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| What remains uncertain are the precise effects
of a strengthened greenhouse effect on global temperatures.
Because there is still much to be learned about how the world’s
climate will react to increased greenhouse gas concentrations,
the range of possible climate futures projected by the IPCC
is an indication of uncertainty about how much the world will
warm over the coming century—not of whether that warming
is happening. |
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| Evidences
and Uncertainties |
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| Measurements from a variety of sources have
suggested that the earth’s average atmospheric temperature
has risen over the last several hundred years—but by how
much? Taking the average temperature of the earth’s atmosphere
is a very difficult measurement problem. First, measurements
must be taken in a large and diverse enough range of locations
to ensure that their average is truly a measure of global temperature
and is not biased toward one region or another. Second, those
locations must be chosen so that individual measurements are
not thrown off by sources of unusually high or low temperatures,
such as cities (which tend to be “heat islands” warmer
than the surrounding landscape). Third, no measuring device
is perfect—all measurements include some amount of error,
or “noise.” Understanding the kinds of errors associated
with different measurement techniques is a key element in evaluating
the accuracy of a given temperature value. In addition, the
study of climate requires measurements over very long time periods,
so sources of paleoclimate data (data on climate from the distant
past) are key to understanding climate change. (See “Global
Stratospheric and Tropospheric Temperature Anomalies (1979–2001)”
on this site to learn more about the problems of measurement.) |
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| How much will global atmospheric temperatures
change over the next century? Two kinds of problems make this
an exceptionally difficult question to answer. (See “Sample
Forecasts of Future Temperature Change” on this site
for some possible answers.) First, the enormous complexity of
the earth’s dynamic climate system—including the interacting
air masses, winds and, ocean currents, and patterns of evaporation
and precipitation—makes long-term climate prediction extremely
problematic. Estimates drawn from reports by the Intergovernmental
Panel on Climate Change (IPCC) project increases in average
global temperatures ranging from 1.4 degrees to 5.8 degrees
C by the year 2100. These numbers may seem small, but because
average global temperatures are actually remarkably stable over
long periods, this range actually represents a very significant
rise in the earth’s temperature over a very short time. |
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Slight changes in temperature may lead
to higher ozone levels near the earth’s surface. This could
significantly increase smog problems in large cities—bad
for all of us, but especially serious for many elderly, ill,
or otherwise physically vulnerable citizens.
Small increases in atmospheric temperatures could also change
the way clouds form and dissipate. Warmer temperatures near
the ground could cause lower clouds to evaporate, letting heat
rise farther into the atmosphere.
As this heated air rises and cools, higher clouds form. But
lower clouds usually reflect sunlight back into space while
higher clouds tend to absorb more heat. More high clouds mean
more heat trapped near the earth’s surface—so small
increases in temperature could set off a cycle in which the
atmosphere holds more and more heat over time.
(This is an example of a positive feedback loop—a system
in which small changes in one direction may set the stage for
later, larger changes in the same direction. But we don’t
yet know whether positive feedback loops like this will dominate
future climate, or whether other factors will prevent patterns
like this from unfolding. ) |
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glossary
terms
