Pollution drying up rainfall

By MICHAEL SMITH, UPI Science News  |  Feb. 17, 2002 at 6:29 PM
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BOSTON, Feb. 17 (UPI) -- New satellite data shows tiny airborne particles are changing rainfall patterns around the world, researchers said Sunday.

The man-made particles, mostly from burning fossil fuels, make it more difficult for clouds to form and less likely to rain if they do form, researchers said at the annual meeting of the American Association for the Advancement of Science.

Because they block sunlight, these tiny particles slow down evaporation from lakes and oceans, said conference participant Daniel Rosenfeld of the Hebrew University of Jerusalem. "So they suppress clouds in the first place," he said.

What's more, he said, the particles are too small to form the seeds of raindrops, "so the clouds that do form ... have a hard time to rain."

The analysis, he said, is based on new data from a joint American-Japanese satellite that uses radar to examine particles in both shallow clouds, near the Earth's surface, and the large, high clouds that contribute most of the rainfall.

Rainfall from even the biggest thunderclouds can be cut in half by such pollution, Rosenfeld said, and it can completely stop rain from shallow clouds.

Sooner or later, he said, the evaporated water does come down as rain, but not where it used to. For instance, he said, the satellite data show rain that used to fall in the tropics is being transported to higher latitudes.

He added the areas most affected are those with the worst air pollution -- and those tend to be the most populated areas, where water is an essential resource.

Man-made pollution also is affecting the monsoons of Asia, according to Veerabhadran Ramanathan, of the Scripps Institution of Oceanography, in La Jolla, Calif. He said a low-level haze over most of the Indian Ocean is blocking sunlight and changing the pattern of evaporation.

As a result, he said, the monsoons of southeast Asia have more rain, while the rest of the region gets less precipitation.

Brazilian researcher Paulo Artaxo, of the University of Sao Paulo, reported a similar discovery in the forests of Amazonia. Between the months of August and October, when farmers burn off woodland, the haze of particles has a dramatic cooling effect on the ground below.

The cooling effect may be as much as 47 degrees Fahrenheit, he said, enough to interfere with evaporation and cloud formation. "To have a cloud form," Artaxo said, "you have to have water vapor from evaporation."

While the environmental emphasis has been on global warming caused by greenhouse gases, these tiny particles -- known as "aerosols" in scientific jargon -- may cause local cooling effects, said Yoram Kaufman of the NASA Goddard Space Flight Center in Greenbelt, Md.

But overall, he said, greenhouse gases remain the greatest concern, because man-made aerosols remain in the air for only a few weeks, while gases such as methane can last decades.

"If we stopped burning fossil fuels tomorrow," he said, "the aerosols would be gone in a week, but the gases would be there for years."

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