Last Year's Volcano Cools This Year's July

YOU may not have noticed, but so far it's a bit cooler this summer than last.

Hot Mt. Pinatubo in the Philippines, which erupted in June of 1991, is the culprit - lowering the earth's average temperature by 1 degree Fahrenheit, with another probable 1 degree to come within the next 12 months. To scientists who constantly measure the temperature of the earth's atmosphere, the change is massive, even though it will be temporary.

The volcano whooshed 20 million tons of sulfur dioxide into the earth's upper stratosphere. The gas spread around the world within two weeks, mixing quickly with water vapor and turning to sulphuric acid in the atmosphere. The billions of sour little drops are now bouncing a percentage of sunlight back into space that would otherwise warm the earth.

"Maximum Pinatubo cooling will be reached within the next 12 months," said Ellsworth Dutton, a scientist with the National Oceanic and Atmospheric Administration's environmental research lab in Boulder, Colo.

After that, the effect will diminish gradually until it is entirely gone, between four and five years from now, Dutton said. Droplets aid in destroying the ozone

One effect of such a massive volcanic eruption as Pinatubo's is additional destruction of the earth's protective ozone layer. The surface of the droplets in the stratosphere, Dutton explains, enable the chemical reactions that deplete ozone to occur faster.

Over the past 100 years, Dutton said, earth's average recorded temperature has increased by 2 degrees. So the `Pinatubo' effect is expected to offset entirely - though only temporarily - this long-term increase.

The recent 1 degree average cooling is spotty. The entire northern hemisphere is cooler, except for one warm area over Europe. The northeastern United States is "lots cooler" than the average temperature for the last 10 years, says Dutton - in some areas by as much as 8 degrees F. But temperature fluctuation could also be the result of shifts in wind patterns.

Scientists accurately predicted the 1 degree of cooling before it happened, said Alan Robock, professor of meteorology at the University of Maryland. This accuracy shows, says Dr. Robock, that, by extension, scientists also know a great deal about the effect of carbon dioxide and other heat-trapping gases on the temperature of the atmosphere - more than is generally acknowledged. This so-called "greenhouse" effect - caused largely from burning fossil fuels - is often cited as cause for the recorded 2- de gree rise. But a serious dispute among scientists remains as to whether enough data exists to prove this theory of global- warming to be correct. Mr. Dutton says there certainly are "similarities between the models used to predict and verify the cooling and those used to predict greenhouse warming" and that this similarity lends "credibility" to the greenhouse theory. El Ninos can make a difference

Both Robock and Dutton note that on April 4, 1982, Mt. El Chichon in Mexico erupted, pouring 7-million tons of sulfur dioxide into the atmosphere - a little over a third of Mt. Pinatubo's discharge. That year, when scientists predicted that the earth's atmosphere would cool, it didn't. Not that they were wrong, say the scientists. The 1982-1983 El Nino, unexpectedly one of the strongest on record, held down the cooling.

El Ninos, which start when trade winds are weaker than normal, feed extra heat into the atmosphere over the Pacific Ocean.

Although an El Nino is active now, its effect on the average temperature is being offset by the Pinatubo eruption.

Things could be worse now. North America had a year without a summer in 1816, after Indonesia's Mt. Tambora in 1815 flung 300 million tons of sulphur compounds into the stratosphere. Lots of New Englanders headed West when it snowed that summer. And Mary Shelley wrote Frankenstein on the shores of Lake Geneva in the summer of 1816, when the bad weather ruined her vacation there with her husband, says Robock.