Module 3
World Climates
A. Climates Classification and Types
By considering temperature and precipitation, both in terms of their averages and
in terms of average maxima, average minima, and seasonal variations, geographers
developed several climate classification systems, including the one portrayed on this
globe ( ⊲). The colors and letters indicate what type of climate characterizes an area.
Examine the color patterns and letters on this globe and compare these to the globes
showing average annual temperature and precipitation (on previous page). What
correlations do you note?
The climate of a region is influenced by many factors discussed in this book,
including latitude and its relationship to Sun angle, elevation, topography, the locations
of semipermanent pressure features, prevailing wind directions, common storm tracks,
sea- surface temperatures, ocean currents, humidity, atmospheric oscillations, and
teleconnections. These and other factors act in concert to produce the characteristic
climates around the world, such as these in Africa and Europe.
Classification is the process of grouping similar items together and separating
dissimilar items. Items that are similar in some ways can be different in others, and items
that are in different groups can have certain similarities. Classifications allow us to
examine general patterns, with the caveat that interpretations derived from classified
groups are best done cautiously. Here, we examine a well-established method of
classifying climates. Climates are classified to let us observe broad patterns and to
simplify communication about the characteristics of a region. Historically there have
been thousands of meteorological stations collecting weather and climatic information,
and in the past few decades this wealth of knowledge has expanded with the addition of
large quantities of remotely sensed information. Classification helps us to seek useful
generalizations from this enormous amount of information. Climate can be classified in
many ways, with the exact nature of the classification depending upon the research
question that is of interest.
Colors on this map of an imaginary country represent mean annual temperatures
(reds above 28°C, blue and purple below 0°C). The black contours represent mean annual
precipitation in millimeters. Meteorological stations are shown as dots. Examine the
patterns on this map and think about how you might classify different parts of the country
(e.g., cold and wet). The southernmost areas are hot, which is an important fact if we
were investigating an outbreak of a heat-related sickness. But we might classify stations
in the southeast differently than those in the southwest because of the differences in
precipitation (and therefore in humidity and perhaps in mold). A study of the timing and
magnitude of floods might focus on (1) regions where winter temperatures reach below
freezing, opening up the possibility of flooding due to spring snowmelt, and (2) the
amount of winter precipitation, which controls how much snow accumulates. Most crops
have fairly well-defined climatological requirements for their growth. A climate
classification could be developed based on “optimal,” “fair,” or “poor” for growing that
crop. For example, the best conditions for coffee are average temperatures of about 20°C
and 1,800–2,800 mm of precipitation. Areas for possible coffee cultivation in this country
would be very limited.
Vladmir Köppen was a botanist interested in the global distribution of vegetation
types. He surmised that the annual temperature and precipitation regimes determined the
types of natural vegetation. Other influences on plant growth, such as which way a slope
faces, the number of consecutive days without precipitation, and the types of soils, have
more localized effects and so were not considered by Köppen. On Köppen’s maps, the
lines separating different types of climate also separate different realms of natural
vegetation. For each climate type, Köppen carefully selected thresholds of temperature
and precipitation that preserved the boundaries for that characteristic assemblage of
vegetation. The original scheme underwent modification as more information on global
climates and vegetation became available. The resulting Köppen climate classification is
the most widely used system; an example is portrayed on this climate map of South
America. In reality, boundaries between climatic types are not static from year to year
and not as clearly delineated as the map would suggest. Instead a boundary between
vegetation and climate types usually occurs across a broad transition zone, known as an
ecotone. In South America, an ecotone occurs between the Amazon rain forest and
mountain climates of the Andes.
Köppen’s system was designed to delineate vegetation realms, so it overlays well
with worldwide natural vegetation zones—biomes. Both climate and vegetation are major
controls on the types of soil that develop, leading to a notable similarity between the
Köppen classification and maps of world soils. These linkages between climate,
vegetation, and soils ensure that this classification is very useful in understanding the
physical geography of our continents at planetary and regional scales. More recent
satellite data availability over oceans allows us to compute the climate type over the
ocean. Köppen was interested in vegetation realms, so he restricted his classification to
the land.
The Köppen climate classification system has five major categories, represented
by the first (capital) letter of the labels on the South America map. These are the
following: A – tropical climates, B – arid climates, C – temperate mid-latitude climates,
D – harsh mid-latitude climates, and E – polar climates, each represented by a photograph
below. The five categories are further subdivided using a succession of criteria of
temperature and the availability of water, as illustrated by the flow chart below. In the A,
C, and D climate types, the second letter is lowercase and indicates whether the climate is
wet year-round (f), has a dry summer (s), has a dry winter (w), or experiences a monsoon
(m). The second letter is uppercase for arid and polar climates. Subsequent pages list key
characteristics of each type of climate, but follow the flow chart below first.
The Köppen climate classification was defined for land areas and generally is
depicted as a map covering a large area and portraying the regional or global distributions
of different climate types. The map on these two pages covers the entire planet and is an
Extended Köppen Classification, because we have extended the land-defined Köppen
classification to encompass similar climatic conditions over the oceans. This allows us to
better explore how different climates relate to regional atmospheric features, like high
pressure in the subtropics. Study the patterns of different climates at the scale of the
entire planet and then focus on a single continent or part of a continent. Refer to tables on
the next two-page spread for a key to the labels. What is the label for the place where you
live?
Each type of climate, as defined by the Köppen climate classification, has specific
characteristics that distinguish it from any other climate type. Once a climate is assigned
to one of the five main groups (A–E), it is then categorized by factors such as whether the
summer is hot, warm, or cool, and whether most precipitation falls in the summer, winter,
throughout the year, or during a monsoon. The table on the right-hand page provides a
reference for characteristics of each climate type. Four globes present the same
information as the flat map on the previous two pages. Carefully observe each globe,
guided by the table, and then read the text blocks in clockwise order around the globes.
The arrangement of climates is least complicated in the oceans, away from the
land. Here, climate types are arranged in belts that roughly parallel latitude, progressing
from A-type (tropical) near the equator to B, C, and E toward the poles. Some climate
types, such as D-types (harsh mid-latitude), are more common over land than over the
oceans, which moderate the temperatures. The patterns of climate types are most complex
on land, especially in areas with large amounts of high relief and variations in
topography. Steep gradients in climate types, as in western Asia, commonly reflect steep
topographic gradients, such as those northeast of the Mediterranean Sea or from the high
Tibetan Plateau to the tropical lowlands of southern India. Note that D-type climates over
landmasses are largely restricted to the Northern Hemisphere, as in Siberia, but an
oceanic belt of D-type climates also encircles Antarctica (not shown in this view).
