What is the difference between weather and climate?
Weather is a short term variation in the atmosphere as measured in part by winds, temperature, amount of sunshine, storms, and precipitation.
Climate is the long term average of
weather patterns. We say that Arizona has a dry climate even though in
some years there is enough rain to produce floods. Florida has a mild winter
climate even though in some years the oranges are threatened by freezing
weather. (Hartmann, 1997)
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How has past climate warming affected human life on Earth?
Some examples of past climate warming include the extinction of many large mammals at the end of the Pleistocene Period when the last ice sheets melted. While these extinctions may have been seriously affected by human hunting, the changing climate may have had an effect as well.
As the last Ice Age ended many large lakes dried up. The Great Salt Lake was the size of the current Lake Michigan. Death Valley and many other western valleys were filled with lakes 50 miles long and 20 to 30 miles wide. All that remains are large evaporite deposits.
Cold adapted organisms shifted their ranges farther north. Spruce trees have moved 1000 miles north during this interglacial. Yellowstone Park personnel have noticed that this process is already underway as plants are dying out in southern part of the park and doing better farther north, indicating the process is now underway.
A small warming period ~1200 AD produced a series of droughts in the American Southwest ( one of which lasted 33 years, according to tree ring data) which continued until the Anasazi gave up their cliff dwellings about 1275 AD.
Between 1934 and 1937 a very short
warming period produced such severe drought in the lower great plains states
(The Dust Bowl years) that people were driven from their land and homes.
(Hartmann, 1997) (Schmidt, 2001)
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How do the sunspot cycles affect our weather?
We are now at the peak of an 11 year
sunspot cycle. These regular periods of the solar cycle do affect the amount
of solar energy that reaches the Earth. We have seen magnetic storms which
have disrupted the electric grids in the Northeastern part of North America
and there is an effect on the world temperature. This temperature variation
however, is too small (about 0.1% greater when the sunspots peak) to have
a significant effect on global climate and it disappears within a year
as the cycle progresses. (Abers, 2001) (Anonymous, 1999) (Hartmann,1997) (Lean and Rind,
1996)
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Are other changes in the sun's radiation responsible for the warming we have experienced so far?
The sun's output is not steady. However, the changes scientists have measured with instruments are so small that it has been referred to as a "solar constant". They were unable to study the variability from Earth (since the atmosphere interfered) until we had satellites in space. Now our information about the sun's output has increased and we know the sun really isn't "constant". Scientists have only been able to observe the sun's variability this way for about 20 years and this is too short a time period for many conclusions other than the 11 year sunspot cycle.
However, there are other measurements
which depend on the amount of sunlight. We can look at 14C
found in tree rings and 10Be
found in polar ice to help us fill in previous time periods. From these
records we have been able to distinguish cyclic variations of about 2300,
210 and 88 years as well as the 11 year sunspot cycle. This reconstruction
suggests the sun has increased its output by about 0.25% in the past 300
years. During the "Maunder Minimum" in the 17th century (~1630 to 1700
AD) the solar output dropped by about the same 0.25%. From what we
know so far the sun will have a very limited effect on the climate
warming problem but it is an area in which we need to learn a lot more
to improve the accuracy of our predictions. (Lean and Rind, 1996)
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Why is Carbon Dioxide a problem? What are other greenhouse gasses?
The major greenhouse gasses are water vapor, carbon dioxide, methane and nitrous oxide. Each of these can be thought of as trapping some of the energy that the Earth would otherwise radiate back out into space. Since this energy doesn't escape it stays in the atmosphere near the Earth and helps to warm the planet.
