA

NORWAY

ICELAND FINLAND

SWEDEN

FIGURE 14.16  The Arctic has borne the brunt of climate change’s impacts so far. As Arctic sea ice melts, it recedes from large areas, as shown by the map indicating the mean minimum summertime extent of sea ice for the recent past, present, and future. Inuit people find it difficult to hunt and travel in their traditional ways, and polar bears starve because they are less able to hunt seals. Human- made structures are damaged as permafrost thaws beneath them: Buildings can lean, buckle, crack, and fall. Map data from National Center for Atmospheric Research and National

Snow and Ice Data Center.

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and upward in elevation. Extreme weather events are project- ed to become more frequent.

We are bound to experience a variety of consequenc- es from anthropogenic climate change in coming years (FIGURE 14.18). Yet by addressing its root causes now, we may still be able to prevent the most severe future impacts.

Are we responsible for climate change? Scientists agree that most or all of today’s global warming is due to the well-documented recent increase in greenhouse gas concentrations in our atmosphere. They also agree that this rise in greenhouse gases results primarily from our com- bustion of fossil fuels for energy and secondarily from the loss of carbon-absorbing vegetation due to deforestation.

Yet despite the overwhelming scientific evidence for climate change and its impacts, many people, especially in the United States, have long tried to deny that it is happen- ing. Indeed, while most nations moved forward to confront climate change through international dialogue, in the United States public discussion of climate change remained mired in debates over whether the phenomenon was real and whether humans were to blame. These debates have been fanned by spokespeople from think tanks and a handful of scientists, many funded by fossil fuel industries. These individuals have aimed to cast doubt on the scientific consensus, and their views are amplified by the American news media, which seeks to present two sides to every issue, even when the sides’ argu- ments are not equally supported by evidence.

In 2009, a hacker illegally broke into computers at the University of East Anglia, U.K., and made public several thousand documents, including over 1,000 private e-mails among a handful of climate scientists. A few of these mes- sages appeared to show questionable behavior in the use of

End-of-Century (2080-2099 average)

Lower Emissions Scenario Projected Temperature Change (°F)

Higher Emissions Scenario Projected Temperature Change (°F)

End-of-Century (2080-2099 average)

2 3 4 5 6 7 8 9 10 >10 (°F)

FIGURE 14.17  Average temperatures across the United States are predicted to rise by 4–6 °F by the end of this century under a low-emissions scenario, and 7–11 °F under a high-emissions scenario. Data from Karl, T.R., et al., eds., 2009. Global climate change impacts in the United States. U.S. Global Change Research Program and Cambridge University Press.

TABLE 14.2 Some Predicted Impacts of Climate Change in the United States

▶ Average temperatures will rise 2.2–6.1 °C (4–11 °F) further by the end of this century.

▶ Droughts and flooding will worsen. ▶ Longer growing seasons and enhanced CO will favor

crops, but more drought, heat stress, pests, and diseases will decrease most yields.

▶ Snowpack will decrease in the West; water shortages will worsen.

▶ Cold-weather illness will decline, but health problems due to heat stress, disease, and pollution will rise. Some tropical diseases will spread north.

▶ Sea level rise and storm surges will erode beaches and destroy coastal wetlands and real estate.

▶ Alpine ecosystems and barrier islands will begin to vanish. ▶ Drought, fire, and pest outbreaks will continue to alter

forests. ▶ Northeast forests will lose sugar maples; Southeast forests

will be invaded by grassland; Southwest ecosystems will turn more desertlike.

▶ Melting permafrost will undermine Alaskan buildings and roads.

Adapted from Karl, T.R., et al., eds., 2009. Global climate change impacts in the United States. U.S. Global Change Research Program and Cambridge University Press.

in most of the United States have already increased by 0.6–1.1 °C (1–2 °F) since the 1960s and 1970s, and are predicted to rise by another 2.2–6.1 °C (4–11 °F) by the end of this century (FIGURE 14.17). Plant communities will likely shift northward

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that a few statements out of thousands in such a vast collabo- rative project would be in error. Yet scientists wanted to assure that the IPCC’s reputation for reliability not be tarnished, and so reforms are underway that should strengthen the IPCC’s process during the preparation of its Fifth Assessment, due out in 2014.