B-type climates are those where precipitation is not abundant enough to replenish
water that could possibly be lost through evapotranspiration (potential
evapotranspiration). B-type climates generally occur in the dry, sinking air that
characterizes the subtropics. Note that B-type climates also occur over the ocean. As
shown by the Indian and Pacific oceans, most A-type climates occur over the ocean, and
these conditions also encompass islands and adjacent landmasses, as in the region of
Papua New Guinea, Indonesia, and the Philippines.
Climates in the ocean are generally asymmetric around continents, as on either
side of Australia, reflecting the asymmetrical circulation of ocean currents, which bring
cooler water and stable atmospheric conditions along one side and warmer water with
unstable conditions along the opposite side. E-type (polar) climates are restricted to
Antarctica and the surrounding ocean and to the Arctic Ocean and the surrounding land.
B. Setting of Tropical Climates
A-type climates in the Köppen climatic classification system are “tropical,”
having consistently warm temperatures all year. Precipitation in these zones is primarily
caused by the convergence of the trade winds along the Intertropical Convergence Zone
(ITCZ), which shifts with the season to locations north and south of the equator. There
are three types of A climates in the Köppen system — Tropical Rain Forest, Tropical
Savanna, and Tropical Monsoon. These globes show the distribution of each of the three
types of Group A (tropical) climates. All three globes show all three types, but each type
is discussed separately below the globes. Note that all three climate types are centered in
the tropics, but they widen or narrow considerably from ocean to ocean and from
continent to continent.
The Tropical Savanna (Aw) climate is named for its typical vegetation of
grasslands and a few scattered trees. It occurs poleward of the other A-type climates and
is therefore subject to the alternating influences of the subtropical highs and the ITCZ.
Temperatures vary throughout the year somewhat more than in other A-type climates. A-
type climates share a trait that we commonly associate with the word tropical — warm
and relatively consistent temperatures throughout the year, a direct consequence of the
way the Sun interacts with our tilted planet. The noontime Sun is generally high over the
tropics, as it migrates during the course of a year, from directly above 23.5° S latitude on
the December Solstice to above 23.5° N latitude on the June Solstice, and back above
23.5° S again by the following December. As a result, it remains high over the region
between these two latitudes (the tropics). This setting causes the areas between the tropics
to be heated with similar intensity throughout the year, unlike the seasonal variations that
occur beyond the tropics. Also, the nearly overhead position of the Sun means the lengths
of day and night vary little seasonally. Locations near the equator experience
approximately 12 hours of sunlight throughout the year, but not all areas in climate
Group A do because they extend away from the equator.
In a Tropical Rain Forest (Af ) climate, precipitation exceeds losses through
evaporation and transpiration in most months of the year. In addition to the lush
vegetation, streams carry large volumes of water from the Af climates. For most months
in a Tropical Monsoon (Am) climate, precipitation exceeds the needs of vegetation. The
excess precipitation stored in the soil during these months allows rain forests to survive a
few relatively dry months. Vegetation becomes slightly shorter and sparser in this zone
compared to Af. The Tropical Savanna climate (Aw) is also known as a Tropical Wet-
Dry climate because about half of the months receive abundant precipitation and the
other half are very dry. The dry season is too long to support forests because the soil
moisture becomes depleted during that time. Instead, the vegetation is grassland with
scattered short, wide trees, such as acacia trees.
In an Af climate, the ITCZ brings rain most days throughout the year, although
rainfall can be heavier in some months than others. Af climates remain under the
influence of the ITCZ, even as it migrates north and south with the seasons. For an Am
climate, the ITCZ is a factor in most months, especially during the summer when it is
typically overhead or very close. During low-Sun-season months, the ITCZ moves too far
away to generate much precipitation. In an Aw climate, the migrating ITCZ brings
abundant summer rain. The low-Sun-season is dry and warm as these areas come under
the influence of the subtropical highs.
Group A climates are all tropical, so there is very little seasonality in temperature.
Precipitation patterns do vary among the three types, displaying some seasonality. Af
climates are closest to the equator and so display the least variation in precipitation over
the course of a year, whereas Aw climates are farthest away from the equator and so
experience the greatest seasonal changes. Am climates are typically in between in
position and character. The plots below are climographs, each showing temperature as a
line graph and amounts of precipitation as a bar graph. Observe the patterns and compare
them with the information above about the ITCZ.
C. Arid and Warm Temperate Climates
Arid Climates occupy a greater portion of Earth’s land surface than any other
climate category. They comprise Group B in the Köppen classification and are
subdivided into desert climates and steppe climates, with the distinction being that deserts
are more arid than steppes. Deserts and steppes are further subclassified into hot or cold
categories — not all deserts or steppes are hot! In the Köppen classification of Group B
climates, potential evapotranspiration is taken into account, in addition to precipitation;
an area is classified as arid only if precipitation does not offset the potential loss of water
through evaporation from the surface and transpiration through leaf surfaces.
A Hot Desert climate (BWh) covers huge areas of Africa (e.g., Sahara), the
Arabian Peninsula, the interior of Australia, and parts of the American Southwest. It also
extends over large areas of subtropical ocean. A Cold Desert climate (BWk) is less
common, occurring mostly in the interior of Asia (like southern Mongolia) and near cold
ocean currents, like the Humboldt Current off the west coast of South America, as shown
here. Hot Steppe climates (BSh) generally surround the hot deserts and represent
transitions to more humid climates, as across the Sahel region south of the Sahara and
north and east of the hot deserts of Australia. Cold Steppes (BSk) are abundant along the
Great Plains and western interior of North America, in Tibet and other parts of central
Asia, and in southern Australia. They cover relatively small parts of the ocean.
An area designated as an arid climate (Group B) in the Köppen classification
system must have the potential to lose more moisture through evapotranspiration (called
potential evapotranspiration) than it receives in precipitation. This condition can result
from a limited amount of precipitation and a high demand for water by having intense
insolation (i.e., high potential evapotranspiration); it is always some combination of the
following factors. The main cause of deserts and other arid areas is their position along
the subtropics, where the descending limb of the Hadley cell brings dry air and high
pressure. Note how the arid lands in the Sahara and the Arabian Peninsula ( ⊲) form an
east-west band along subtropical latitudes, a manifestation of the Hadley-caused high
pressure. The descending air generally means that even if the air is humid, it cannot rise
easily, which limits formation of clouds and precipitation. Arid lands of Australia and the
American Southwest occupy similar subtropical settings.
Other desert climate areas (BWh and BWk) and steppe climate areas (BSh and
BSk) owe their existence partly or mostly to their proximity to cold ocean currents, which
limit the amount of moisture in the air and promote atmospheric stability. One such area
is the Namib Desert of southwest Africa. Many arid (Group B) areas are dry because they
are in the interior of a continent, far from oceanic moisture sources, as in the case of the
western interior of the U.S.