The problem with Carbon Dioxide is how long it stays in the atmosphere. If water vapor is added today it will leave the atmosphere (probably as rain) within a few weeks. The methane and nitrous oxide also leave the atmosphere within relatively short periods of time. Some authors have suggested that it may take centuries to thousands of years for the carbon dioxide now being put into the atmosphere to be taken out. The major sinks (a term that refers to ways to remove CO 2 from the atmosphere) are plants, soil and the ocean. The more forests we cut down (for example the extensive cutting of the rain forests) and plants we pave over, the fewer there are to remove the excess carbon dioxide gas. The soil can take up some of this excess but whenever it is plowed it will release it to the atmosphere again. The ocean can absorb much more but it does so by chemical reactions and these operate on a very slow time table. It is estimated to take thousands of years for the ocean to come into chemical equilibrium with our current amount of Carbon Dioxide. Today's carbon dioxide levels are higher than they have been in the last 400,00 years
From ice core measurements and instrument
records on Mauna Loa, a mountain top in Hawaii (away from any city influences)
we know the amount of CO2 in
the atmosphere has risen from about 250 ppm (parts per million) to 360
ppm today. This rate of increase is steadily rising and is due primarily
to the burning of fossil fuels (wood, coal, petroleum and natural gas)
since the beginning of the industrial revolution(~1750 AD)
( Hartmann, 1997) (Hill,
2001)
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Couldn't we just plant more trees to solve the problem?
Trees, and plants in general, are the
easiest source of removal of carbon dioxide from the atmosphere. One problem,
however is that it doesn't stay removed. Each year as deciduous trees drop
their leaves their carbon dioxide is returned to the atmosphere in the
respiration of the scavengers who "decay" the fallen leaves. There would
be a short term improvement if more trees are planted but as these come
to the end of their lifetimes, fall, and begin the decay process their
carbon dioxide will also be returned to the atmosphere. There is not enough
permanent change to compensate for the predicted warming. Additionally, forest
fires, when they occur would return all of the carbon dioxide to the atmosphere
in a very short time. (Kasting, 1998) (Wetherald, 2001)
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What caused the Ice Ages?
In 1924 Milutin Milankovitch proposed
a cause for the Ice Ages which has received some more recent support from
scientific studies. He related the amount of insolation (solar energy)
received at the higher latitudes to the advance and retreat of the ice
sheets. To account for these changes in the sun's output he defined cycles
in the Earth's orbit which would affect the amount of energy received at
the surface:
| 1) A 23,000
year cycle - this cycle relates to the day of the year
in which the Earth is closest to the sun. This is now January 3rd. When
this day is in the northern summer it will ensure more melting and reduce
the possibility that ice will melt during the summer.
2) A 41,000 year cycle - this is the cycle involving the tilt of the Earth on its axis. This tilt varies from 22º - 24.5º. We are currently at 23.5º. The greater the tilt the more sunlight will reach the higher latitudes. 3) A 100,000 year cycle, and
a 400,000 year cycle - variations in the shape of the Earth's
orbit away from a perfect circle.
|
When these cycles converge to reduce the amount of insolation received by the higher latitudes in the summer so that the snow does not melt from year to year, an ice age may begin. The last greatest advance of the ice, ~18,000 years ago occurred at the lowest point in summer insolation. Furthermore, the ending of the ice sheets coincided with the increasing summer insolation which was about 10,000 years ago.
While these cycles match the more recent
geologic events it does not account for some periods in the past,
such as the Mesozoic (from ~ 250 million years ago to ~65 million years
ago) when the climate was almost universally warm. This leads us to conclude
that there are still other factors which influence the ice
ages. ( Crowley, 1996) (Hartmann, 1997)
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Since we are now in an interglacial period, shouldn't we expect this kind of warming as a natural event?
In the past 500,000 years ice ages have occurred four times. Each ice age was followed by a period of rapid (geologically speaking) warming. As the ice caps melted the sea level rose and climatic belts moved to the north - just as we are observing today.
The global average temperature differences between ice age climates and interglacial periods is ~ 5º C. This change normally occurs over a period of ~ 25,000 years or 1º C every 5,000 years. (We know this slow change was not a simple linear relationship. There have been periods of up to 500 years that were a little warmer ( the Medieval Optimum, ~ 750 AD - 1150 AD) and a little cooler ( the little ice age from ~ 1350 AD to 1850 AD). In these cases however, the adjustment was less than 1o C and this occurred over several centuries. What concerns many atmospheric scientists about the current changes are 1) the RATE of current change and 2) the amount of committed warming already in the system.
