Heat traps

Aerosols trap heat and increase temperatures

Published: Saturday 15 September 2001

black carbon aerosol pollution, produced by humans, can impact global climate as well as seasonal cycles of rainfall shows new research based upon satellite data and a multi-national field experiment.

Aerosols that contain black carbon both absorb and reflect incoming sunlight, therefore they can exert a regional cooling influence on Earth's surface that is about three times greater than the warming effect of greenhouse gases. But even as these aerosols reduce by as much as 10 per cent the amount of sunlight reaching the surface, they increase the solar energy absorbed in the atmosphere by 50 per cent -- thus making it possible to both cool the surface and warm the atmosphere. Scientists are concerned that this heating may perturb atmospheric circulation and rainfall patterns.

"When we combined the satellite measurements with surface measurements, we found that the reduction of sunlight reaching the surface was three times larger than the amount of sunlight reflected back to space," says V Ramanathan, director of the Center for Clouds, Chemistry and Climate at the Scripps Institution of Oceanography at the University of California, San Diego. "Averaged over the entire northern Indian Ocean, the man-made pollutants reflected more solar radiation back to space than normal, but they absorbed up to twice as much radiation in the atmosphere."

Data for the investigation were collected during the Indian Ocean Experiment (indoex) -- an international, multi-agency measurement campaign conducted from January through March in the years 1997, 1998 and 1999.

In contrast to the results of the new study earlier global warming research has suggested that aerosols make our world 'brighter' by reflecting more sunlight back to space, thereby helping to counteract the greenhouse effect. Most natural aerosols scatter and reflect sunlight back to space, thereby making our planet brighter. However, human-produced black carbon aerosol absorbs more light than it reflects, thereby making our planet darker. "Ultimately, we want to determine if our planet as a whole is getting brighter or darker," says Ramanathan. "A large reduction of sunlight reaching Earth's surface has implications for the hydrological cycle because of the close tie between heat and evaporation," Ramanathan says. "It could change the heating system of the atmosphere and disturb the climate system in ways we don't understand now. We don't know, for example, how this might affect the monsoon season."

While Ramanathan admits that scientists don't know the net effect of bright and dark aerosols on global climate, they have been able to show that aerosols have a net cooling effect on the surface and they now know the magnitude of that cooling over a large region.

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