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soils in many locations, including a potential 10 to 30 percent decline
in summer soil moisture over much of America.
 There is some evidence that precipitation severity has increased somewhat
during this century. The cloud record is less certain. But greater cloud
cover could explain why warming seems to have increased most at nighttime
and winter.
During the day, clouds keep the land cooler by shading the surface,
while at night they form a barrier that confines heat close to the surface.
Many experts believe that even a few degrees would have a dramatic effect
on the intensity of heat waves of the sort that killed more than 400 people
in Chicago in 1995. It also would jack up the heat index, which combines
heat and humidity.
According to the National Oceanic and Atmospheric Administration (NOAA),
downtown Washington "currently has an average July heat index of 85°F,
but if CO2 levels reach 550 ppm [from the present 360 parts per million]this
could increase to 95°F." Few in the area would wish July days to
feel 10 degrees hotter.
Q: How much hotter will
it become_
A: That ultimately depends
on how much more waste gas we dump into the atmosphere, as well as whatever
natural changes the planet is undergoing. Many scientists believe that Earth
alternates between hot times and ice ages over periods of tens of thousands
of years, owing to periodic changes in the tilt of the planet's axis, which
affects the amount of sunlight that strikes higher latitudes.
But no matter what happens, we're very probably facing a century of
warming. According to the Intergovernmental Panel on Climate Change (IPCC),
a United Nations-sponsored group comprising about 2,000 scientists from
dozens of countries, average global surface temperatures will rise between
1°C and 3.5°C (about 2 to 7°F) by the year 2100 unless very
appreciable reductions are made in greenhouse-gas output within the next
few decades.
There's a long lag time because some gases, such as CO2, can endure
for 100 or more years in the air once in circulation, as well as other reasons
involving the physics of oceans.
By comparison, Earth has warmed about 5°C since the last ice age
ended about 10,000 years ago. Thus the .5°C warming this century has
occurred about 10 times as fast.
Most of the increase is expected to occur between 40 degrees north,
the latitude of Philadelphia, and 70 degrees, the northernmost extremes
of Canada and Siberia. That is where most of the surface warming has taken
place in the 20th century.
Q: How much have green-house
gases increased_
A: About 1750, when the industrial
revolution was starting to hit its stride, the air contained carbon dioxide
at about 280 parts per million by volume. Now it's near 360 ppm, an increase
of about 30 percent. The IPCC estimates that, even if annual emissions remain
constant for 100 years at 1994 levels, CO2 concentrations still would reach
500 ppm by 2100, doubling the pre-industrial level. It could be as high
as 700 ppm.
By contrast, analyses of air trapped deep in ice sheets indicate that, during
the last glacier heyday about 20,000 years ago, CO2 concentrations were
about 40 percent below the preindustrial level.
Methane levels have more than doubled since pre-industrial times. Nitrous
oxide (N2O) concentrations have increased about 8 percent. And all of the
halocarbons have been added in the last few decades.
Increased CO2, scientists believe, is responsible for about 60 percent of
the enhanced greenhouse warming detected so far.
Because the effect of CO2 is so large, most current concern focuses on it.
Humanity puts about 7 billion tons (Gt) of carbon into the atmosphere annually:
5 to 6 Gt from burning fossil fuels, producing cement and other gassy activities
and another 1 to 2 Gt from slash-and-burn deforestation. Since about 740
Gt already are in the air, that's an addition of about 1 percent each year,
though recent growth rates are higher.
Q: What are the greatest
areas of uncertainty_
A: When dealing with something
as inherently irregular as weather, practically everything is uncertain.
But three issues are particularly troublesome:
Clouds. Further greenhouse warming is expected to increase
the average amount of clouds, but its effect is unclear. Clouds may be involved
in a sort of planetary feedback mechanism that has kept us cooler or hotter
than we would have been.
In addition, some polluting aerosols emitted by industry-notably smoke
and sulfur compounds that also contribute to acid rain-actually may make
clouds shinier. The more aerosols they carry, the greater their reflectiveness
and the less light they let through. According to NOAA, aerosols may have
offset about 20 percent of enhanced greenhouse warming that would have occurred.
Plants. Terrestrial vegetation makes its living by pulling
carbon dioxide from the air and turning it into "biomass," thus
serving as a carbon sink. There is some indication that the more CO2 in
the air, the faster plants will grow, at least to a point. This fertilizing
effect, seen in many commercial greenhouses that keep CO2 levels at 600
parts per million or higher, is being studied intensively, as is the question
of how long such a bio-boom might last. Nor is it clear how
global warming might affect food production. Few experts think that total
worldwide yield would decrease. But yield could increase as much as 40 percent
in some areas and decrease by the same amount in others.