The climographs below convey temperatures (the lines) and precipitation (bar
graphs) in areas designated Group B (desert and steppe climates). As we might expect,
precipitation totals are very small, and what precipitation falls evaporates quickly in the
dry air, leaving a parched landscape. Steppe climates are more abundant and far more
heavily populated than deserts. They are transition zones between desert climates and
more humid climates. Precipitation in such environments is variable from year to year,
with some years receiving desert-like precipitation totals and others experiencing
precipitation more characteristic of humid climates. Like natural vegetation, human
populations in these regions must be able to adapt to periodic dry spells and severe water
shortages.
Temperate mid-latitude climates, designated as Group C of the Köppen system,
experience moderate temperatures and precipitation and so are sites of particularly
intense human activity. Some temperate climates are relatively warm, because they occur
mostly in subtropical latitudes. These include Humid Subtropical, Mediterranean, and
Temperate Monsoon climates. The globes below depict distributions of six temperate
mid-latitude climate types. These include Humid Subtropical climate (Cfa), two
Mediterranean climates characterized by dry summers (Csa and Csb), and three other
climates in which precipitation is predominantly in summer due to continental monsoon
effects, for which they receive the “w” or “winter dry” designation (Cwa, Cwb, and
Cwc).
Humid Subtropical climates (Cfa) are wet year-round and have hot summers with
relatively short and intermittent cold seasons. They generally occur on the east coasts of
continents and adjacent oceans—southeastern South America (shown here), southeastern
U.S., eastern Asia (including much of Japan and part of China), and eastern Australia.
Since Cfa climates are defined as having hot, rather than warm, summers, their marine
extent is somewhat limited. Mediterranean climates experience wet winters but dry and
hot (Csa), or dry and warm (Csb), summers. These are pleasant climates for vacationers
or retirees. In addition to partly encircling the Mediterranean Sea, they generally occur
near the west coasts of continents and in adjacent oceans, including along the Pacific
coast of the U.S., southwestern South America, southern Eurasia, and the southwestern
tips of Africa and Australia. Coastal California is famous for its moderate, Mediterranean
climate. Three other warm temperate climate types experience large variations in
precipitation between seasons, generally associated with shifts in wind direction—a
monsoon. Places with Temperate Monsoon climates receive most of their rainfall in the
summer but are dry in the winter, and they can have hot (Cwa), warm (Cwb), or very
rarely, mild (Cwc) summers. They are best known from India to Southeast Asia, but they
also occur in southern South America, southern Africa, and in northeastern Australia.
On this globe showing average annual temperature, the Mediterranean (the nearly
enclosed sea in the center) is a zone of transition, between very hot temperatures of the
subtropics to the south and colder temperatures farther north. This characterizes a
temperate climate—it is intermediate between tropical and harsh climates. Summers are
warm to hot, because the Sun is high and day lengths are moderately long. Coastal areas
have temperatures that are moderated by nearby bodies of water. Summers are influenced
exclusively by tropical air masses, especially maritime tropical air masses and less often
by continental tropical air masses. In winter, temperate climates are influenced by polar
and tropical air masses, but the polar air masses are usually moderated by the time they
reach these latitudes.
Temperate climates, especially those in or near the subtropics, are affected greatly
by subtropical highs, like the BermudaAzores High in the north Atlantic and the South
Atlantic High. When these high-pressure areas are strong and nearby, they suppress the
formation of clouds and precipitation. Precipitation can occur more easily when the highs
are farther away and when they are weaker. . Subtropical highs display a very important
asymmetry—sinking motions descend to the surface on the eastern side, which coincides
with the western side of adjacent continents. However, the sinking does not extend to the
surface on the western side, which corresponds to the eastern side of continents.
The “tilting” of the subtropical highs is caused primarily by the difference in
ocean currents on each side of the ocean. Cold currents on the eastern sides of ocean
basins stabilize the atmosphere and allow sinking of air to the surface. Warm currents on
the western margins destabilize the atmosphere by adding energy to the surface, thereby
encouraging rising atmospheric motion. Similar processes occur in both the Northern and
Southern hemispheres, because in both settings cold currents are on the east side of the
ocean and warm currents are on the west side.
Hadley cells migrate and change width seasonally, following the position of
maximum insolation northward during northern summer and southward in southern
summer. Since the subtropical highs represent the descending limb of the Hadley cell,
they shift and change in intensity and size too, as shown here. In both hemispheres, the
subtropical highs expand and make their closest approach to regions of temperate
climates during the summer (June in the Northern Hemisphere and December in the
Southern Hemisphere). The strong sinking action on the eastern side of a high, when it
makes its closest approach in summer, stifles rain on the west coasts of the continents.
Mediterranean (Csa, Csb) climates are characterized by this lack of rain in the summer.
Because sinking associated with a subtropical high does not extend to the surface
on its western sides, the east sides of continents are places with Humid Subtropical
climates. Here, clouds can grow vertically and summer rain falls in places like
southeastern South America or the southeastern U.S. On this figure, note that
Mediterranean climates (in light and dark pink) are more common on the west coasts of
continents, but Humid Subtropical climates (shown in light green) are more common
along the east coasts of continents—a direct response to the asymmetry of the subtropical
highs.
With the change in seasons, the Hadley cells migrate toward the opposite
hemisphere (e.g., into the Southern Hemisphere during December, the northern winter).
The subtropical high in the hemisphere experiencing winter also shifts toward the equator
and becomes smaller and less intense, so it no longer blocks migrating weather systems
or the southern advance of polar air masses. As a result, frontal precipitation from mid-
latitude cyclones spreads across regions of temperate climate during the winter. Thus, a
Mediterranean climate owes its characteristic dry and warm or hot summer, but cooler,
wet winter, to its subtropical latitude and to the interactions between migrating
subtropical highs and ocean currents— a lot of things need to happen for a nice day on a
California beach.
D. Mid Latitude, Subarctic, and Polar Climates
Non-arid, mid-latitude climates tend to experience a relatively even distribution of
precipitation year-round. Some are dominated by maritime air masses that moderate
temperature swings and result in relatively mild winters and summers; they are within
Group C in the Köppen designation. Other mid-latitude climates involve more severe
winters because they are dominated by continental air masses, and these fall into the D
classification. Mid-latitude regions are affected by westerlies, and precipitation patterns
reflect the role of mid-latitude cyclones.
The globes below portray the distributions of three mid-latitude climate types —
Marine West Coast climate (Cfb and Cfc), Humid Continental climate (Dfa and Dfb), and
Continental Monsoon climate (Dwa and Dwb). As you can observe, most are in cold
places. The Marine West Coast climates (Cfb and Cfc) occur over or near oceans at
moderately high latitudes, like the southwest coast of Canada. An abundant supply of
moisture provides precipitation throughout the year. The influence of the ocean means
that summers are warm (Cfb) or cool (Cfc), and winters are cold but not severe. Such
climates occur in northern and southern parts of oceans and across most of western
Europe.
