Recent Scientific Accomplishments in Observing,
Understanding, and Predicting Climate Change over the Next Few
- Modeling studies predict that atmospheric aerosols, largely emitted
from human activities, exert a non-uniform cooling effect over the globe. On
average, this effect may be counterbalancing about half of the expected
warming induced by increases in atmospheric greenhouse gas concentrations.
- The decline in snow cover extent in the Northern Hemisphere by about 10
percent over the past 20 years has resulted in a further warming of surface
air temperatures, which is consistent with the observed trend of increasing
water vapor concentration during this time period. Other evidence of warming
includes a decrease in Arctic sea ice, continued melting of alpine glaciers,
and a rise of sea level.
- Information on past changes in the Earth's climate (from ice cores, lake
level data, and other indicators) suggests that the melting of very large
icebergs associated with glacial retreat can perturb ocean circulation
patterns and result in relatively abrupt climate shifts over time periods as
short as decades.
- Oceanographic observations along the central North Atlantic have revealed
a distinct warming in the upper 2500 meters over the past 35 years, suggesting
either a large-scale warming in the ocean or a shifting of large masses of
- Results of measurements of carbon exchange at a site on the Arctic tundra
indicate that the tundra may have shifted from being a net sink for
atmospheric carbon dioxide in the 1970's to a net source of CO2 in
the 1990's. Because of the large reservoir of carbon in tundra soils, the
release of carbon to the atmosphere from tundra ecosystems could have a major
positive feedback effect on the atmospheric CO2 concentration and
on climate change.
- Combined satellite, aircraft, and surface measurements indicate that
significantly more solar radiation may be absorbed by clouds than is currently
predicted by general circulation models. The interpretation of these
measurements is inconsistent with current understanding and therefore requires
further observational confirmation. If confirmed, these new findings will
require reanalysis of the Earth's radiation balance and the role of clouds in
climate change and could result in significantly improved climate models.
- Results of analyses of the relationship between the growth of boreal
forest trees in northern and central Alaska and climate show that, over the
past 100 years, tree growth at first accelerated but then stabilized as
climate continued to warm. These results indicate that the response of boreal
forest tree growth to climate change may not be simple and linear.
- New research indicates that some species could potentially benefit from
increased atmospheric concentrations of CO2. Experiments on the
interactive effects of exposing agricultural crop species to different
mixtures of atmospheric gases suggest that elevated CO2
concentrations may, for selected species, mitigate the damaging effects of
elevated ozone (O3) concentrations. However some species, such as aspens,
become more sensitive to increased concentrations of O3 if CO2
concentrations are elevated.
- Potential economic impacts of a global warming scenario on U.S.
agriculture were estimated using an approach that accounts for not only the
direct impacts of climate change on crop yields but also potential adaptations
(e.g., crop substitution) by farmers in response to different climates.
Results show a significantly lower overall economic impact of global warming
compared to that estimated by using a simple production-function approach that
does not account for adaptive, profit-maximizing farmers. The results also
suggest that global warming could be beneficial to U.S. agricultural
productivity if irrigated water is not limited.
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Last updated 04/12/96