The .5º C rise we saw in the last century should have taken 2,500 years. The rate of change is continuing to rise. Many wonder if the rapid rate of these changes will allow ecosystems time to adapt or if widespread extinctions might result.
The second part of the concern involves
"committed warming". This refers to the fact that the atmospheric response
to the increasing CO2 lags behind
the measured amount. Carbon dioxide can remain in the atmosphere more than
a century once it is released from its storage in fuels, plants or rock
(making Portland cement releases a ton of CO 2
for every ton of cement made). Also, due to the great heat capacity of
the ocean, it is very slow to respond to temperature changes. Current computer
models suggest that if we stopped all additional emissions of CO2
right
now, it will take about 200 years or more for the atmosphere/ocean system to reach
a new equilibrium and that this new temperature would average 1º C
higher. However, this rise of 1additional degree presumes that all emissions of
carbon dioxide stop right now. For each day that goes by the future warming
increases. (Barron, 1995) (Crowley, 1996)
(Hartmann, 1997) (Wetherald,2001)
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How do the computer models which predict global warming work?
A model of this type is called a General Circulation Model (GCM). Different Universities and governments have developed their own model to help with their research. Essentially what each model does is divide the Earth into a series of boxes and then use the mathematics, chemistry and physics of the atmosphere to figure out what is happening in that box. As the results are calculated for each box the model adjusts the neighboring boxes to determine the effects as time goes forward. More recent advances have included a coupling of computer models of the ocean as well as the atmosphere.
There are several problems which limit
the accuracy of the models. The first is that the computers can only work
with those actions in the atmosphere and ocean which are currently known
- and there is a lot we still don't know. One of the areas which is poorly
understood is the behavior (mathematically) of clouds and precipitation. How
each research team develops an algorithm to handle this problem probably creates
most of the difference in the results of each model. The second problem is that the
system is so complex that our most advanced, parallel programmed, super
computers cannot handle the problem within the budgets of time that researchers
are allotted unless they restrict the number of boxes (currently they are
about 350 miles on each side - all of New England is in one box) and consider
only the most significant processes. At this current scale it takes
about 90 hours of continuous running to make one calculation. To use a
finer grid would allow you to include more local topography and conditions
but it would tie up the biggest super computer for more than a month to
make the calculations. (Barton, 1995) (Hartmann, 1997) (Wetherald, 2001)
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Do all of the computer models agree with each other?
No, each model has been developed by a group of researchers to be used on the problems in which they are interested and within the budget and type of computers they have to use. These groups, however, do all talk to each other and share new developments. Each model may divide its "boxes" a little differently, make different assumptions for starting conditions and use slightly different time intervals between each advance in the calculations. This will produce slightly different results. The amazing thing is that they all are very close to each other. (Barton, 1995) (Spahr, 2001) (Wetherald, 2001) ]
This figure represents the sum of seven different General Circulation Models. Each model may have a different number for a specific time but the general trend is consistent from model to model (from Barton,1995)
How do we know that anything the computer predicts might occur if we don't know all of the processes at work in the atmosphere and ocean?
Each of the computer models is tested against past weather conditions to see if it will predict the conditions we know have already happened. Models are also compared to one another and while a specific part of the outcome (a given year or place) might not match, the general trends are in the same direction.
Some parts of the problem are known with greater certainty than others. Cooling in the stratosphere (from about 6 to 30 miles about the earth) due to ozone loss is now being observed just as it was predicted. The increase in Earth's surface temperature will continue to be above normal background levels but the amount of this rise varies among the models. Each group of scientists makes an assumption about how much more carbon dioxide will be added to the atmosphere( increasing the temperature). Also, how much sulfate aerosols (acid rain) will be there to cool it. Some models suggest that we will work to eliminate the acid rain and then the warming will increase. Other models suggest that the level will be the same as now and the temperature rise will not be as high.
