WASHINGTON -- Mount Pinatubo's eruption in June had a profound effect on global climate, blocking out 2 percent of light from the sun and lowering global temperatures by perhaps a half-degree Fahrenheit, a group of scientists said yesterday.
The Philippine volcano shot a plume of sulfuric acid into the upper atmosphere that is cooling the planet so effectively, they said, that it has overshadowed the warming from this winter's El Nino, a periodic increase in the surface temperature of the equatorial Pacific.
For the next two or three years, Pinatubo is also expected to dwarf the impact of civilization's millions of smokestacks, tailpipes and rice fields on global weather patterns. Its slowly dissipating acid cloud is reflecting twice as much heat as is beingtrapped by the greenhouse gases produced by industry and agriculture, said James E. Hansen of NASA Goddard Institute for Space Studies in New York.
"I think we will see a temporary reversal of the global warming trend we've seen in the last couple of decades," Dr. Hansen predicted.
Scientists described the latest findings on Pinatubo's impact at a press conference here yesterday, the 12th anniversary of the eruption ofMount St. Helens in Washington state. At the same time, the American Geophysical Union released a special report on volcanoes and climate.
The researchers said Pinatubo, which spewed more climate-modifying chemicals into the stratosphere than any volcano since Krakatau erupted in 1883 in Indonesia, was an important test for theoretical models of the way greenhouse gases may be causing global warming.
"Pinatubo is nature's own great climate experiment," Dr. Hansen said.
That experiment helped prove, scientists said, that sulfur is the key ingredient for altering climate, not ash or dust -- meaning that sulfur-poor eruptions such as Mount St. Helens in 1980 had little effect on global weather patterns, while Pinatubo had a big impact.
Studies of the Philippine volcano produced some surprising conclusions.
Alan Robock, a meteorologist with the University of Maryland, said that although Pinatubo is lowering the average global temperature, it has also increased the flow of the jet stream, causing a milder-than-normal winter of 1991-1992 in Europe and North America.
Those moderate temperatures, he added, may have helped reformers in the former Soviet Union by averting famine there.
Richard P. Turco of the University of California at Los Angeles said thatwhile Pinatubo ejected massive amounts of ozone-destroying chlorine into the atmosphere, much of the volcano-produced chemical was quickly scrubbed out by ash particles.
But the volcano's huge sulfuric acid cloud provided a kind of nTC platform, he said, where chlorine from chlorofluorocarbons and other man-made chemicals could attack ozone.
Partly as a result of the volcano, 4 percent of the globe's stratospheric ozone was destroyed this past winter, with depletion as high as 15 percent to 25 percent near the poles. (The ozone layer partially replenishes itself annually, but overall levels are falling from year to year.)
Stratospheric ozone is important because it blocks harmful ultraviolet radiation, which can cause skin cancers and cataracts. But scientists said Pinatubo's sulfuric acid cloud apparently reflected a lot of ultraviolet radiation back into space -- reducing the impact of this year's ozone loss on human health.
Scientists are piecing together a volcanic history of the Earth, and comparing it with climate data from tree rings and ice cores, in order to better understand how eruptions alter global weather and, eventually, human cultures.
These data indicate, for example, that a major eruption occurred in 536 A.D., although the volcano has never been identified. Michael G. L. Baillie, a geophysicist at Queen's University, Belfast, has theorized that the blast triggered famines in Europe and Asia and a plague that reached the Mediterranean in 542 A.D.
So far, scientists said, the largest eruption identified was one that created Lake Toba, in Sumatra, 73,400 years ago. The blast was 8,000 times bigger than Mount St. Helens, probably produced a worldwide volcanic winter and sent global temperatures plunging up to 9 degrees Fahrenheit for several years.
The study of Pinatubo's sulfur cloud has provided new evidence for researchers in several fields.
Some scientists now believe that the mass extinction that wiped out the dinosaurs around 64.5 million years ago was triggered by an asteroid that hit the Yucatan peninsula of Mexico, leaving a crater 113 miles wide. A similar crater, centered in Baltimore, would extend from south of Alexandria, Va., to Newark, Del.
Dr. Haraldur Sigurdsson of the University of Rhode Island said that, unfortunately for the dinosaurs, the asteroid hit a certain type of sulfur-rich rock that covers only about 1.3 percent of the Earth's surface.