Aerosols: critical questions for climate.

Aerosols: Critical question for climate

In the 1970s, some scientists suggested that particles of air pollution called aerosols could help Earth slip into an ice age by blocking sunlight and cooling the planet's surface. But as global temperatures climbed and concern grew over the threat of greenhouse warming, most research on the aerosol-climate link entered a deep freeze. That was a major mistake, two climate experts say.

The paucity of aerosol data greatly hampers efforts to predict how growing concentrations of greenhouse gases might change the climate, assert James E. Hansen and Andrew A. Lacis of NASA's Goddard Institute for Space Studies in New York City. Pollutant aerosols might be slowing the greenhouse warming and altering rainfall patterns in certain areas of the world today, they note in the Aug. 23 NATURE.

"I think we don't know the importance that aerosols have because we don't have the data on them. That's a very important point because we're not gathering the data, either," Hansen told SCIENCE NEWS. One of the more outspoken researchers in the climate field, Hansen set politicians and the media abuzz in 1988 when he became the first leading scientist to express near-certainty that the greenhouse warming had already started.

The term aerosol refers to tiny liquid and solid particles entering the atmosphere from natural sources and from "anthropogenic" sources such as fossil-fuel combustion and the burning of plants. The highest concentrations occur in the industrialized regions of the Northern Hemisphere. Coal burning represents the major U.S. source, emitting sulfur dioxide that turns into sulfate aerosols.

Aerosols reflect sunlight back toward space, thereby cooling Earth's surface. They also cool the planet indirectly by helping water vapor condense into cloud droplets, making the clouds more reflective to sunlight. And recent experiments demonstrated that anthropogenic aerosols can also increase cloud reflectivity by inhibiting rainfall (SN: 8/12/89, p.106).

In their analysis of existing data. Hansen and Lacis calculate that anthropogenic aerosols could play a major or minor role in the climate, depending on their levels and their impact on clouds. "To some extent, aerosols must partially cancel the greenhouse effect. But we really don't know to what extent. It could be anything from zero to 50 percent," says Hansen. That means anthropogenic aerosols could exert a cooling effect about half as strong as the warming force from the buildup of greenhouse gases.

Joyce Penner of the Lawrence Livermore (Calif.) National Laboratory goes farther, suggesting that the current aerosol impact might even balance the effect of greenhouse gases. Penner told SCIENCE NEWS she bases her estimate on highly simplified computer simulations tracing the effects of aerosols from burning grasslands and tropical forests.

Hansen thinks the aerosol question is as significant as that concerning the role of clouds, often cited as the wild card in today's climate models. Other climate experts may quibble with that assertion, but they agree that aerosols inject serious uncertainty into their forecasts. Despite the unknowns, Hansen says policymakers shouldn't wait until it's too late to prevent global warming. Instead, he recommends creating programs that address the greenhouse issue and other problems at the same time. Although President Bush has endorsed this philosophy, Hansen says the administration is not doing enough to improve energy efficiency and develop alternative energy sources.

In their report, Hansen and Lacis make an appeal for efforts to monitor aerosols and to study their effects on clouds. But that appeal comes too late for the U.S. aerosol-monitoring network. The National Oceanic and Atmospheric Administration decided in June to abandon the project because of inadequate funding.