Science
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Nov18_climate_and_change.pptxClimate
Weather averaged over a very long period of time. Typically 30 years.
Includes temperature and precipitation (rain and/or snow).
Sometimes can include humidity and wind.
Not the same as weather on a given day.
Weather forecasting can predict weather for up to a week (usually)
Climate forecasting tries to predict what the average weather will be 50-100 years from now.
A very different process.
Before we do climate change, we have to do present day climate
Weather vs. Climate: Terminology
It usually snows 4 or 5 times in a winter in MD. Last year it snowed twice
DC summers are always hot and sticky, but last summer it rained almost every day.
This October was the warmest October in DC history.
Climate words: “usually” or “always” or “generally” or “you can expect” or “as a rule”
Weather is what you actually get
A trickier example: Since 1990, we’ve been getting milder winters and hotter summers in DC
that we would expect. Weather or Climate?
It might be climate change because we’re talking about a longer period of time
Climate weather
Climate weather
Weather
How to classify climate: Direct vs. Indirect Rays
Lowest latitudes (between 23.5 degrees N and S) always receives direct rays Tropical
What is Maryland?
Between 23.5 and 66 degrees: sometimes receive direct rays Temperate (distinct seasons)
Polar regions never receive direct rays Polar
temperate
Step 2 in classifying climates
Temperate climates further break out into 2 categories
mild winters: subtropics (average winter temperature above freezing)
cold winters: continental (average winter temperature below freezing)
What is Maryland?
Final Step: Precipitation: wet or dry.
Areas of the world with too little rainfall for trees.
Arid climates
Mostly Subtropical- except for near Pennsylvania border
Other subtropical cities: Atlanta, Raleigh, Houston. Miami is tropical. NY City is continental, Philly is borderline
Summary of 5 basic climate categories
Tropical no winters (only Miami comes close to this)
Temperate subtropical/mild winters (southern US, including DC)
Temperate continental/cold winters (northern US, most of Canada)
Polar no summers (parts of northern Canada, Alaska)
Arid usually warm, little rain (worlds great deserts)
Boundaries are determined based on specific temperature/rainfall limits and plant growth.
To be “arid”, usually too dry for trees
To be “polar”, too cold for trees
Four Factors that influence temperature
Latitude: The further from the equator, the more indirect rays and colder.
DC vs. Boston
2. Altitude: Temperatures decrease with increasing altitude in the troposphere. Higher elevations
are colder. Baltimore vs. Garrett County in western MD (they’re at the same latitude)
3. Distance from large bodies of water.
Water moderates climate. Doesn’t warm up as much in summer.
Doesn’t cool down as much in winter. San Francisco vs. St. Louis (at same lat and altitude)
4. Ocean currents. We haven’t talked about this yet.
Surface Ocean Currents
Oceans also travel in circles. Blown by the wind. A circular pattern in the ocean water motion is called a gyre.
This high pressure system
causes the N. Atlantic gyre
The North Atlantic gyre
Note the differences in temperature; red is warm water
blue is cold. Where are the beaches warmer: DC or San Francisco?
Gulf Stream. Western part of the North Atlantic gyre, driven by prevailing winds
Red is warm water
Blue is cold
This is why the oceans in
NC snd SC are so warm
(w/ palm trees)compared
with DE and MD
This current also slowed up
colonial ships sailing to NY
from England- Ben Franklin
figured it out by making
temperature measurements.
Direction of
current
Ocean currents (Gulf Stream- part of N. Atlantic gyre) keep Europe’s climate warmer than it should be for its latitude
London falls in the temperate subtropical category (category #2),
Canada at the same latitude is continental cold winter (category #3)
50oN latitude
Halifax
Polar front, rising air wet
Descending air Dry
Descending air Dry
Tropical Convection Wet
Global wind patterns and precipitation: alternate
Descending air Dry
Distribution of world’s arid climates
Generally not at the equator, but 20-30o latitude away where the subtropical high pressure zones are found.
Google Maps shows the zones w/ colors
Equatorial tropical rain forests
(dark green)
Subtropical arid high pressure
Zones (except where they
Get hurricanes/typhoons)
S. Hem. subtropical arid high
Pressure zones
Prevailing winds
What happens when the wind comes in off of the Pacific Ocean?
