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Dirt in the Greenhouse

OK, you read it here first: next winter will probably be another chiller.

No, this prediction doesn't come from the new edition of the Farmer's Almanac. It isn't based on the latest satellite images captured here at the Geophysical Institute. Rather, it's an informed guess made by experienced volcano watchers. They're worried that the eruption of Mount Pinatubo in the Phillipines amounts to a gigantic switch, turning down the global thermostat.

They have some cause for concern. In retrospect, atmospheric scientists judge that the 1982 eruption of El Chichon volcano in Mexico kept so much of the sun's energy from warming earth that the planetary surface temperature cooled by several tenths of a degree Centigrade for more than a year. According to Science magazine, Pinatubo's outpouring looks far larger, and seems likely to have a greater effect on the atmosphere.

Anyone who has seen photographs of gritty, drifted volcanic ash covering structures and vehicles at Clark Air Force Base near the eruption might come up with a possible reason for cooling. Mount Pinatubo is putting out vast quantities of ash, and sunlight can' t penetrate well through ash-filled air. The ash plume is like an enormous parasol, shading the ground and sea below.

But volcanic ash, even when it is hurled high into the stratosphere, falls out of the air fairly quickly. We'll be dusting the last flecks of Mount Pinatubo's eruption off our shelves in a few months at most, probably after enjoying unusually vivid sunsets for several weeks.

The real villain in volcanic climate cooling is sulfuric acid. A big volcanic eruption pours tons of sulfur dioxide gas into the atmosphere, where it combines with water vapor to become tiny droplets of sulfuric acid. These acid aerosol particles are so fine that they can stay aloft for between one and three years--and they make a very effective sun block.

As of the end of June, Pinatubo's sulfur dioxide output had reached 15 million tons, give or take 5 million--nearly twice what Ed Chichon poured into the atmosphere. This estimate is based on measurements made by the Total Ozone Mapping Spectrometer (TOMS) aboard the Nimbus-7 satellite. The spectrometer was not designed to look for volcanic clouds, but the sulfur dioxide appears in its measurements as interference in the ozone signal that TOMS does measure.

The usefulness of TOMS for this purpose hints at another worry Mount Pinatubo has given atmospheric scientists: the sulfuric acid aerosol helps destroy stratospheric ozone. Researchers at the National Oceanic and Atmospheric Administration have calculated that Ed Chichon's aerosol could have led to the destruction of 15 percent of the ozone over the middle latitudes of the Northern Hemisphere. They think that the acid droplets serve the same purpose as the water droplets do in clouds within the Antarctic ozone hole, providing a surface on which the destructive chemical reactions can occur.

Thus, on two fronts, a once-obscure Philippine volcano may be putting glitches in carefully measured and observed atmospheric problems. Its gases may lead to a temporary but sharp worsening of stratospheric ozone loss. They may also slow---or stop, or reverse---the greenhouse heating that has increased average global temperature by half a degree Centigrade during the 20th century. It's enough to make atmospheric scientists wish their laboratory wasn't quite as big (and as uncontrolled) as all outdoors.