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Greenhouse snow: melting the preconceptions.

Scientists have a difficult time running controlled experiments on the climate, so they turn to computer models as surrogates. By tinkering with these mathematical replicas, researchers can gain insight into how the real climate works. Now, a study of 17 such computer models teaches that snow and global warming may interact in unexpected ways.

Researchers generally assume that decreases in total snow coverage would amplify a climate warning caused by greenhouse-gas pollution. According to the standard theory, a temperature increase would reduce the area of the globe covered by reflective, white snow, making Earth's surface darker. The newly uncovered areas would absorb more sunlight than before, heating up and thereby intensifying the greenhouse warming.

But a comparison of the world's best general circulation models suggests a more complex snow scenario, says Robert D. Cess of the State University of New York at Stony Brook. Cess led a group of 33 scientists from eight nations in a study addressing this question.

While most of the models behaved as theory would predict, some did not. In five out of 17, the reduction in snow cover elicited a negative feedback--a reaction that counteracted a small part of the greenhouse warming, the scientists report in the Aug. 23 SCIENCE.

The negative feedback arose because the retreat of the snow cover generated indirect effects. Some models showed that clouds increased over areas once covered by snow, helping to block out incoming solar radiation. In other instances, the snow loss caused Earth to emit more heat toward space -- another factor that would work toward cooling the planet.

The indirect effects did not always weaken global warming. In some models, they strongly amplified the original temperature increase.

The newly found indirect effects, especially the greater heat release, caught researchers off guard. "I don't think anyone anticipated that," says Cess. Researchers now must assess whether such effects actually occur in the real world, he says.

Global circulation models contain mathematical equations that simulate how various aspects of the climate evolve over time. In global warming studies, scientists usually double the preindustrial level of carbon dioxide in the atmosphere and then watch the computerized climate respond. The forecasts resulting from these simulations have varied widely: Some models show a modest warming of about 1.5[degrees]C; others predict a temperature rise more than three times greater. These inconsistencies have fueled political debate over the seriousness of global warming.

A study led by Cess in 1989 indicated that clouds deserve most of the blame for the divergence among forecasts (SN: 8/12/89, p.106). Climate modelers readily acknowledge that the present job of simulating cloud behavior. The latest comparison now suggests that the issue of snow cover also drives some of the disagreement models, albeit to a lesser degree than clouds.
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Title Annotation:study of 17 computer models of the climate
Author:Monastersky, Richard
Publication:Science News
Date:Aug 24, 1991
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