Global surface temperature has been measured since 1880 at a network of
ground-based and ocean-based sites. Over the last century, the average
surface temperature of the Earth has increased by about 1.0o
F. The eleven warmest years this century have all occurred since 1980,
with 1995 the warmest on record (Figure 7).
The higher latitudes have warmed more than the equatorial regions.
Beginning in 1979, satellites have been used to measure the temperature
of the atmosphere up to a height of 30,000 feet. The long-term surface
record and the recent satellite observations differ, but that fact is not
surprising: the two techniques measure the temperature of different parts
of the Earth system (the surface, and various layers of the atmosphere).
In addition to this, a variety of factors, such as the presence of
airborne materials from the 1991 eruption of the volcano Mt. Pinatubo,
affect each record in a different way. Satellite observations were
initially interpreted as showing a slight cooling, but more recent
analyses accounting for natural, short-term fluctuations imply warming,
just as the ground-based measurements have indicated over a longer time
period. As more data from the satellite record become available, and as
the quality of measurements is improved, comparison of these two records
should yield additional insights.
What does warming do? A warmer Earth speeds up the global water cycle:
the exchange of water among the oceans, atmosphere, and land. Higher
temperatures cause more evaporation, and soils will tend to dry out
faster. Increased amounts of water in the atmosphere will mean more rain
or snow overall.
We may be seeing the first signs of changes in the water cycle. Since
the beginning of the century, precipitation in the United States has
increased by about 6 percent, while the frequency of intense
precipitation events (heavy downpours of more than two inches per day)
has increased by 20 percent. Such events can cause flooding, soil
erosion, and even loss of life (Figures 8 and
9 ). In some midcontinental areas,
increased evaporation has led to drought because the heavy rains fell
There is also evidence that ecosystems are reacting to warming. Between
1981 and 1991, the length of the growing season in the northern high
latitudes (between 45oand 70o N) increased by a
total of up to twelve days, as documented by satellite imagery. Greening
in spring and summer occurred up to eight days earlier, and vegetation
continued to photosynthesize an estimated four days longer.
Global mean sea level has risen 4 to 10 inches over the last 100 years,
mainly because water expands when heated. The melting of glaciers, which
has occurred worldwide over the last century, also contributes to the
rise. Formerly frozen soils (permafrost) in the Alaskan and Siberian
arctic have also begun to melt, damaging both ecosystems and
infrastructure. Melting and tundra warming will also lead to decay of
organic matter and the release of trapped carbon and methane, creating an
additional source of greenhouse gases.