Ice. Although many glaciers certainly are melting, the
Greenland and Antarctic ice masses appear fairly stable. If they were to
begin melting in earnest, the result would be catastrophic, including sea-level
changes measured in yards.
Q: Would global warming affect
sea level_
A: Yes, and that may be underway.
During this century, sea level has risen about 4 to 10 inches, probably
in part from melting of glaciers, many of which are noticeably losing weight.
But thermal expansion probably is the primary cause. The ocean, like most
things, swells when heated. Whatever the reason, sea level has been rising
at about 2 millimeters (about 0.08 inches) a year recently.
Is greater trouble on the way by 2100_ The IPCC projects a sea-level
rise of about 50 centimeters (20 inches), with a range of 15 cm to 90 cm.
The mid-range 50-cm estimate would leave about 92 million people at considerable
risk of flooding every year.
The top end would require redrawing a lot of maps. For example, a sea-level
rise of 1 meter (39.37 inches) would put 6 percent of the Netherlands and
17.5 percent of Bangladesh under water. Not surprisingly, lowland nations
are profoundly worried about greenhouse warming. [TOP]
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Q: What are the principal greenhouse
gases, and what are they doing in the air_
 A:
Most infrared trapping is due to water vapor.
It may seem that people have little control over its concentration in the
atmosphere. But if the planet warms considerably because of increases in
man-made greenhouse gases, even more water vapor could enter the atmosphere,
leading to yet more warming in a "feedback" effect.
Next in importance is carbon dioxide, copiously produced by the burning
of fossil fuels and wood, as well as by thousands of natural processes including
earthworm and termite digestion, decomposition, oceanic emission and respiration
of living things.
Both water vapor and CO2 absorb at many IR wavelengths between 3 and
30 microns, but neither absorbs much between 8 and 12 microns, which accounts
for a great deal of the Earth's surface radiation. That potential escape
route, however, is plugged by another class of greenhouse gases that is
highly absorbing in the 8-to-12 micron range, most notably methane, nitrous
oxide, and halocarbons, which include chlorofluorocarbons (CFCs) and their
relatives.
Methane is emitted by termites, landfills, wetlands, rice paddies, large-animal
digestive systems (ours included), coal mining, natural gas leaks, and vegetation
burning in the tropics, among other causes. Per molecule, it's about 60
times more effective at absorbing Earth's surface radiation than is CO2.
Nitrous oxide is produced by soils, fertilizer application, combustion,
and various industrial processes. Per molecule, it's about 270 times as
effective as CO2 at trapping IR.
Only halocarbons have no natural source. CFCs were widely used as refrigerants
and in making plastic foams before an international treaty called the Montreal
Protocol constrained their production in 1987. The treaty does not affect
many of their chemical kin and replacement compounds known as "HFCs"
and "HCFCs." Some halocarbons are thousands of times more effective
per molecule at trapping heat than CO2.
|
| Q: Why is there so much disagreement
among climate scientists_ |
A: Actually,
there is surprisingly little disagreement. A few touchy questions are hotly
debated. For example, ground-based and ocean-surface sensors show consistent
warming over the last 20 years or so. But satellites that monitor temperature
in the lower air, as well as data from balloons, indicate a very slight
cooling trend. This debate, long underway, will continue.
But much of what may look like disagreement actually is a matter of
the daunting uncertainties involved. Some projections can be done by simply
extrapolating trends (such as rainfall changes over the last century) into
the future. But major climate predictions come from computer simulations
of the atmosphere. |
Chis Calwell
A high-altitude glacier melts in the Canadian
Rockies: scientists see such-melt downs as an early warning signal of climate
change.
|
These models have proven quite good at "predicting" past weather
patterns from initial conditions, the only way to test their accuracy. But
their conclusions depend critically on uncertain assumptions, such as how
oceans interact with the atmosphere and how rising temperatures affect cloud
behavior.
Even more difficult is the fact that some hugely important weather phenomena,
such as the shading action of clouds, can't be simulated well in big climate
models. Most divide the atmosphere into squares larger than 100 miles on
a side. Nearly all cloud formations occur on smaller scales.
Finally, some htmlects of future climate are ambiguous, especially in
localized regions. |
 "A warmer global
climate will dramatically alter America's wilderness. The Grand Canyon,
Yosemite, Yellowstone and many other national treasures could cease to be
the wild places we know and love.
As the climate continues to heat up, forests may be reduced to pastures,
pastures to desert, and desert to lifeless places of sand and rock. Coastal
treasures such as the Everglades will be flooded by ocean waters, while
Arctic tundra may become forested bogs.
Clean drinking water, precious wildlife habitat, places to hike, places
we love could be lost forever.
Yet there is another choice. By acting today, we can leave a legacy that
guarantees the children of the next millennium the right to experience the
wonder and beauty of America's wilderness."
William H. Meadows
President, The Wilderness Society |
|
