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.
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."
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.
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 "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.