RESPONDING TO CLIMATE CHANGE Today, most of the world’s people recognize that our fossil fuel consumption is altering the planet that our children will inherit. From this point onward, our society will be focusing on the difficult question of how best to respond to the chal- lenges of climate change. The good news is that everyone— not just leaders in government and business, but everyday people, and especially today’s youth—can play a part in this all-important search for solutions.

data and the treatment of other researchers. Climate-change deniers named the incident “Climategate” and used it to ac- cuse the entire scientific establishment of wrongdoing and conspiracy. The news media disseminated the story widely. However, subsequent investigations into the affair by several independent panels cleared the climate scientists, finding no evidence of wrongdoing. Each panel concluded that some in- dividuals may have exercised poor taste or judgment and that some practices could be improved, but they also found that many media accounts trumpeting the news had misrepre- sented the content of the e-mails. The panels agreed that the hacked messages among a few individuals in no way called into question the vast array of research results compiled by thousands of hard-working independent climate scientists over several decades.

Questions were raised the same year about some of the IPCC report’s conclusions, and subsequent inquiry revealed that several statements were inadequately backed by evidence or otherwise misstated or misleading. It is hardly surprising

Coral bleaching

Stronger hurricanes?

Sea level rise

Spread of tropical diseases

Shift of agricultural zones

Growth in per capita consumption

Human population growth

Crop failures

Flooding

Droughts

Social disruption

Health impacts

Economic loss

Causes Consequences

As you progress through this chapter, try to identify as many solutions to anthropogenic climate change as you can. What could you personally do to help address this issue? Consider how each action or solution might affect items in the concept map above.

Deforestation

Fossil fuel use

Warming atmosphere

Greenhouse gas

emissions Storm surges and coastal erosion

Regional climate change

(e.g., cold Europe)

Change in ocean currents

Loss of biodiversity and ecosystem

services Shift of organisms

and ecological communities

Glacier and icecap melting

Solutions

Anthropogenic climate change • increased air

temperatures

• increased ocean temperatures

• altered rainfall and ENSO patterns

FIGURE 14.18  Human-induced global climate change stems from several causes (ovals on left) and results in a diver- sity of consequences (boxes on right) for ecological systems and human well-being. Arrows in this concept map lead from causes to consequences. Note that items grouped within outlined boxes do not necessarily share any special relation- ship; the outlined boxes are intended merely to streamline the figure.

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Electricity generation From cooking to heating to lighting, much of what we do each day depends on electricity. Fossil fuel combustion generates 70% of U.S. electricity, and coal accounts for most of the resulting emissions.

We can reduce our use of fossil fuels by encouraging conservation and efficiency (pp. 343–345). Power producers can use approaches such as cogeneration (p. 344) to produce fewer emissions per unit energy generated. Manufacturers can produce and consumers can use technologies such as compact fluorescent lightbulbs; high-efficiency appliances; and energy- efficient windows, ducts, insulation, and heating and cooling systems. In addition, each of us can make lifestyle choices to reduce electricity consumption.

We can also reduce greenhouse gas emissions by switching to cleaner energy sources. Natural gas generates the same amount of energy as coal, with half the emissions. Cleaner still are alternatives to fossil fuels, including nu- clear power (pp. 345–351), bioenergy, hydroelectric power, geothermal power, solar photovoltaic cells, and wind power (Chapter 16).

While our society transitions to clean and renewable en- ergy alternatives, we are also attempting to capture emissions before they leak to the atmosphere. Carbon capture refers to technologies or approaches that remove carbon dioxide from power plant emissions. The next step is carbon seques- tration, or carbon storage, in which the carbon is seques- tered, or stored, under pressure in deep salt mines, depleted oil or gas deposits, or other underground reservoirs (see Figure 15.12, p. 337). However, we are a long way from de- veloping adequate technology and secure storage space to ac- complish this without leakage—and it is questionable whether we would ever be able to sequester enough carbon to make a sizeable dent in our emissions.

Transportation The average American family makes 10 trips by car each day, and U.S. taxpayers spend over $200 mil- lion per day on road construction and repairs for the nation’s 250 million registered automobiles. Unfortunately, the typi- cal automobile is highly inefficient. Over 85% of the fuel you pump into your gas tank does something other than move your car down the road (FIGURE 14.19).

Shall we pursue mitigation or adaptation? We can respond to climate change in two fundamental ways. One is to pursue actions that reduce greenhouse gas emis- sions, so as to lessen the severity of climate change. This strategy is called mitigation because the aim is to mitigate, or alleviate, the problem. Examples include improving energy efficiency, switching to clean and renewable energy sources, preventing deforestation, recovering landfill gas, and encour- aging farm practices that protect soil quality.

Alternatively, we can pursue strategies to cushion our- selves from the impacts of climate change. This strategy is called adaptation because the goal is to adapt to change. Erecting a seawall like the Maldives’ Great Wall of Malé is one example of adaptation. Other examples include restrict- ing coastal development; adjusting farming practices to cope with drought; and modifying water management practices to deal with reduced river flows, glacial outburst floods, or salt contamination of groundwater.