Humid Continental climates (Dfa and Dfb) occupy the interior of continents, but
only in the Northern Hemisphere—southern continents are not wide enough at these
latitudes to allow these climates. Humid Continental climates are wet year-round and
feature hot (Dfa) or warm (Dfb) summers but long and cold (Dfa) to severe (Dfb)
winters. They are the climates of southern Russia and the upper Midwest and Great Lakes
region of the U.S. Continental Monsoon climates (Dwa and Dwb) are restricted in
distribution, occurring mostly in Asia, in the interior of China and north of the Korean
Peninsula. In both locations, precipitation varies greatly during the year, partly because of
nearby monsoons. As the name implies, Dwa climates have dry winters and hot, wet
summers. Dwa occurs farther north and has warm (not hot) summers.
The mid-latitude westerlies bring a parade of mid-latitude cyclones to areas of
Marine West Coast climates (Cfb and Cfc) and Humid Continental climates (Dfa and
Dfb). This ensures abundant precipitation throughout the year in marine climates and
adequate precipitation throughout the year in continental ones. Humid Continental
climates of Asia experience little precipitation in winter, but the climates are not arid
because potential evapotranspiration is low in winter there, so water is not removed
readily from the surface. What other factors might affect the precipitation climatology of
Marine West Coast and Humid Continental climates?
The two mid-latitude marine climates (Cfb and Cfc) are either over relatively cool
oceans or over land areas where winds bring oceanic air across a continent, as along the
Pacific coast of Canada and in western Europe. Even though the subtropical high-
pressure zones are far away, they still make their nearest approach to these regions in the
summer and suppress precipitation somewhat, particularly on the southern fringes of the
Marine West Coast climate. As a result, precipitation tends to peak in October (when the
oceans are warmest) and December (when the subtropical high is farthest away) in
Northern Hemisphere areas under these two marine climates. Humid Continental (Dfa,
Dfb) climates receive precipitation throughout the year, including snow in the colder
months. Summers are somewhat wetter than the rest of the year in most Humid
Continental climates because of convective thunderstorms on hot summer afternoons and
the high water-vapor capacity in warm air. In North America, the clockwise flow of Gulf
of Mexico moisture around the expanded Bermuda-Azores High adds to the summer
precipitation potential. In winter, the combination of moisture from the Great Lakes and
bitter cold air results in heavy snowfall (lake-effect snows) on lands downwind of the
lakes.
The mid-latitude westerlies, including the polar front jet stream (shown here in
idealized form), push maritime polar air masses from the Atlantic deep into the interior of
Europe. This is possible in part because south of Scotland, Europe lacks north-south-
oriented mountain ranges to confine the moist air to the coastal zones. As a result, most
of northwestern Europe is in a Marine West Coast climate (Cfb). The effects of this moist
air fade farther eastward across the continent, allowing continental air masses to exert
more control. As a result, countries of eastern Europe, like Belarus and Ukraine,
experience Humid Continental climates (Dfb). The east-west oriented Alps often prevent
Arctic and polar air masses from reaching southern Europe. Nothing prevents such air
masses from penetrating farther southward in eastern Europe, so the Humid Continental
climates extend farther south in this region. Mid-latitude cyclones that occur over these
continental-climate areas produce less precipitation than might be expected because of
the increased distance from the source of moisture (i.e., the Atlantic Ocean and
Mediterranean Sea).
Subpolar and adjacent climates occur at very high latitudes in both the Northern
and Southern Hemispheres. Climates of polar regions, assigned to Group E in the Köppen
system, are extremely cold and are subdivided into Ice Cap (EF) and Tundra climates
(ET). Farther from the pole, the Tundra climate gives way to other Group D climates —
the Subarctic climate (Dfc, Dfd), and Subarctic Monsoon (Dwc, Dwd) climate. The
Subarctic and Group E climates are characterized by frigid temperatures and low
precipitation totals. The Subarctic climate (Dfc and Dfd) is located in a band away from
the poles. The line separating it from more polar climates, the 10°C (50°F) isotherm in
the warmest summer month, separates a climate that provides enough energy for trees to
survive from one that does not. This boundary, called the tree line, is visible from
satellites. The Subarctic climate occurs farther south at high elevations. A related
subarctic climate, the Subarctic Monsoon climate (Dwc and Dwd) in Asia, receives even
less winter precipitation than places with a Subarctic climate.
The polar (Group E) climates, Ice Cap (EF) and Tundra (ET), are centered around
both poles (although only the North Pole is shown on this globe). In the Northern
Hemisphere, the Ice Cap climate is restricted to Greenland, but the Tundra climate covers
the Arctic Ocean, high Arctic islands, parts of the North Atlantic, and northernmost North
America, Asia, and Europe. Antarctica is mostly covered by ice and snow and so
possesses an Ice Cap climate. The surrounding ocean is classified as a Subarctic climate,
and true tundra (polar vegetation with no trees) occurs in some islands and other ice-free
lands.
Polar and adjacent Subarctic climates are characterized by low precipitation,
except locally. This is largely because these are very cold places, as depicted on this
globe of average annual temperatures. Very low temperatures have a number of
implications for precipitation. Very cold air has a low water-vapor capacity, so cold air
can carry only limited amounts of water vapor. Without much water vapor, it is difficult
to form clouds and even more difficult to generate precipitation. Also, much energy goes
into latent heat during melting of ice and thawing of upper parts of the soil during the
summer, so the surface doesn’t heat up enough to generate convective precipitation.
For most of the year, subarctic and polar regions lie far from the zone where
warm and cold air masses meet, so frontal precipitation is minimal. Tropical cyclones can
never penetrate far enough poleward and inland to influence Subarctic, Subarctic
Monsoon, and polar climates. Another limiting factor is that much of the region of
Subarctic and polar climates is inland, or near frozen seas. Humidity flowing into the area
must primarily originate over unfrozen seas away from the pole, but persistent high
pressure over the pole causes wind to generally blow away from the poles, driving any
moist air away. The high pressure and sinking air also limit atmospheric instability. The
most likely scenario to produce precipitation (especially over land) occurs in
summertime, when the boundary separating cold and warm air masses—the polar front—
makes its nearest approach to subarctic and polar latitudes. Marine areas often have
precipitation peaks in fall, as the seasonal warming is delayed over oceans.
E. Air Quality
Air Pollution consists of gases, liquids, and solids introduced into the atmosphere
by human activities and deemed to be detrimental to humans and other creatures, plants,
or other aspects of ecosystems. Most air pollution consists of noxious gases and liquids,
car exhaust, and smoke and soot from industrial activities and fires. Air pollution can be
in the form of molecules of gas, tiny drops of liquids, or solid particles that are small
enough to be lifted into the air. The table at the bottom of the page lists some common air
pollutants.
