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May 11, 2000

John Bluck

NASA Ames Research Center, Moffett Field, CA

650/604-5026 or 650/604-9000

Mario Aguilera

Scripps Institution of Oceanography, La Jolla, CA

Phone: 858-822-0660



A common pollutant that has been around for thousands of years has been newly identified as a potentially major contributor to global climate change, according to a paper published in the May 12 issue of the journal Science.

Scientists at NASA and the Scripps Institution of Oceanography report that airborne black soot has the capacity to raise regional temperatures far more than carbon dioxide, a major greenhouse gas that also results from combustion. According to the research team, the intense sunlight of the tropics heats the soot present in polluted air. This heating burns off the flat tops of shallow cumulus clouds for hundreds of miles downwind of pollution sources. With less cloud cover reflecting sunlight back to space, there is increased solar energy that reaches the Earth’s surface and the lower atmosphere. This can significantly heat the atmosphere and oceans, according to the new findings.

"Aerosol pollution can increase or decrease cloudiness, depending on the weather and the particular ingredients of the pollution," said Andy Ackerman, lead author of the paper and scientist at NASA's Ames Research Center in California's Silicon Valley. "This newly discovered mechanism amounts to a heating effect over the Indian Ocean that is 3 to 5 times as strong as the global effect of increases in carbon dioxide since pre-industrial times," he said.

The research team used measurements of the dark haze covering vast areas of the Indian Ocean (during the dry monsoon in Feb.-Mar., 1998 and 1999) as input to a sophisticated computer model of tropical clouds. Researchers obtained the measurements during the Indian Ocean Experiment (INDOEX).

To their surprise, researchers found the cloud-burning effect of soot in the haze to be much stronger than the globally averaged greenhouse effect due to increases in atmospheric carbon dioxide since the 1800s.

It is only the soot component of pollution that causes this newly discovered cloud-burning effect. Prior research on aerosol-cloud-climate effects focused largely on other ingredients of aerosol pollution. These components were found to increase cloudiness and oppose greenhouse warming. This occurred because increased amounts of water-soluble aerosols produce more numerous and smaller cloud droplets. Such droplets reflect sunlight more efficiently and are less likely to result in rain.

"While this is an important finding, we should recognize that it is a theoretical-model calculation which must be tested against actual measurements," said V. Ramanathan, co-author of the paper and director of the Center for Clouds, Chemistry, and Climate at Scripps Institution of Oceanography in La Jolla, California. "Much additional field work remains to be completed," he said.

Solar heating measured during the most recent experiment in the tropics is not considered to be unique or specific to a given time and place, according to researchers. On the contrary, the authors noted that comparable amounts of soot have been measured by previous researchers in other polluted air masses such as those off the mid-Atlantic coast of the US.

The authors of the Science paper expect that their recent finding will motivate a new direction of research into aerosol-cloud-climate interactions. It may well lead to further refinements in global climate models and enhance our ability to predict future weather patterns.

In addition to Ackerman and Ramanathan, the authors of the Science paper include: O. Brian Toon, University of Colorado, Boulder, CO;

D. E. Stevens, Lawrence Livermore National Laboratory, Livermore, CA;

A. J. Heymsfield, National Center for Atmospheric Research, Boulder, CO; and E. J. Welton, Science Systems and Applications, Greenbelt, MD. INDOEX is a cooperative program involving scientists from the United States, Europe, India and the Maldives. The National Science Foundation, the U.S, Department of Energy, the National Oceanic and Atmospheric Administration and NASA funded the U.S. part of INDOEX.



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