We need to pursue adaptation because even if we could halt all our emissions right now, the greenhouse gas pollution already in the atmosphere would continue driving global warm- ing until the planet’s systems reach a new equilibrium, with temperature rising an estimated 0.6 °C (1.0 °F) more by the end of the century. Because we will face this change no matter what we do, it will be wise to develop ways to minimize its impacts.

We also need to pursue mitigation, because if we do noth- ing to diminish climate change, it will eventually overwhelm any efforts we might make to adapt. The sooner we begin re- ducing our emissions, the lower the level at which they will peak, and the less we will alter climate. We will spend the re- mainder of our chapter examining approaches for the mitiga- tion of climate change.

We are developing solutions in electricity generation and transportation The generation of electricity produces the largest portion (40%) of U.S. carbon dioxide emissions, and transportation is not far behind.

100%Gas 14% Moving car

62% Engine heat loss, friction, inef�ciencies

17% Idling

5% Drive train friction and inef�ciencies

2% Running

accessories: water pump, stereo, etc.

FIGURE 14.19  Conventional automobiles are inefficient. Only about 13–14% of the energy from a tank of gas actually moves the typical car down the road, while almost 85% of useful energy is lost, prima- rily as heat, according to the U.S. Department of Energy.

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and regulations? Should it impose no policies and hope that private enterprise will develop solutions on its own? Should it take the middle ground and design programs that give private entities financial incentives to reduce emissions? This debate has been vigorous in the United States, where many business leaders and politicians have opposed all government action to address climate change, fearing that emissions reductions will impose economic costs on industry and consumers.

In 2007, the U.S. Supreme Court ruled that carbon diox- ide was a pollutant that the Environmental Protection Agen- cy (EPA) should regulate under the Clean Air Act (p. 283). When Barack Obama became president, he instead urged that Congress craft legislation to address emissions. In 2009, the House of Representatives passed legislation to create a cap- and-trade system (pp. 108–109) in which industries and utili- ties would compete to reduce emissions for financial gain, and under which emissions were mandated to decrease 17% by 2020. However, similar legislation did not pass in the Senate. As a result, responsibility for addressing emissions passed to the EPA, which embarked on the complex process of develop- ing regulations beginning in 2011. The EPA plans to phase in emissions limits on industry and utilities gradually over many years, hoping to spur energy efficiency retrofits and renewable energy use at a pace that will minimize economic impacts and political opposition.

The Kyoto Protocol sought to limit emissions Climate change is a global problem, so global cooperation is needed to forge effective solutions. This is why the world’s policymakers have tried to tackle climate change with interna- tional treaties. In 1992, most of the world’s nations signed the

More aerodynamic designs, increased engine efficiency, and improved tire design can help make our vehicles more fuel-efficient (p. 344). Indeed, the vehicles of many nations are more fuel-efficient than those of the United States. As gasoline prices rise, consumer demand for fuel-efficient automobiles will intensify. Advancing technology is also bringing us al- ternatives to the traditional combustion-engine automobile. These include gasoline-electric hybrid vehicles (p. 344); fully electric vehicles; alternative fuels such as compressed natural gas and biodiesel (p. 361); and hydrogen fuel cells (p. 375).

We can also make lifestyle choices that reduce our reliance on cars. Some people are choosing to live nearer to their work- places. Others use mass transit such as buses, subway trains, and light rail. Still others bike or walk to work or on errands. Making automobile-oriented cities and suburbs more friendly to pedestrian and bicycle traffic and improving people’s access to public transportation stand as central challenges for city and regional planners (pp. 404–407).

We will need multiple strategies Advances in agriculture, forestry, and waste management can help us mitigate climate change. In agriculture, sustainable management of cropland and rangeland enables soil to store more carbon. New techniques reduce the emission of meth- ane from rice cultivation and from cattle and their manure and lessen nitrous oxide emissions from fertilizer. We can also grow renewable biofuel crops, although whether these decrease or increase emissions is an active area of research (pp. 358–361).

In forest management, preserving existing forests, refor- esting cleared areas, and pursuing sustainable forestry prac- tices (Chapter 9) all help to absorb carbon dioxide from the air. Waste managers are doing their part to cut emissions by treating wastewater (pp. 270, 272–273), generating energy from waste in incinerators (p. 385), and recovering methane seeping from landfills (pp. 385–386). Individuals, communi- ties, and waste haulers can encourage waste reduction, recy- cling, composting, and the reuse of materials and products (pp. 386–390).

We should not expect to find a single “magic bullet” for mitigating climate change. Reducing emissions will require many steps by people and institutions across many sectors of our economy. However, most reductions can be achieved using current technology—and we can begin implementing these changes right away. Environmental scientists Stephen Pacala and Robert Socolow advise that we follow some age- old wisdom: When the job is big, break it into small parts. Pa- cala and Socolow identify 15 strategies (TABLE 14.3) that could each eliminate 1 billion tons of carbon per year by 2050 if de- ployed at a large scale. Achieving just 7 of these 15 aims would stabilize our CO2 emissions at current levels. If we achieve more, then we reduce emissions.

