Warmer clouds could keep Earth cooler.

Warmer clouds could keep Earth cooler

Having refined the way computers simulate clouds, a team of British scientists reports that the greenhouse world of tomorrow may not produce temperatures as warm as predicted.

The new work, detailed in the Sept. 14 NATURE, illustrates one of the special problems that currently limit the credibility of long-term climate predictions (SN: 8/12/89, p. 106). "There is still major uncertainty about the magnitude of greenhouse warming, and a lot of the uncertainty comes from uncertainty about how to include the clouds," says David A. Randall, an atmospheric scientist at Colorado State University in Fort Collins.

Climate experts rely on complex computer models of the atmosphere and oceans, called global circulation models, to forecast how accumulating greenhouse gases such as carbon dioxide will affect the planet. Representing clouds in the standard way, the model at the British Meteorological Office in Bracknell predicts the world will warm 5.7 [degrees] C by the time carbon dioxide concentrations double, which scientists expect in the middle of the next century. When J.F.B. Mitchell and his colleagues altered the cloud portion of this model, the predicted warming dropped to 2.7 [degrees] C.

The difference stems from the ways in which the model predicts cloud properties. The revised cloud treatment requires the model to simulate the amount of liquid water inside a cloud. This technique is more detailed than the standard one, which bypasses a calculation of the cloud's liquid water.

The new simulations show that water in clouds -- either ice or liquid water -- could play an important role in limiting global warming because clouds reflect solar radiation that would otherwise warm Earth's surface. Ice particles fall out of clouds more quickly than do water droplets, which tend to be smaller, says Mitchell. When a cloud loses its water in either form, it thins and eventually disappears.

Many clouds in the world's midlatitudes form at temperatures just below freezing, so even a slight global warming could change ice clouds into ones that contain liquid water. Because the water clouds last longer, a warmer world would reflect away more sunlight than today's planet -- an effect that would check rising temperatures, according to the new forecast.

Though more sophisticated than the conventional cloud treatment, Mitchell's method does not necessarily yield a truer picture of the feedback clouds will exert on climate change, says Randall. A recent study of 14 different general circulation models shows wide variation in their predictions of future temperatures, precisely because they disagree on cloud feedback. In a commentary accompanying Mitchell's report, Anthony Slingo of the National Center for Atmospheric Research in Boulder, Colo., says that both the new simulations and the comparative study of models demonstrate the need to understand cloud feedback before models can make reliable predictions.

Earlier this month, the President's Office of Science and Technology Policy highlighted the importance of cloud studies when it offered its plan for U.S. research on global change for fiscal year 1990. The report lists understanding the role of clouds as the top scientific priority.