Much air pollution is in the form of gases, many of which are invisible. Some
gases combine with other chemical compounds to produce visible smog. Air pollution
can also be tiny drops of liquid, including those in visible steam and more noxious liquids
derived from sulfur dioxide (SO2 ) and other chemicals. Steam is white to gray, but other
chemicals and particles in the mix can turn the steam brown or black. Air pollution can
also consist of solid particles, including dust, whether it is from a dirt road or from the
coal being hauled by this truck. Tiny drops of liquids and solids in the atmosphere are
called aerosols, but not all aerosols are caused by human activities. Air pollution, whether
it is as a gas, liquid, or solid, can be introduced into the atmosphere in various ways. One
major source of air pollution is from automobile exhaust. Large quantities of air pollution
also arise from industrial activities, such as power-generating stations, factories,
petroleum refineries, wastewater treatment plants, and mining. Less industrial human
activities, such as burning forests and other vegetation, also introduce gases, especially
carbon dioxide (CO2 ) and solid particulates, into the air.
A family of atmospheric pollutants known as nitrogen oxides (NOx ) includes
NO2 (nitrites) and NO3 (nitrates). These pollutants can have numerous environmental
and epidemiological consequences once they enter the atmosphere, precipitate, and run
off from land surfaces. Automobile engines are the primary producers of NOx in the
atmosphere. This map of NOx concentrations by county shows that the largest
metropolises and the sprawling Sunbelt cities that grew during the automobile era have
the highest concentrations. Sulfur dioxide (SO2 ) enters the atmosphere primarily through
combustion of fossil fuels, particularly low-quality coal. SO2 causes respiratory
infections and diseases, and it damages vegetation and crops. In the 1990s and earlier, the
coal-burning industrial regions in the eastern U.S. had the highest SO2 concentrations in
the country. Smaller red and orange spots in the Southwest, such as on the borders of
Arizona, also mark the sites of coal-burning power plants, some of which have caused air
pollution in the Grand Canyon and nearby parks.
Many factors influence whether an area has relatively clear, pollution-free air or is
heavily polluted. Air pollution starts with a source of pollution, but whether the pollution
is dispersed (less concentrated but covering a larger area) or is concentrated, like in a
single valley, depends on many topographic and atmospheric factors. Source—Air
pollution comes from some type of source. If it comes from a single, relatively localized
site, such as from a single smokestack, it is a point source. If it comes from multiple
sources it is a non-point source. How concentrated pollution is depends on whether it is a
non-point or point source and how much pollution is actually emitted. Once air pollution
is in the air, it is subject to various processes in the atmosphere, especially wind and
vertical motion. Winds blow air pollution away from the source, perhaps clearing it from
one side of a valley but concentrating it elsewhere. Stable, sinking air, as during a
temperature inversion, can trap pollution close to the source and the ground, and a lack of
wind limits dispersal. The shape of Earth’s surface greatly influences winds close to the
ground, so wind can trap or disperse air pollution. Shorelines can also influence pollution
distribution, because water and land respond differently to heating, cooling, and winds.
Los Angeles, California, has both of these influences—high mountains that trap polluted
air in the basin, and a nearby shoreline with inland sea breezes.
The climate of a place is a combination of the regional atmospheric conditions,
such as those expressed by the Köppen climate classification, and local effects caused by
topography, local wind directions, pollution, and other factors. These result in local
variations, commonly called the local climate or the microclimate. The local climate is
greatly influenced by how urbanized the area is. The constructed features of cities and
towns influence how much insolation heats the surface, which in turn heats the air, and
how this heat is retained and released. Cities affect their local climate by being warmer
than rural areas, especially at night, a phenomenon known as the urban heat island (UHI).
Some emitted pollutants react with sunlight, water, or other contaminants to
create a new pollutant — a secondary pollutant — that can be more harmful than the
original. Climate and physical geography of Earth’s surface play a direct role in the
degree to which these pollutants are concentrated, but so do the spatial distribution of
population density and activities of people, which are part of human geography.
Sprawling cities and industrial areas are the largest sources of air pollution in the U.S.,
where air quality has improved steadily since passage of the original Clean Air Act in
1970.
Sunlight and volatile organic compounds can react with NOx to generate
secondary pollutants, including ozone (O3). The same O3 that protects us when it is in
the stratosphere is toxic near the ground. It eats away things, such as leaf cuticles and
human respiratory tissue. Sunny climates are the most susceptible to O3 pollution,
particularly in summer when the Sun angle is high and daylight hours are abundant.
Atmospheric circulation moves this O3 from one location and altitude to another.
Atmospheric water can react with SO2 or NOx to produce precipitation that is acidic (has
a low pH). This map of average pH of precipitation in the U.S. shows that humid climates
in industrial areas are particularly prone to acidic precipitation (acid rain).
When a city is first built or enlarges, changes in land use and associated human
construction replace whatever was there previously — natural lands, parks, undeveloped
lots, agricultural fields, or some other kind of open space. Buildings, concrete, sidewalks,
and glass windows have different albedos and thermal properties than the open space
they replaced. As a result, the urban area becomes warmer than the surrounding rural area
— the city is an urban heat island (UHI). This figure illustrates some ways in which a
UHI forms. Buildings and other constructed features interact in a complex way with
incoming shortwave radiation, generally decreasing the albedo in some places, like from
dark asphalt, and increasing it in others, such as from light-colored rooftops and
reflective glass. Buildings that are close to one another can change the albedo and cause
the radiation to reflect from one surface to another.
Building materials are designed to allow slow absorption of insolation during the
hot daytime to prevent heat from penetrating the interiors of homes and other inside
spaces. At night, when outdoor temperatures fall, the energy stored in the materials is
slowly released to the atmosphere as longwave radiation. The buildings can hinder the
escape of this longwave energy out to space, keeping the nighttime temperatures warmer.
Roads, gutters, and storm drains remove water from the surface quickly after rain falls,
limiting the amount of water that infiltrates the ground. This prevents water from being
evaporated and transpired at the site, and instead allows insolation to raise the
temperature of the ground, rather than being converted into latent heat during
evaporation. Also, urban areas have fewer tree-shaded areas and less standing water
compared with rural areas, so this further allows direct heating of the ground.
Urban areas also have a high concentration of waste heat from industrial,
domestic, and transportation sources, such as cars, furnaces, lighting, electrical devices of
all sorts, and, in colder climates, burning fireplaces. Even air conditioners generate more
heat than they remove, adding more waste heat to the local environment The UHI is
affected by the size of the city, amount and location of green space, and economic
activities. Whereas an urban area is influenced by all of the factors described above, less-
developed or more rural areas are relatively cooler overall, because they contain more
open space, natural or cultivated vegetation, and surface water. Surface water helps limit
warming because it results in more insolation being converted to latent heat rather than
warming the surface (sensible heat). This map of the conterminous United States ( ⊲)
shows how much warmer different urban areas are compared to adjacent, less developed
areas, such as grasslands, forests, and farms. The map was generated mostly using
satellite data, including the type of land cover (e.g., concrete and asphalt versus natural
vegetation). Can you identify what cities generate the largest UHI? Is there an urban heat
island in the place where you live or go to school? Can you feel a difference between
temperatures in the center of the city and those you experience when you travel just
outside a city?