What role should government play? Even if people agree on strategies and technologies to reduce emissions, they may disagree on how to encourage those strategies and technologies. A major debate is what role gov- ernment should play: Should it mandate change through laws

TABLE 14.3 Fifteen Ways to Eliminate 1 Billion Tons of Carbon Per Year by 2050

▶ Double the fuel economy of cars. ▶ Halve the miles driven by cars. ▶ Maximize efficiency in all buildings. ▶ Double the efficiency of coal-powered plants. ▶ Switch from coal to natural gas at 1,400 plants. ▶ Capture and store carbon from 800 coal plants. ▶ Capture and store carbon at hydrogen plants. ▶ Capture and store carbon from 180 “synfuels” plants. ▶ Increase hydrogen fuel production by 10 times. ▶ Triple the world’s nuclear capacity. ▶ Increase wind power capacity by 50 times. ▶ Increase solar power capacity by 700 times. ▶ Increase ethanol production by 50 times. ▶ Halt tropical deforestation, and double reforestation. ▶ Adopt conservation tillage on all croplands. Adapted from Pacala, S., and R. Socolow, 2004. Stabilization wedges: Solving the climate problem for the next 50 years with current technologies. Science 305: 968–972.

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Climate negotiations have progressed from Copenhagen to Cancun to South Africa In recent years, representatives of the world’s nations have met at a series of conferences, trying to design a treaty to take effect once the Kyoto Protocol ends in 2012. These climate negotiators could not reach consensus at their 2009 meet- ing in Copenhagen, Denmark. Here, the world’s two biggest greenhouse gas emitters, China and the United States, did not offer enough to satisfy other nations (FIGURE 14.20). China promised steep emissions cuts but proved unwilling to allow international monitoring to confirm them. U.S. President Obama chose not to promise more than the U.S. Congress had already agreed to. The conference ended amid discord, without specific targets or solid commitments.

The process got back on track in Cancun, Mexico, in 2010, where nations held productive discussion and fleshed out proposals from the Copenhagen conference. Developed nations promised to pay developing nations to help with their mitigation and adaptation efforts—up to $100 billion per year by 2020—through a fund overseen by the World Bank. Nations broadly agreed upon a plan, nicknamed “REDD” (p. 192), to help tropical nations reduce forest loss. Developed nations agreed to transfer clean energy technology to devel- oping nations. And rapidly industrializing nations such as China and India agreed in principle to emission targets and international monitoring. Participants also shared examples of ways they are reducing greenhouse gas pollution. For ex- ample, China is accelerating its renewable energy efforts and Brazil announced plans to limit deforestation and encourage no-till agriculture (pp. 133, 143).

Climate negotiators have one last chance to create a suc- cessor treaty to the Kyoto Protocol in December 2011, in Durban, South Africa. However, reaching consensus among 200 nations through the treaty process is a daunting chal- lenge, and the political steps outlined so far will not be ad- equate to halt climate change. As a result, experts predict that most success in mitigating climate change will come from

U.N. Framework Convention on Climate Change (FCCC), which outlined a plan to reduce greenhouse gas emissions to 1990 levels by the year 2000 through a voluntary approach. Emissions kept rising, however, so nations came together to forge a binding treaty to require emissions reductions. Drafted in 1997 in Kyoto, Japan, the Kyoto Protocol mandated sig- natory nations, by the period 2008–2012, to reduce emissions of six greenhouse gases to levels below those of 1990 (TABLE 14.4). This treaty took effect in 2005 after Russia became the 127th nation to ratify it.

The United States was the only developed nation not to ratify the Kyoto Protocol. Because the United States emits one-fifth of the world’s greenhouse gases, its refusal to join this global effort generated widespread resentment and un- dermined the treaty’s effectiveness. U.S. leaders called the treaty unfair because it required industrialized nations to re- duce emissions but did not require the same of rapidly indus- trializing nations such as China and India. Proponents of the Kyoto Protocol countered that the differential requirements were justified because industrialized nations created the cur- rent problem and thus should take the lead in resolving it.

As of 2009 (the most recent year with full international data), nations that signed the Kyoto Protocol had reduced their emissions by 11.3% from 1990 levels. However, much of this reduction was due to economic contraction in Russia and former Soviet-Bloc nations following the breakup of the Soviet Union. When these nations are factored out, the remaining signatories showed a 2.1% increase in emissions from 1990 to 2009. Moreover, emissions continued rising from 2009 to 2011 as nations emerged from economic recession.

