Is greenhouse effect offset by tiny air pollution particles?

Upton, NY - The Earth's climate may be more vulnerable to humankind's influences than we think, warn a scientist from the U.S. Department of Energy's Brookhaven National Laboratory and a German colleague. Then again, it might not. In either case, they argue, major policies on global climate change are being considered with incomplete information.

In a scientific commentary published in the May 24 issue of Science, Brookhaven atmospheric chemist Stephen Schwartz and biogeochemist Meinrat Andreae of Germany's Max Planck Institute of Chemistry report that uncertainty runs high when scientists attempt to predict how tiny air pollution particles, known as aerosols, influence the Earth's climate.

They also criticize a recent National Research Council (NRC) report for not recognizing the urgency of reducing this uncertainty, especially in the face of widespread resistance to limits on emissions of greenhouse gases based on limited evidence of their climatic influence.

Atmospheric aerosols, such as smoke, smog, dust and volcanic ash, come from both natural and anthropogenic, or manmade, sources. Together, the particles form a haze, which restricts visibility and, when seen from an airplane, makes the planet 'brighter' than it would otherwise be.

Schwartz has dubbed this phenomenon the "whitehouse effect," to contrast it with the "greenhouse effect," the influence of carbon dioxide and other atmospheric gases on the climate.

"The difference between these two effects is the difference between living in a greenhouse, which traps the sun's heat, and living in a white-painted house, which reflects it," said Schwartz. "In the whitehouse effect, aerosol particles reflect the sun's rays and keep them out of the atmosphere, causing a cooling effect. In the greenhouse effect, warming occurs because atmospheric gas molecules trap infrared rays that would otherwise escape to space."

Although the greenhouse effect is quantitatively well understood, the whitehouse effect is not. "The two effects are canceling each other out to some extent, but the aerosols' impact is highly uncertain," Schwartz said. "We just don't know yet, and not knowing is a risky basis for making decisions or postponing them"

Another major difference between the whitehouse and greenhouse effects, Schwartz said, is the lifespan of the pollutants that cause them. While aerosols hang around in the atmosphere for only about a week before being washed out by precipitation, greenhouse gases persist for decades or even centuries. So, each week's worth of aerosols is in essence counteracting an ever-mounting mass of carbon dioxide and other gases built up from years before.

If whitehouse aerosols are reflecting enough light to offset nearly all the effects of the greenhouse gases, and if the remaining greenhouse effect is causing the global warming now being seen, say the researchers, then the climate is much more susceptible to artificially caused change, or "forcing," than is now commonly thought.

On the other hand, if the whitehouse effect is offsetting only a small portion of the greenhouse effect, and the remaining greenhouse forcing is responsible for the global warming seen to date, then perhaps the climate is somewhat able to roll with the punches of humankind's activities.

Schwartz and Andreae acknowledge that the NRC's recent report, A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change, is on the right track. But, they state, the new research effort recommended by the report falls far short of what is needed to understand this complex phenomenon.

To gauge aerosols' influences on the Earth's climate, computer models used to predict climate change need to account for the behavior of the tiny particles. But, the researchers say, the underlying knowledge base that computer modelers must draw upon in order to factor aerosols into their models is lacking in many respects. More information is needed about the chemical and physical properties and processes of aerosols, including their ability to modify clouds.

Schwartz has been examining the whitehouse effect for nearly a decade. In 1992, he and six colleagues published a report in Science, 'Climate Forcing by Atmospheric Aerosols,' that provided some of the first global-scale estimates of the phenomenon. He is also author of numerous other scientific papers on this phenomenon and, in 1994, testified on the whitehouse effect before a hearing of the Senate Committee on Energy and Natural Resources.

Brookhaven National Laboratory carries out basic and applied research in the physical, biomedical and environmental sciences and in selected energy technologies. Brookhaven is operated by Associated Universities, Inc., a nonprofit research management organization, under contract with the U.S. Department of Energy.

-- 30 --