SAN FRANSICO — Sulfurous ocean gases condensing into tiny aerosol particles over the Southern Pacific may seem far removed from the halls of government power. But they have a direct bearing on multibillion-dollar decisions that governments are considering as they try to respond to public pressure to "do something" about possible global warming.
Aerosols are the great unknown in computer-based climate forecasts. While "greenhouse gases" such as carbon dioxide and methane trap outgoing heat and warm the lower atmosphere, aerosol particles scatter sunshine back into space and cool things down. Climate modelers have a fair idea how to simulate greenhouse warming using computers. But they know zip about simulating the climatic influence of aerosol particles, say researchers studying the effect.
That's why aerosol chemist Timothy Bates at the National Oceanic and Atmospheric Administration's lab in Seattle says he can't have "real strong confidence" in climate modeling today. Dr. Bates is one of hundreds of atmospheric chemists engaged in an intensive international program to learn to make more accurate climate models. Some of them presented early fruits of their research this week at the fall meeting of the American Geophysical Union.
As Bates and several colleagues explained, understanding the aerosol effect is substantially more complicated than dealing with greenhouse warming. Many aerosol particles are formed by sulfur dioxide emissions from burning fossil fuel. Others form from sulfurous volcanic gases and natural emissions of dimethyl sulfide by microscopic marine organisms.
Uneven aerosol distribution complicates climate simulation, explains oceanographer Barry Huebert, of the University of Hawaii in Honolulu. Greenhouse gases are relatively long-lived and spread throughout the atmosphere; their warming effects act globally. Aerosols are short-lived; their cooling acts regionally over areas smaller than continents. Thus the influence of greenhouse warming versus aerosol cooling differs substantially from region to region. This pattern of warmer or cooler regions, in turn, affects where storminess develops and how areas of relative dryness and wetness are distributed.
Dr. Huebert says aerosols are probably the reason we haven't yet seen the warming climate modelers have predicted. He adds that if one removed aerosols from a region - by eliminating sulfur dioxide emissions, for example - temperatures there might shoot up.
The International Global Atmospheric Chemistry Program now under way plans to study the details of the aerosol effect in the Pacific and Atlantic Oceans, over China and Europe. The first study was done a year ago south of Australia; the results pin down "a key element in our understanding of what's going on in this region," says University of Hawaii oceanographer Anthony Clarke - namely, how the aerosol particles form from naturally emitted sulfurous gas from the sea.
Rather than forming near the sea surface, as scientists had expected, the gases rise to high altitudes in clouds, then move into cold air where they condense into new particles and drift down to the lower atmosphere.
This kind of detailed knowledge has to be learned about many aerosol processes in many parts of the world before climate modeling can be improved. Until that is done, the scientists here said, governments won't know what would happen if they took costly aggressive action - either to drastically curb sulfur dioxide pollution, which might be a good thing, or to cut down on carbon dioxide emissions by imposing a stiff "carbon tax" on fossil fuels.