FIGURE 14.20  At the Copenhagen climate conference in 2009, these activists showed the negotiating delegates their support for island nations such as Tuvalu and the Maldives and for the goal of bringing the atmospheric carbon dioxide level down to 350 parts per million.

TABLE 14.4 Emissions Reductions Required and Achieved

Nation Required change, 1990–2008/20121

Observed change, 1990–20092

Russia 0.0% �35.6%3

Germany �21.0% �26.3%

United Kingdom �12.5% �26.9%

France 0.0% �7.7%

Italy �6.5% �5.4%

Japan �6.0% �4.5%

United States4 �7.0% �7.2%

Canada �6.0% �16.9% 1 Percentage decrease in emissions (carbon-equivalents of six green- house gases) from 1990 to period 2008–2012, as mandated under the Kyoto Protocol.

2 Actual percentage change in emissions (carbon-equivalents of six greenhouse gases) from 1990 to 2009. Negative values indicate decreases; positive values indicate increases. Values do not include influences of land use and forest cover.

3 Russia’s decrease was due mainly to economic contraction following the breakup of the Soviet Union.

4 The United States has not ratified the Kyoto Protocol but was as- signed a reduction requirement and reports its emissions, which are included here for comparison.

Data from U.N. Framework Convention on Climate Change, National Greenhouse Gas Inventory Reports, 2011.

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states had signed on to the U.S. Mayors Climate Protec- tion Agreement, initiated by Mayor Greg Nickels of Seat- tle (FIGURE 14.22A). Under this agreement, mayors commit their cities to pursue policies to “meet or beat” Kyoto Pro- tocol guidelines.

At the state level, the boldest action so far has come in California. In 2006 that state’s legislature worked with Governor Arnold Schwarzenegger (FIGURE 14.22B) to pass the Global Warming Solutions Act, which aims to cut California’s greenhouse gas emissions 25% by the year 2020. This law was the first state legislation with penalties for noncompliance, and it followed earlier efforts in California to mandate higher fuel efficiency for automobiles.

Action was also taken by 10 northeastern states that launched the Regional Greenhouse Gas Initiative (RGGI) in 2007. In this effort, Connecticut, Delaware, Maine, Mary- land, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island, and Vermont set up a cap-and-trade program for carbon emissions from power plants. A simi- lar effort, the Western Climate Initiative, involves Arizona, British Columbia, California, Manitoba, Montana, New Mexico, Ontario, Oregon, Quebec, Utah, and Washington. These emissions trading programs (pp. 108–109) show how government can engage the market economy to pursue pub- lic policy goals.

Market mechanisms are being used to address climate change Permit trading programs (pp. 108–109) aim to harness the eco- nomic efficiency of market capitalism to achieve public policy goals while allowing business, industry, or utilities flexibility in how they meet those goals. Supporters of permit trading programs argue that they provide the fairest, least expensive, and most effective method of reducing emissions. Polluters choose how to cut their emissions and have financial incen- tives to reduce emissions below the legally required amount

technological advances, carbon trading markets, and national, regional, and local initiatives. Business and industry are ac- celerating renewable energy and energy efficiency efforts, and policymakers are looking for ways to help the private sector generate productive solutions.

Will emissions cuts hurt the economy? The U.S. Senate has opposed emissions reductions out of fear that they will dampen the U.S. economy. China, India, and other industrializing nations have so far resisted emis- sions cuts under the same assumption. This assumption is understandable, given that so much of our economy depends on fossil fuels. Yet nations such as Germany, England, and France have reduced their emissions since 1990 while en- hancing their technologically advanced economies and pro- viding their citizens standards of living comparable to those of U.S. citizens.

Because resource use and per capita emissions are high in the United States and other industrialized nations, govern- ments and industries there often feel they have more to lose economically from limiting emissions than developing na- tions do. However, industrialized nations are also the ones most likely to gain economically from major energy transi- tions, because they are best positioned to invent, develop, and market new technologies to power the world in a post-fossil- fuel era. Germany, Japan, and China have realized this, and are now leading the world in production, deployment, and sales of solar energy technology (FIGURE 14.21). If the United States does not act quickly to develop energy technologies for the future, then the future could belong to nations like China, Germany, and Japan.

States and cities are advancing climate change policy In the absence of action by the U.S. federal government to address climate change, state and local governments across the country are advancing policies to limit emissions. By 2010, mayors from over 1,000 cities from all 50 U.S.

(a) Greg Nickels (b) Arnold Schwarzenegger

FIGURE 14.22  In the absence of leadership at the federal level, elected officials at the state and local levels have taken charge. Seattle mayor Greg Nickels (a) convinced over 1,000 U.S. mayors to commit to fighting climate change, while California governor Arnold Schwarzenegger (b) promoted ambitious steps to lower his state’s greenhouse emissions.

