Recent Scientific Accomplishments in Observing, Understanding, and Predicting Climate Change over the Next Few Decades

• 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.

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  • Climate Models Predict Cooling Effect of Aerosols

• 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.

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  • Reduction in Northern Hemispheric Snow Cover Over the Last 20 Years and Enhanced Surface Warming in Northern Latitudes

• 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.

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  • Sediment Record from Central Asia Reveals that the Climatic Response as the Oceans and Ice Sheets for Over 250,000 Years

  • Massive Iceberg Discharges Linked to Past Global Climate Change

• 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 ocean water.

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  • Changes in Atlantic Ocean Temperatures

• 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.

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  • Arctic Ecosystems May Be a Source of CO₂

• 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.

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  • New Discrepancy Noted Between Predicted and Observed Absorption of Solar Radiation by Clouds

• 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.

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  • Boreal Forest Growth Rate Stalled

• 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.

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  • Elevated CO₂ Found to Partially Protect Some Crops From the Effects of Air Pollution

• 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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  • World Agriculture Can Better Adapt Than Previously Estimated If Crop Production Can Shift

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  Last updated 04/12/96