As the Earth's average temperature has risen in the last half-century with the buildup of greenhouse gases in the atmosphere, many scientists have come to see clouds, because they reflect so much of the sun's heat into space, as the biggest puzzle in interpreting the planet's changing climate picture.
But new evidence suggests that the current stew of airborne chemicals and particles might be giving clouds stronger cooling properties than previously thought, said Robert Charlson, a University of Washington atmospheric chemist.
"Clouds are a devilishly difficult but extremely important aspect of the Earth's climate system," said Charlson, lead author of a paper in the June 15 edition of the journal Science that details the new evidence.
Clouds are formed as water droplets condense around particles in the atmosphere. Previous studies have shown that when the number of particles increases because of emissions from human activities, for example there still remains the same amount of water to spread among them. The result is more and smaller droplets, which creates more total surface area within the cloud to reflect sunlight.
The new evidence implies that droplet formation also is influenced by several other factors, including the presence of soluble gases and organic pollutants (for instance, stearic acid from a variety of sources, from forest fires to backyard grills) that are only slightly water-soluble. That means even more droplets, giving the resulting clouds even more cooling capacity.
The Earth's surface in regions where this happens will be cooled even more than would happen with normal cloud cover, a finding that could carry broad ramifications, Charlson said. The phenomenon can affect regions as large as the eastern third of the United States, the European continent or the Szechwan Basin of China.
"Roughly half the Earth is covered with clouds at any one time," he said. "Of that, some percentage is influenced by products of human
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Contact: Vince Stricherz
vinces@u.washington.edu
206-543-2580
University of Washington
14-Jun-2001