P
Montana
SD
Wind flow over mountains: Orographic Lifting orography dealing with mountains
Hawaii has some of the wettest and driest climates in the US because of its mountains.
Windward: wet California/Oregon mountains Leeward: dry Nevada/Utah/Idaho/Montana
Summary: Definitions learn distinction between weather and climate
Five general climate categories
Tropical
Temperate/subtropical
Temperate/continental
Polar
Arid
Four factors which control average temperatures
latitude, altitude, location near water, ocean/wind patterns
Four factors which lead to rising air precipitation
convection
forced upward at frontal boundary
at low pressure (cyclones)
forced over mountains- orographic lifting
Processes which control the current climate and past climate change
Tilt of Earth’s axis. Obliquity.
Brightness of the sun
Continental Drift
4. Magnitude of the greenhouse effect
1. First, remember our axis?
2.4 degrees, every 41,000 years. One major reason for ice ages!
The term is obliquity. Oblique means “high angle”
Changes in the tilt of earth’s axis linked with recent ice ages
And the more ice melts away in summer
Not relevant to present climate change- this takes many thousands of years (and we were getting cooler!)
2. The sun does change brightness…. But only a little
less than 1/10 of 1 percent.
And its gone in the opposite direction from
global temperatures in the last 60 years.
Sun has gotten a little dimmer recently!!
Atmospheric temperatures have been only up for
over a 100 years
3. Continental Drift in the Cenozoic Era #1
Tropical waters of Pacific can flow into Atlantic
and keep Atlantic hotter than at present. When N. America
connects to S. America this warming effect stops
cooler
Warm water
blocked
3. Continental Drift in the Cenozoic Era #2
.
India crashes into Asia makes Himalayas which block warm
south winds Northern Asia gets colder
colder
Warm winds
blocked
Combined effects: Winters in N. Atlantic and Northern Asia get
much colder snow and ice can build up
Note: this took millions of years!! Irrelevant to our present day changing climate!!!
4. The Greenhouse Effect: A natural phenomenon
What are “greenhouse gases”?
Its like a blanket preventing heat escape
Greenhouse Gases: only a tiny fraction of our atmosphere
Effective greenhouse gases absorb heat (infrared radiation). Have at least 3 atoms.
H2O
CO2
CH4 (methane)
Ineffective greenhouse gases are transparent to heat. Only have 2 atoms
O2
N2
Earth’s atmosphere is 99% composed of gas that is transparent to heat!
Our greenhouse effect is from the remaining 1%.
But that 1% keeps us from having mostly ice ages. Greenhouse effect is useful for life.
A tale of two planets
Why? (not because its closer to the sun shouldn’t make that much diff.)
A tale of two planets
Venus’ atmosphere is all CO2 and thicker than ours runaway greenhouse!
The greenhouse effect is old physics: 1859
Actually, maybe older. Tyndall may have based his work on this paper from 1856
Human emissions of carbon turn into CO2
Temperatures have increased by 1.0-1.5oC (like 2oF) since 1900
Average over 130 years
Colder than the 130 average
Warmer than the 130 average
Present day
The change of CO2 over the past 800,000 years
Note: subtle point…. In recent years, CO2 drives the temperature. But during the big ice ages
the cause and effect is reversed….. Cold temperatures cause lower CO2
This is why you need computer models to sort these cause effect relationships
Its more than just increasing surface temperatures
One slightly hopeful trend: Coal use is decreasing
Coal is one of the main sources of carbon- eliminating coal will be good progress to slowing the rise of CO2
U.S. has led the way in
using less coal! We use
40% less than 8 years ago!
Summary: Responses to counter-arguments
Greenhouse effect not real
Has been studied for over a 100 years, both on Earth and on Venus (and Mars). A natural effect that
helps control temperature on a planet
Climate always changes naturally, so what?
Natural changes, like continental drift or earth’s obliquity only occur over thousands or millions of years.
Yes, the sun can change brightness, but only a very small fraction of a percent- not enough to cause the changes
that we see. And in the last 50-60 years its going the opposite way!
How do you know its human activities?
Carbon dioxide concentrations haven’t been this high for more than a million years. Steep increases started after
Industrial Revolution and invention of engines. Industrial output and atmospheric CO2 track very well.
The temperature data are unreliable
Many other indicators of earth’s atmosphere, ocean and surface are consistent.
Glaciers all around the world are shrinking. Sea levels are rising.