Over the last 150 years, people have measured atmospheric temperatures. This
record, albeit short in geological terms, shows an overall increase in temperatures —
global warming. There is currently much scientific and political discussion of this topic.
What is the evidence that Earth’s climate is changing? Earth’s climate has been changing
since the planet formed 4.55 billion years ago, varying over timescales of decades to
millions of years. Climate change can include global trends in warming, cooling,
precipitation, wind directions, and other related measures. Global warming means
increasing global atmospheric and oceanic temperatures from some point in the past to
the present, usually compared to an arbitrary mean global temperature (e.g., averaged
from 1961 to 1990). Scientists examine various records of Earth’s climate to investigate
past changes. In addition to direct measurements, we infer past climatic conditions from
other types of observations, called proxy evidence.
Thermometers provide a direct measurement of air temperature. This record
shows an average variation in temperature for the last 140 years. According to this
record, it appears that average air temperatures have increased over the last century. From
the 1940s to the 1970s, the data show a relatively cool period. Another direct
measurement of temperature is sea-surface temperature (SST), which is collected from
buoys, ships, and more recently by satellites. Observe the SST graph below and then
compare it to the air-temperature data to the left. Glaciers flow from areas of snow
accumulation to lower elevations. The dynamics and energy flow of glacier movement
and retreat are well understood. Most scientists interpret changes in the lengths of
glaciers to be related to changes in atmospheric temperature and in the amount of
precipitation. The combined data from glaciers around the world are interpreted as a
warming trend beginning before the turn of the 20th century. Note that changes in glacier
length started around 1850, and much melting occurred prior to 1940. Recent warming
has been accompanied by another increase in the rate of melting. Continental glaciers on
Greenland and Antarctica have yearly layers that record winter precipitation and summer
dust accumulations. Scientists extract ice cores by drilling, and then they chemically
analyze the gases and ice of these cores in refrigerated laboratories. Air trapped in tiny
bubbles provides samples of the atmosphere (including CO2 concentrations) back to at
least 100,000 years ago. Oxygen and hydrogen isotopes in ice provide a proxy for
temperature.
We can use other types of proxies, such as sediments deposited in lakes and
oceans, minerals precipitated in caves, and tree rings. Tree-ring growth is partly
dependent on climate. Some trees can grow for 300 years or more, and the thickness of
tree rings of successive populations from temperature- and precipitation-sensitive forests
can provide a climate record going back hundreds or thousands of years. The National
Academy of Sciences (NAS) published the curves in the chart to the right as part of a
report to Congress. The report summarizes and compares the various temperature records
shown on earlier graphs and some reconstructions produced by combining multiple types
of proxies. The plot shows direct measurements, such as the temperature record, as well
as the various types of proxy data, each in a different color. The curves have been
smoothed to emphasize overall variations and show strong similarities in shape.
Comparing the different types of data strengthens the case that global warming has
occurred since the mid-1800s. The NAS concluded that Earth’s atmosphere has warmed
0.6 C° in the last 100 years. Increasingly, satellites are used to measure and monitor
changes in temperature and other expressions of climate with instruments designed to
measure different aspects of change. For example, some satellites measure temperature,
moisture, or cloud cover in the lower atmosphere, whereas others measure temperature,
abundance of different gases, and attributes of higher parts of the atmosphere, like the
stratosphere.
F. Climate Change
Climate change is always occurring, including global warming since the mid-
1800s. There are many natural causes of climate change, including changes in Earth’s
orbit around the Sun and changes in solar activity. Many scientists propose that human
activities, including the burning of fossil fuels and the clearing of forests, contribute to
climate change by releasing greenhouse gases to the atmosphere. Other factors may lead
to global cooling, including ash from large volcanic eruptions and an increase in certain
aerosols in the atmosphere. Here, we examine some of the factors that can influence our
climate.
Earth’s surface temperatures are dominated by energy from the Sun. Insolation
heats the oceans, land, and atmo sphere, but several factors influence how much of this
energy reaches the surface and how much is retained. Nearly all of Earth’s heating at the
surface comes from insolation, which heats the atmosphere, land, and oceans. Most of
this energy escapes eventually back into space in the form of longwave infrared energy.
The rest is delayed by interactions with Earth, keeping the planet warm by a process
called the greenhouse effect. The amount of insolation hitting Earth varies regularly, by a
small amount, due to orbital fluctuations and changes in the Sun’s energy output, as
expressed by changes in sunspot activity. Sunspots are the darker areas that, on average,
appear and disappear on the surface of the Sun in an 11-year solar cycle.
Some insolation is absorbed by the atmosphere (shown as an orange disk in the
figure), and some is reflected off the atmosphere. Much of the reflected insolation returns
to space without heating Earth or its atmosphere, as depicted by the blue arrows.
Insolation is absorbed by clouds, by soot from burning, and by fine particles (aerosols),
which are produced by volcanoes, industry, and automobiles. Some of this absorbed
energy radiates back into space as infrared (longwave) energy (shown by the wavy red
arrow). Clouds and particles also reflect some insolation.
Some insolation is reflected back to space from the land surface and oceans. Ice in
continental glaciers is an effective reflector. As glaciers melt, darker land or ocean is
uncovered. This increases the amount of energy that is absorbed by Earth and
subsequently re-radiated back to the atmosphere. Some insolation is absorbed by the land
and the oceans, both of which then radiate infrared (longwave) energy back into the
atmosphere. Some of this infrared energy is absorbed by atmospheric gases, such as
water vapor (H2O), carbon dioxide (CO2 ), methane (CH4 ), and nitrous oxide (N2O),
which are called greenhouse gases. Some portion of these gases is produced naturally and
some is produced by human activities.
Earth’s orbit is affected almost exclusively by the Sun and Moon, causing it to
cycle from more elliptical to more circular paths with one cycle completed over about
100,000 years. This influences Earth’s climate, but the current global warming has
occurred in less than 200 years. This is too short a time period for orbital changes to have
caused all of the observed warming. The tilt and direction of Earth’s axis of rotation also
change, which changes the contrast of the seasons, but these effects occur over tens of
thousands of years. The Sun is a candidate for causing climate change. Solar activity, as
presented by the number of sunspots, clearly correlates to earlier changes in temperature,
but less so to recent warming. Cosmogenic isotopes in ice cores and tree rings (proxies)
suggest that the Sun’s energy emission has varied by only 1% over the last 1,000 years.
Both data sets are relevant when evaluating the Sun’s role in recent warming.