FIGURE 14.21  Workers at a Chinese factory produce photo- voltaic solar panels (p. 365). China is racing to develop renewable energy technology and is on track to surpass the United States in becoming a leader in green energy technology.

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Carbon taxes are another option As the world’s carbon trading markets show mixed results, a number of economists, scientists, and policymakers are say- ing that cap-and-trade systems are not effective enough, don’t work quickly enough, or leave too much to chance. Many of these critics would prefer that governments enact a carbon tax instead. In this approach, governments charge pollut- ers a fee for each unit of greenhouse gases they emit. This gives polluters a clear financial incentive to reduce emissions: If they can find ways to reduce their emissions, they reduce their tax and save money. Carbon taxes have so far been es- tablished in several European nations, in the Canadian prov- ince of British Columbia, and in Boulder, Colorado.

The downside of a carbon tax is that most polluters sim- ply pass the cost along to consumers by charging higher pric- es for the products or services they sell. Proponents of carbon taxes have responded by proposing an approach called fee- and-dividend. In this approach, funds from the carbon tax, or “fee,” paid to government by polluters are transferred as a tax refund, or “dividend,” to taxpayers. This way, if polluters pass their costs along to consumers, those consumers will be reimbursed by the tax refund they receive. In theory, the sys- tem should provide polluters a financial incentive to reduce emissions while imposing no financial burden on taxpayers.

(FIGURE 14.23). As an example of how a cap-and-trade program for carbon emissions can work, consider the Re- gional Greenhouse Gas Initiative:

1. Each state decided what polluting sources it would re- quire to participate.

2. Each state set a cap on the total CO2 emissions it would allow, equal to its 2009 level.

3. Each state distributed to each polluter one permit for each ton it emits, up to the amount of the cap.

4. Each state will lower its cap gradually—10% by 2018. 5. Sources with too few permits to cover their pollution must

find ways to reduce their emissions, buy permits from other sources, or pay for credits through a carbon offset project (p. 322). Sources with excess permits may sell them.

6. Any source emitting more than its permitted amount will face penalties.

Once up and running, it is hoped that the system will be self- sustaining. The price of a permit is meant to fluctuate freely in the market, creating the same kinds of financial incentives as any other commodity that is bought and sold.

The world’s largest cap-and-trade program is the European Union Emission Trading Scheme. This market got off to a suc- cessful start in 2005—until investors discovered that national governments had allocated too many permits to their indus- tries. The overallocation gave companies little incentive to re- duce emissions, so permits lost their value, and prices in the market fell to 1/100th of their high value. Europeans tried to correct these problems by making emitters pay for permits and setting emissions caps across the entire European Union while expanding the program to include more greenhouse gases, more emissions sources, and additional members. In the long run, permits will be valuable and the market will work only if policies are in place to limit emissions.

Government establishes cap on emissions from all plants1

Plant A succeeds in cutting emissions

Plant B fails to cut emissions3

Plant A profits from selling allowances to Plant B

Allowances

$$$$$

FIGURE 14.23  In a cap-and-trade emissions trading system, � government first sets an overall cap on emissions. As polluting facilities respond, some will have better success reducing emissions than others. In this figure, � Plant A succeeds in cutting its emissions well below the cap, whereas � Plant B fails to cut its emissions at all. As a result, � Plant B must pay money to Plant A to purchase allowances that Plant A is no longer using. Plant A profits from this sale, and the government cap is met, reducing pollution overall. Over time, the cap can be lowered to achieve further emissions cuts.

Cap-and-Trade or a Carbon Tax? What advantages and disadvantages do you see in using a cap-and-trade system to reduce green- house gas emissions? What pros and cons do you

see in using carbon taxes to achieve this goal? What do you think of the idea of a “fee-and-dividend” program? If you were a U.S. senator, what type of policy would you support in order to address emissions in the United States, and why?

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can find ways to reduce their carbon footprints directly. An excellent example is Pearson Education, the publisher of your textbook. In 2009 Pearson achieved carbon-neutrality after a concerted two-year effort. Pearson reduced its energy con- sumption and carbon footprint by 12% by upgrading build- ings for energy efficiency, designing more efficient computer servers, reducing the number of vehicles in its fleets, increas- ing the proportion of hybrid vehicles, and cutting back on employee business travel while enhancing the use of video conferencing. Pearson eliminated a further 47% of its emis- sions by purchasing clean renewable energy instead of fossil fuel energy and by installing a large solar panel array at one of its sites in New Jersey. To offset the remaining 41% of its emissions, the company is funding a number of programs to preserve forest and replant trees in various areas of the world, from England to Costa Rica.

