Climate Warms as Black Soot Traps Sun's Heat
By Cat Lazaroff
LA JOLLA, California, May 15, 2000 (ENS) - Soot, a common
pollutant that has been around for thousands of years, may be a
major contributor to global climate change. Scientists have found
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.
Soot is formed from incompletely burned fuels and wastes. Forest
fires produce soot, as do coal burning power plants, charcoal
burners, diesel engines and trash incinerators.
A research team from the National Aeronautic and Atmospheric
Administration (NASA) and the Scripps Institution of Oceanography
have found that 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,
increased solar energy reaches the Earth's surface and the lower
atmosphere. This can significantly heat the atmosphere and
oceans, the team reports in the May 12 issue of the journal
"Science."
"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 three to five 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 as input to a sophisticated
computer model of tropical clouds. Researchers obtained the
measurements, taken during the dry monsoon in February through
March, 1998 and 1999, 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 interactions
between aerosols, clouds and climate focused 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.
The cloud burning effects of soot are not unique to the tropics,
the researchers say. Comparable amounts of soot have been
measured in other polluted air masses, such as those off the
mid-Atlantic coast of the United States.
In another paper, published in the May 4 edition of the journal
"Nature," V. Ramanathan and S.K. Satheesh of the Center for
Clouds, Chemistry and Climate (C4) at Scripps show that particles
of soot produced in southern Asia are absorbing significant
amounts of sunlight, leading to higher atmospheric temperatures.
"The atmospheric heating over the northern Indian Ocean is
surprisingly large compared to other oceanic regions and is
comparable in magnitude with that observed over the coastal
regions of the Atlantic Ocean," said Ramanathan.
The "Nature" authors propose that the disruption caused by the
soot aerosols may have several consequences for the region's
climate, including slowing down the natural hydrological cycle
and breaking up cloud cover. Although the researchers documented
aerosol particles such as sulfate, nitrate, organics, and ash,
the sunlight absorption was largely due to combustion derived
soot.
Ramanathan, who also coauthored the "Science" paper, warned that
both studies must be backed up with further observations. "While
this is an important finding, we should recognize that it is a
theoretical model calculation which must be tested against actual
measurements. Much additional field work remains to be
completed," he said.
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 lead to further
refinements in global climate models and enhance our ability to
predict future weather patterns.
Article by Environment News Service (ENS)
http://www.ens-news.com/ens/may2000/2000L-05-15-06.html
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