. Several gases in Earth’s atmosphere absorb infrared radiation emitted by Earth.
This causes them to vibrate and heat up, and then to emit infrared radiation. This
radiation can escape into space or be absorbed by other greenhouse gases, mostly lower
in the atmosphere where the concentration of greenhouse gases is the highest. Since
1957, atmospheric scientists have collected air samples on the high peaks of Hawaii. The
data for Mauna Loa are plotted on this graph, which shows CO2 content in the
atmosphere as a function of time. The con centration of CO2 in air, represented by the red
line, has increased by 20% in the last 40 years. Most of this increase is attributed to
humans, especially through the burning of fossil fuels.
Our oceans are a huge reservoir for dissolved gases and other chemical
components, including CO2 . As the CO2 of our atmosphere increases, a large amount of
this goes into the oceans, where it affects seawater chemistry, especially the acidity. This
ocean acidification and its impact on sea life lead to concerns about increases in
atmospheric CO2 . Seawater can dissolve less CO2 when it is warm, so it releases CO2
back to the atmosphere as it warms, as is occurring now. So some increase in the
atmospheric CO2 could be a result of recent warming.
Climate change has many impacts, ranging from obvious ones like an increase in
global temperatures, to less obvious ones, such as a possible increase in malaria and other
diseases. Some of these consequences are highly probable, whereas others are very
speculative. On these pages, we briefly introduce some of the most likely results of
climate change, specifically those related to global warming. Abundant information is
available elsewhere, including reports by governmental and nongovernmental
organizations. The most detailed and widely cited reports are those by the United
Nations-sponsored Intergovernmental Panel on Climate Change (IPCC). The figure
below illustrates some consequences of global warming, but many more are possible.
Examine the figure and think about what you know about the feature being depicted and
how it might respond to an increase in average temperatures.
The most obvious result of global warming would be an increase in global
temperatures. Computer models of climate indicate that many areas, especially those near
the poles, will indeed increase in temperature, but other regions might get colder. Higher
temperatures would be predicted to cause more melting of snow and ice, resulting in
glaciers melting back and becoming less extensive, as has been observed. Higher
temperatures and altered ocean circulation could lead to more drought in some places and
even the expansion of desert areas, the process of desertification. Changes in global
temperatures and accompanying changes in precipitation patterns can affect the
distribution of communities of plants and animals. In response to warming, such
communities may shift to higher elevations or higher latitudes to stay within an optimal
temperature range. Climate change is predicted to decrease the geographic range of some
communities, while increasing the range of others. Included in this consideration are
croplands, some of which will benefit from global warming, while others will suffer.
An increase in global temperatures is predicted to increase evaporation from
warmer surface waters. This would increase both humidity and water-vapor capacity of
the air. Warmer temperatures and more humidity may make the atmosphere less stable
and increase precipitation. More precipitation, along with more melting of glaciers,
would lead to more runoff from streams, with an associated increase in the amount of
flooding. Increased runoff also means a larger influx of fresh water into the ocean.
Climate change has the potential to change the frequency and intensity of severe weather.
For various reasons, global warming can either increase or decrease the frequency of
certain types of severe storms. These are presented on the next page. Global warming is
predicted, and observed, to cause an increase in global sea-surface temperatures (SST)
and a decrease in the amount of sea ice in the Arctic.
The various consequences have complex interactions, as between SST, stability,
humidity, precipitation, runoff, and influx of fresh water into the ocean. Such interactions
could result in other changes, like a change in ocean currents, including those involved in
the thermohaline conveyor, an important moderator of global climate. The computer-
generated images below show the decrease in the amount of sea ice in the Arctic from
1979 (left globe) to 2015 (right globe), which encompasses a period of global and
regional warming. The amount of Arctic ice displays some year-to-year increases but
shows an overall decrease with time. Since the middle of the 1800s and before, sea level
has been slowly increasing, rising 0.2 meters in 200 years. Currently, there is scientific
debate about whether the rate of sea level rise is remaining the same or is accelerating
with time.
In 2005, an unprecedented 28 named tropical storms formed in the Atlantic, and
19 formed in 2012. Yet we have also had many quiet years in recent times. Patterns such
as these have led people to wonder whether we are moving into an era of more numerous
and destructive tropical cyclones, with the increase attributed to climate change. What
does the science say about these important questions? Observed increases in the
frequency of all forms of severe weather are at least partially the result of Doppler radar
and other improved remote-sensing technology, which allow us to detect storms more
easily. One approach is to compile data on severe storms over time, to examine whether
the frequency and severity of storms correlates with observations of climate change. But
remember that simple correlation does not demonstrate causation.
Observational and modeling studies suggest that climate change will increase
SST, which in turn might increase the intensity and duration of the strongest storms.
Warming might also lengthen the severe tropical cyclone season. In theory, increased
SST would allow an increase in evaporated water, which increases the amount of latent
heat in the atmosphere. Since latent-energy release—when the evaporated water
condenses or deposits—is a key ingredient for any type of severe weather, increased
evaporation can result in more energy for storms. The end result is larger, taller clouds
fueling more powerful storms. Warmer sea and land surfaces are also likely to affect
atmospheric stability. Recall that with warmer air near the surface, the environmental
lapse rate will steepen. A steep environmental lapse rate enhances the ability of air to
rise, increasing the instability of the atmosphere. In other words, warmer air near the
surface will contribute to taller clouds as long as moisture is present. Global warming
could cause changes to upper-level circulation, which in turn could cause underlying
areas to experience more frequent precipitation or drought. Changes in upper-level
circulation patterns might make the upper levels of the atmosphere more or less favorable
for cyclone formation. Specifically, if climate change displaces or decreases the flow of
upper-level westerlies, fewer developing tropical cyclones may have their tops sheared
off as they move westward. Alternatively, global warming could increase vertical wind
shear, which might cause mid-latitude storms to be more severe. What do the data say?
The graph below plots the number of Atlantic storms for each year. The time
periods represented on the graph correspond to the interval of time when global
temperatures have demonstrably increased (although in detail, temperatures have gone up
and down during several approximately 30-year cycles). Research using these and similar
data suggests that short-lived Atlantic tropical cyclones may have become more frequent
over the years, but moderate-duration storms have not. The research on this question is
ongoing, with some studies predicting an increase in intensity, while others find a
decrease in intensity. Other studies acknowledge that changes in tropical cyclones under
climate change scenarios will be small in relation to other factors that create variability in
tropical cyclones, such as ENSO. Another question to evaluate is whether global
warming has caused an increase in the number of mid-latitude cyclones or tornadoes.
Some scientists have proposed that warming provides additional energy that might
enhance the environment for such severe storms. Examine the graph below, which plots
the number of tornadoes per year during the time when most warming occurred. The data
indicate that the number of strong-to-violent tornadoes in the U.S. has not increased over
time. One possible explanation is that increasing surface temperatures would presumably
be accompanied by increases in upper-level temperatures, leading to no net change in
stability, as the adiabatically rising air and the air around it have both warmed.