Should we engineer the climate? What if all our efforts to reduce emissions are not adequate to rein in climate change? As severe climate change begins look- ing more likely, some scientists and engineers are reluctantly considering drastic, assertive steps to alter Earth’s climate in a last-ditch attempt to reverse global warming—an approach called geoengineering (FIGURE 14.24).

Carbon offsets are popular Emissions trading programs generally allow participants to buy carbon offsets, voluntary payments intended to enable another entity to reduce emissions that one is unable to re- duce oneself. The payment thus offsets one’s own emissions. For example, a coal-burning power plant could pay a reforest- ation project to plant trees that will soak up as much carbon dioxide as the coal plant emits. Or a university could fund the development of clean renewable energy projects to make up for fossil fuel energy the university uses. Carbon offsets have fast become popular among utilities, businesses, universi- ties, governments, and individuals trying to achieve carbon- neutrality, a state in which no net carbon is emitted.

In principle, carbon offsets seem a great idea, but rigor- ous oversight is needed to make sure that the offset money actually accomplishes what it is intended for—and that offsets fund only emissions reductions that would not occur other- wise. Efforts to create a transparent and enforceable system to assess offsets are ongoing.

Corporations are going carbon-neutral Carbon offsets are a major route toward carbon-neutrality among businesses and corporations seeking to make their practices more sustainable (pp. 95–96), but corporations also

Place space mirrors in orbit

Fertilize ocean with iron to spur plankton blooms

Inject sulfate aerosols into stratosphere

Erect land-based mirrors

Seed clouds with seawater mist

Capture carbon with arti�cial trees

Store carbon underground

Restore forests

FIGURE 14.24  Geoengineering proposals seek to cool the planet’s climate by removing carbon dioxide from the air or reflecting sunlight away from Earth. Long considered fringe science or science fiction, geoengineering is now being considered by researchers who fear our efforts to reduce emissions will be too little and too late. However, most geoengineering ideas are untested, would take years to develop, might not work well enough, or might lead to other envi- ronmental impacts—so they are not a substitute for reducing emissions.

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You can reduce your carbon footprint Government policies, corporate actions, international trea- ties, technological innovations—and perhaps even geoengi- neering—will all play roles in mitigating climate change. But in the end the most influential factor may be the collective actions of millions of regular people. Just as we each have an ecological footprint (p. 4), we each have a carbon footprint that expresses the amount of carbon we are responsible for emitting. To help reduce emissions, each of us can take steps in our everyday lives—from turning off lights and choosing appliances to deciding where to live and how to get to work.

College students are vital to driving the personal and so- cietal changes needed to reduce carbon footprints and address climate change. Today a groundswell of interest is sweeping across campuses (pp. 414–415). This was evident on January 31, 2008, when over 1,900 schools participated in the Focus the Nation teach-in on global warming. Young people have played a large role in subsequent grassroots events, including 350.org’s International Day of Climate Action—kicked off by the Maldives’ underwater cabinet meeting—which featured 5,200 events in 181 nations and was called “the most wide- spread day of political action in the planet’s history.” Global climate change may be the biggest challenge we face, but with concerted action we can still avert the most severe impacts. Through outreach, education, innovation, and lifestyle choic- es, today’s youth have the power to turn the tables on climate change and help bring about a bright future for humanity and our planet.

One main geoengineering approach would be to suck car- bon dioxide out of the air. We might enhance photosynthesis in natural systems by planting trees or by fertilizing ocean phytoplankton with nutrients such as iron. A more high-tech method would be to design “artificial trees,” structures that chemically filter CO2 from the air.

The second main geoengineering approach would be to block sunlight before it reaches Earth, thus cooling the planet. People have proposed deflecting sunlight by injecting sulfates or other fine dust particles into the stratosphere, by seeding clouds with seawater, or by deploying fleets of reflecting mir- rors on land, at sea, or in orbit in space.

Scientists have long been reluctant even to discuss the no- tion of geoengineering. The potential methods are technically daunting and would take years or decades to develop, and they could pose unforeseen environmental risks. Moreover, blocking sunlight does not reduce greenhouse gas concentra- tions, so problems such as ocean acidification would contin- ue. In addition, many experts are wary of promulgating hope for easy technological fixes, lest politicians lose their incentive to try to reduce emissions.

However, as climate change intensifies, more scientists are becoming willing to contemplate geoengineering as a back-up plan in case our efforts to reduce emissions fall short. As a result, some researchers and scientific institu- tions are beginning to assess the risks and benefits of geo- engineering options, so that we can be ready to take action in future years if climate change becomes severe enough to justify it.

