Polluted clouds might bring patchy cooling in a warming world

June 14, 2001

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 activity, particularly in the Northern Hemisphere, where most people live."

Charlson was joined in the work by John Seinfeld and Athanasios Nenes of the California Institute of Technology; Markku Kulmala at the University of Helsinki; Ari Laaksonen at the University of Kuopio, Finland; and Maria Cristina Facchini at Italy's CNR-Instituto di Scienze dell' Atmosfera e dell'Oceano.

"This work modifies the original 1920s and '30s theory to include soluble gases, such as nitric acid gas, and surface active organic materials, like smoke from forest fires and garbage incinerators," Charlson said.

The findings come from a computer model developed at Caltech that shows how the reflectivity, or albedo, of clouds is changed when various factors are introduced. The evidence raises questions about how clouds affect global climate change, but it doesn't answer them, Charlson said.

In their formation, "clean" clouds reach a point at which droplets pass a certain size threshold and begin to gather more water and grow spontaneously to ever-larger sizes. But the new study shows the chemical effects of pollution can trigger the formation of clouds or fog with tinier droplets, as small as a micron (one-thousandth of a millimeter) that are limited in their growth because of the substances going into them.

"When you get so much pollution in the air, it influences the formation of clouds," Charlson said.
For more information, contact Charlson at (206) 543-2537, or Seinfeld at (626) 395-4635 or seinfeld@caltech.edu

University of Washington

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