Computer models are used to investigate issues related to climate, like global
warming. The simplest climate models compute energy (temperature), water, or
momentum for a small area over a limited time. The most complicated models use
numerical methods and principles of physics to compute atmospheric conditions at many
vertical levels across the entire Earth, for a long period of time. These are called general
circulation models (GCMs). A key issue in climate modeling and prediction is
understanding the nature of positive and negative feedbacks in the climate system.
Among the most important — and complicated — aspects of global climate
models are feedbacks. A feedback is the way a system responds to a change in
conditions, which in turn acts to amplify or dampen that change. Most climate scientists
would agree that feedbacks are among the least understood parts of the global climate
system. These two figures were presented and discussed elsewhere, but they provide a
useful review of feedbacks in the context of modeling. The left figure illustrates that we
can think of natural phenomena as systems, containing matter and energy, with individual
parts that interact in some ways more than others. In studying and modeling a system, we
want to understand the inputs to some part of the system, in this case the input of
moisture into the atmosphere from the water and ice (shown by the upward arrows).
Then, we want to model, using physical principles, the response of this change to the
system, such as to increased precipitation. One way the system can respond to the
imposed changes is through positive and negative feedbacks. A positive feedback
amplifies those changes (causes more change in the same direction), while a negative
feedback drives the system in the opposite direction, undoing some or all of the changes.
In the figure shown here, a decrease in the amount of sunlight and an accompanying
decrease in temperature cause an increase in the amount of snow and ice, which due to its
high albedo reflects more insolation, causing even more cooling, a positive feedback.
GCMs consider feedbacks, but they are difficult to understand and model because
a chain reaction of effects may result from a single change. Shown below are some
important feedbacks modeled in modern GCMs. Some of these feedbacks are not as
straightforward, nor as well understood, as these simple figures imply. Polar permafrost
contains methane, a greenhouse gas. Warming the permafrost releases the methane,
which causes more warming by absorbing more outgoing longwave radiation. Warming
the oceans evaporates more water, the most abundant greenhouse gas. But more water
vapor should produce more low-level clouds, which reflect insolation. More water vapor
and more clouds, therefore, could cause cooling, a negative feedback.
Carbon dioxide (CO2 ) goes in and out of the oceans, which hold over 50 times
more CO2 than the atmosphere. Any CO2 that is emitted into the atmosphere but goes
into the oceans reduces the rate of atmospheric warming. If the oceans warm, however,
CO2 in the oceans may eventually be released back to the atmosphere through other
processes, leading to more warming, a positive feedback.
Global, energy, hydrologic systems, and biologic systems are interdependent,
linked by processes, like photosynthesis, evapotranspiration, and albedo, through a
system of complicated feedbacks. How does the climate stay at conditions favorable for
life, despite the fact that it is always changing? Alternatively, can the biosphere help
sustain a favorable environment through such self-regulating processes? This possibility
is demonstrated in an Earth-like planet populated only by black or white daisies — Daisy
World. Daisy World illustrates the connections between various components of the
climate, including life.
On this hypothetical planet, Daisy World, there are two kinds of daisies—white
daisies and black daisies. The black daisies have a low albedo, so they absorb more
insolation than do white ones. In hot or bright conditions, however, the dark color is a
disadvantage because the black daisies retain so much insolation that they overheat. The
white daisies reflect more insolation, and so they do better in hot environments, but not as
well in cold ones, where they do not get enough energy. There are no “daisy-eaters,” so
the relative survival rates of the two daisies depend only on how they interact with the
climate.
After observing the setup of Daisy World, examine the figure below, which
illustrates how energy, water, and the daisies interact to keep the climate within certain
bounds of temperature and water vapor. The letter and number on each arrow refer to text
adjacent to the figure. E arrows involve energy, W involves water, and B refers to the
biosphere (daisies). Refer to chapters 2, 3, and 4 to review key topics discussed below.
Variables in Daisy World — and on Earth — all interact with, and adjust to, one other.
Some adjustments are almost instantaneous on an annual scale, like changes in planetary
albedo, while other linkages, such as the daisy growth rate, take time to fully impact the
system. This diagram illustrates some of the ways in which Daisy World could evolve
through time, through positive and negative feedbacks between energy, water, and
biology. In many natural systems the variables are constantly adjusting to one another
because of time lags between the input and response.
The air above black daisies is always warmer than above white daisies because of
the black daisies’ lower albedo (25%) and ability to retain insolation compared to that of
white daisies (75%). Planetary temperatures tend toward those associated with the
prevailing daisy type (warmer over black daisies), although some of the planet is covered
by bare ground. As temperatures above each type of daisy approach or diverge from the
optimum (22.5°C here), growth rates increase or decrease. Mortality rates of all daisies
remain fixed, thus proportions of the planet covered by each daisy do not change
instantaneously. However, there is an accelerated changeover from white to black daisies
as higher temperature above black daisies approaches the optimum, while temperatures
above white daisies are getting colder, diverging from the optimum. Higher proportions
of black daisies reduce planetary albedo and rapidly warm the planet.
Suppose that an area has a higher proportion of black daisies. What do you think
will happen to temperature and atmospheric moisture in this scenario? The black daisies
have a lower albedo, so they reflect less insolation back to space and retain more of the
energy, heating up the surface. If water is available near the surface, a warmer planet may
lead to more evaporation, more water vapor in the air, more clouds, a more active
hydrologic cycle, and a higher proportion of the planet covered in clouds. Greater
cloudiness increases planetary albedo and reduces the insolation reaching the daisies.
However, water vapor is a greenhouse gas, thus the presence of additional cloud cover,
while increasing global albedo, also restricts cooling from the loss of outgoing longwave
radiation.
Eventually, the planet may get too hot for the heat-absorbing black daisies, so
more black daisies die, causing the planet to cool, which then favors the white daisies.
Suppose instead that an area has a higher proportion of white daisies. Predict what will
happen to temperatures and atmospheric moisture in this scenario. The white daisies have
a higher albedo, so they reflect more insolation back to space, retaining less of the
energy. This causes the surface to cool. A cooler planet has less evaporation, less water
vapor, fewer clouds, and a less active hydrologic cycle. Less of the planet is covered in
clouds, and there is less heat-trapping greenhouse gas. The proportion of white daisies
continues to grow. Following the peak abundance of white daisies, the high surface
albedo (white daisies) and other factors cause a decrease in temperatures, decreasing the
abundance of warm-adapted white daisies. At this point, the growth rate of white daisies
declines at an ever-increasing rate, while that of the black daisies grows. A repetition of
the cycle has then been initiated. Does Earth function in a similar way, with positive and
negative feedbacks that keep the systems within certain bounds? We hope so!
