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.

incinerator

Incinerators like this one in Chicago can release soot from incompletely burned wastes (Photo courtesy Lake Michigan Federation)
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."

cessna

The INDOEX project uses ground, ship and airplane based instruments to gather information in the Indian Ocean (Photo courtesy INDOEX)
"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.

clouds

Heated soot particles may dissipate cumulus clouds (Photo courtesy NOAA)
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.

truck

Tractor-trailer rigs that run on diesel emit soot from unburned fuel (Photo courtesy Mack Trucks Inc.)
"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.

power plant

Coal burning power plants release particles of soot (Photo courtesy National Park Service, Indiana Dunes National Lakeshore)
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.