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

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

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

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.