T E S T I N G Y O U R C O M P R E H E N S I O N

1. What happens to solar radiation after it reaches Earth? How do greenhouse gases warm the lower atmosphere?

2. Why is carbon dioxide considered the main greenhouse gas? Why are carbon dioxide concentrations increasing in the atmosphere?

3. How do scientists study the ancient atmosphere? De- scribe what a proxy indicator is, and give two examples.

4. List five major trends in climate that scientists have doc- umented so far.

5. Now list five future trends that researchers are predict- ing, along with their potential impacts.

6. Describe how rising sea levels, caused by global warm- ing, can create problems for people. How may climate change affect marine ecosystems?

7. How might a warmer climate affect agriculture? How is it affecting distributions of plants and animals? How might it affect human health?

8. What are the two largest sources of greenhouse gas emissions in the United States? How can we reduce these emissions?

9. What roles have international treaties played in address- ing climate change? Give two specific examples.

10. Describe one market-based approach for reducing green- house gas emissions. Explain one reason it may work well and one reason it may not work well.

� CONCLUSION Many factors inf luence Earth’s climate, and human activi- ties have come to play a major role. Climate change is well underway, and further greenhouse gas emissions will in- tensify global warming and cause increasingly severe and diverse impacts. Sea level rise and other consequences of global climate change will affect locations worldwide from the Maldives to Bangladesh to Alaska to Florida. As

scientists and policymakers come to better understand an- thropogenic climate change and its environmental, eco- nomic, and social consequences, more and more of them are urging immediate action. Reducing greenhouse gas emissions and taking other steps to mitigate and adapt to climate change represents our society’s foremost challenge in the coming years. 323

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Kyoto Protocol was not fully effective, and most are committed to creating a stronger agreement. The U.S. government has instructed you to take a leading role in designing the new treaty and to engage constructively with other nations’ representatives while protecting your nation’s economic and political interests. What type of agreement will you try to shape? Describe at least three components that you would propose or agree to, and at least one that you would oppose.

5. THINK IT THROUGH You have just been elected gov- ernor of a medium-sized U.S. state. Polls show that the public wants you to take bold action to reduce green- house gas emissions. However, polls also show that the public does not want gasoline or electricity prices to rise. Carbon-emitting industries in your state are wary of emissions reductions being required of them but are will- ing to explore ideas with you. Your state legislature will support you in your efforts as long as you remain popu- lar with voters. The state to your west has just passed am- bitious legislation mandating steep greenhouse gas emis- sions reductions. The state to your east has just joined a new regional emissions trading consortium. What ac- tions will you take in your first year as governor?

1. Some people argue that we need “more proof” or “ better science” before we commit to substantial changes in our energy economy. How much certainty do you think we need before we should take action regarding climate change? How much certainty do you need in your own life before you make a major decision? Should nations and elected officials follow a different standard? Do you believe that the precautionary principle (pp. 152, 222) is an appropriate standard in the case of global climate change? Why or why not?

2. Describe several ways in which we can reduce green- house gas emissions from transportation. Which ap- proach do you think is most realistic, which approach do you think is least realistic, and why?

3. Suppose that you would like to make your own lifestyle carbon-neutral and that you aim to begin by reducing the emissions you are responsible for by 25%. What three actions would you take first to achieve this reduction?

4. THINK IT THROUGH You have been appointed as the United States representative to an international con- ference to negotiate terms of a treaty to take hold after the Kyoto Protocol ends. All nations recognize that the

S E E K I N G S O L U T I O N S

C A L C U L A T I N G E C O L O G I C A L F O O T P R I N T S

Global climate change is something to which we all contrib- ute, because fossil fuel combustion plays such a large role in supporting the lifestyles we lead. Conversely, as individuals, each one of us can help to mitigate climate change through personal decisions and actions in how we live our lives. Several online calculators enable you to calculate your own personal carbon footprint, the amount of carbon emissions for which you are responsible. Go to www.nature.org/initia- tives/climatechange/calculator, take the quiz, and enter the relevant data in the table.

Carbon footprint (tons per person per year)

World average

U.S. average

Your footprint

Your footprint with three changes

1. How does your personal carbon footprint compare to that of the average U.S. resident? How does it compare to that of the average person in the world? Why do you think your footprint differs from these in the ways it does?

2. As you took the quiz and noted the impacts of vari- ous choices and activities, which one surprised you the most?

3. Think of three changes you could make in your lifestyle that would lower your carbon footprint. Now take the footprint quiz again, incorporating these three changes. Enter your resulting footprint in the table. By how much did you reduce your yearly emissions?

4. What do you think would be an admirable yet realistic goal for you to set as a target value for your own foot- print? Would you choose to purchase carbon offsets to help reduce your impact? Why or why not?

Go to www.masteringenvironmentalscience.com for homework assignments, practice quizzes, Pearson eText, and more.

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