Global temperatures are rising.
Observations collected over the last century suggest that the average land
surface temperature has risen 0.45-0.6°C (0.8-1.0°F) in the last century.
The surface
|
- red circles
reflect warming - blue
circles reflect
cooling
- Note:
cooling in Southeast U.S. may be due to sulfate aerosol influence
|
of the ocean has also been warming at a
similar rate. Studies that combine land and sea measurements have generally
estimated that global temperatures have warmed 0.3-0.6°C (0.5-1.0°F) in the
last century. About two-thirds of this warming took place between 1900 and
1940. Global temperatures declined slightly from the 1940s through the 1970s;
but have risen more rapidly during the last 25 years than in the period before
1940.
Surface temperatures are not rising
uniformly. Night-time low temperatures are rising on average about twice as
rapidly as daytime highs. The winters in areas between 50 and 70° North
Latitude (the latitude of Canada and Alaska) are warming relatively fast,
while summer temperatures show little trend. Urban areas are warming somewhat
more rapidly than rural areas, because of both the changes in land cover and
the consumption of energy that take place in densely developed areas (a
feature known as the "urban heat island" effect).
In
the United States, temperatures in the last 50 years have cooled in the East
while warming in the West. Over the last 100 years, the pattern is similar,
except that New England is warmer than 100 years ago because it warmed more in
the first half of the 20th century by more
than it cooled in the second half. This pattern of warming and cooling may be
part of a worldwide pattern: while most of the earth has warmed, the regions
that are downwind from major sources of sulfur dioxide emissions have
generally cooled (see the discussion on sulfates in the Atmospheric Change
section). This pattern is evident when one compares the two world maps below.
The first map of the world shows the areas that have warmed and cooled from
1951-93. The second map of the world shows the amount of incoming solar
radiation blocked by the cloud of atmospheric sulfates downwind from
industrial emissions of sulfur dioxide.
Although scientists have
incontrovertible evidence that the surfaces of the land and oceans have been
warming, some scientists are not yet convinced that the atmosphere is also
warming. The sattelite data do not show a warming trend; however, the 1979-97
data series may be too short to show a trend in atmospheric temperature.
Part of the reason that satellites do
not show a warming trend may be a coincidence regarding the year that NASA
began to collect this data. Balloon data, which shows the same absence of
warming over the 1979-97 period, shows a significant warming trend from 1958
to 1997. Measurement errors associated with the new technology, and cyclical
variations in temperature due to El Ninos (see glossary),
may also be responsible for the lack of a warming trend. Nevertheless, to many
scientists, the absence of a warming trend in the satellite data provides an
important caution that there is still much to learn about the global climate.