Some events, like volcanic eruptions, are not included
in the models at all and these do have a cooling effect that is often felt
for several years. This may suggest the model is wrong until the effect
wears off and the warming then catches up with the predictions. This happened
when Mt. Pinatubo erupted in 1991. The models were predicting increasing
warming and for several years it was actually cooler. Then the sulfur aerosols
were washed out of the atmosphere and the end of the decade was the warmest
recorded. However, the eruption of Mt. Pinatubo has been used in
retrospect to further fine tune various models. After the eruptions scientists
used the amount of material that was erupted by the volcano in their models and
checked to see if they would "predict" what really happened. Most came
very close and the differences are used to further fine-tune the model. (Barron, 1995) (Hartmann, 1997) (Wetherald,
2001)
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According to the computer models, what would happen in North America if the amount of CO2 in the atmosphere doubles from its pre-industrial revolution levels?
Five major effects have been identified which would result from this change.
First, the average surface temperature of Earth would rise. This rise will be greater at the poles (high latitudes) than in the equatorial regions. In the polar regions the melting snow and sea ice makes the surface darker and would reflect less sunlight back into space. This darker earth and sea will absorb more heat and this will in turn cause more melting then more heat absorption. This process is called the snowcover/albedo feedback mechanism. Higher latitude regions are modeled to rise about 9º C and the tropics about 2º C. At the latitude (30º - 50º N) of the United States warming of ~ 3º C is anticipated.
Second, in the mid-latitudes (the U.S.) changes in soil moisture would be equivalent to the loss of up to 5 cm. of rainfall, with the American Mid-west drying out the most. The current rain patterns will move further north often missing the Plains states. Less precipitation will fall as snow. Since the snow melts slower it soaks into the ground in greater amounts. When the winter does not produce as much snow (the precipitation falls as rain instead), runoff increases and water is sent through rivers to the sea instead of into the future groundwater supply. It also means that the soil will begin drying earlier in the year putting great stress on agriculture. In the American West, water supplies are dependant on snowfall. Rain water runs off and must be passed through reservoirs and dams early in the season to prevent floods later. With the slower melting snows, the reservoir capacity needed can be anticipated and when they fill the extra allowed to pass through.
Third, agricultural production, especially of cereal grains, will be seriously affected as the climatic belts move north. Canada has great land masses and the forests could be cut down to accommodate grain production but the northern soils are not as rich as those of the American prairie. The great glaciers of the ice ages scoured these soils and greatly reduced their thickness. Cool weather, permafrost and more acidic soils have prevented the soil enhancing organisms from working their magic as well. Food production would probably suffer. However, some plants (the C3 ) may do better with the higher carbon dioxide levels if they can tolerate the changing moisture and temperature conditions. Other plants (ginko's have been used in measuring fossil carbon dioxide levels this way) have reduced the number of stomata on the underside of the leaf during times of high CO 2. This reduces the amount of moisture a plant loses, this would allow it to survive in drier conditions.
Fourth, The rise of mean sea level would affect the low lying, Atlantic Barrier Islands as well as the Gulf of Mexico coastline. Cities like New Orleans, which is already below sea level, would be increasingly threatened. In the past 100 years sea level has risen 10 - 25 cm. (4 - 10 inches). Since the ocean is so slow to respond to increasing temperature much of this change may be the result of the ending of the "Little Ice Age" about 1850. As water warms it occupies a greater volume - try gently heating a completely full pan, long before boiling it will overflow. Even without melting the continental ice masses, the warming sea will rise and encroach upon the land. If the glaciers melt as well, this rise will be increased.
Lastly, human and domestic animal health
may be adversely affected by the changing climatic conditions. Most insects
increase their activity in warmer weather. More generations are born and
fewer may be killed off if winter weather does not have many days below
freezing. Tropical diseases may spread north to parts of the United States
which have been free of these because the vectors could not survive long
in the present climate. (Hartmann, 1997) (Epstein, 1997)
(Kasting, 1998) (Rockwell, 1998) (Rosenzweig and Hillel, 1995) (Wetherald,
2001)
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