Pinatubo's impact spreads around the globe.

More than 10,000 kilometers lie between the United States and the Philippines, but the eruption of Mt. Pinatubo in June may increase the risk of skin cancer in North America next summer, according to some preliminary estimates of the volcano's effect on stratospheric ozone. Scientists also expect the eruption to cool the globe for the next few years -- a climatic twist that would complicate efforts to discern whether greenhouse-gas pollution is currently warming the Earth.

Pinatubo can wreak such distant and long-lasting effects because its eruption lofted millions of tons of sulfur dioxide gas into the stratosphere. As winds blew the volcanic cloud westward, the gas molecules reacted with water in the atmosphere to form tiny droplets, or aerosols, of sulfuric acid, which will stay in the stratosphere for two to three years before they fall.

Some computer models of atmospheric chemistry suggest that this huge increase in sulfur dioxide aerosols could thin the protective ozone layer, allowing more harmful ultraviolet radiation to reach Earth's surface. "We found a substantial ozone decrease, especially in the mid- and high-latitudes, and especially in winter," says Guy P. Brasseur, director of the atmospheric chemistry division at the National Center for Atmospheric Research (NCAR) in Boulder, Colo.

Scientists believe sulfuric acid aerosols affect ozone levels through a complex cascade of events. The aerosols provide tiny surfaces on which certain nitrogen molecules can react. These reactions alter the chemistry of the stratosphere, causing "safe" chlorine molecules to transform into ones that can destroy ozone.

Satellite and aircraft measurements indicate Pinatubo was probably the largest volcanic eruption of the century, belching out at least twice as much sulfur dioxide as the 1982 eruption of Mexico's El Chichon. Using these data, Brasseur's model calculates that the aerosol increase will cause a 15 percent reduction in midlatitude ozone values during winter. Brausseur cautions, however, that these predictions include significant uncertainties.

Susan Solomon, an atmospheric chemist at the National Oceanic and Atmospheric Administration in Boulder, used a different model to calculate the effect of the volcanic aerosols. Although she says it's too early to discuss specific results, she hints that her model predicts an ozone decrease even greater than that calculated by Brasseur.

Unlike Solomon, however, Brasseur also found substantial ozone depletions--on the order of 6 to 8 percent--in the mid-latitudes during summer, when ultraviolet levels reach their yearly maximum. Sasha Madronich of NCAR calculates that such thinning would allow summertime levels of ultraviolet radiation to reach 18 percent higher than normal in the northern mid-latitudes, which includes much of the United States, europe and the Soviet Union. The boost in radiation would increase skin cancer risk and could generate several thousand additional cases of melanoma in the United States alone over the next few decades, estimates Madronich.

Other atmospheric chemists, however, say discussions of health problems are premature because scientists remain unsure whether the eruption will spur any significant ozone decrease. "We don't really have the models or the measurements that would allow us to confidently make such predictions," says Michael J. Prather, an atmospheric chemist at NASA's Goddard Institute for Space Studies in New York City. According to Prather, only about half of the atmospheric-research community is currently betting the volcano will significantly decrease ozone levels.

The answer should become clearer this winter, after the aerosols spread toward the North Pole. Long before the eruption, scientists began planning a major research campaign to probe the fate of ozone int he stratosphere over the northern mid-latitudes and polar regions. They want to understand why wintertime ozone levels have declined by 6 to 8 percent over the mid-latitudes during the last decade. The planned measurements should also detect effects of the volcanic aerosols, providing the data needed to make better predictions of the possible summertime ozone thinning, Prather says.

The same veil of sulfuric acid aerosols that threatens ozone also reflects sunlight back toward space, slightly dimming light reaching the Earth's surface. Climate experts say the Pinatubo aerosols will lower average world temperatures during the next several years, temporarily reversing the warming trend of the last two decades.

Global temperatures may not fall significantly in 1991 and 1992, because an El Nino developing in the Pacific will help warm the Earth, slightly offsetting the aerosol cooling. But when the El Nino wanes in about a year, Pinatubo aerosols could chill the Earth by about 0.5 [degrees]C, says James Hansen of NASA's Goddard Institute.

Before the eruption, Hansen and others had predicted that the behavior of global temperatures during the 1990s would provide a good test for theories forecasting a global warming from greenhouse-gas pollution. Because Pinatubo's cooling should temporarily mask any long-term warming trend, the eruption will make the climate record more complex and difficult to interpret. But even with several years of cooling, the 1990s should still turn out warmer than the 1980s if the greenhouse warming is currently underway, Hansen says.