FOR climatologist James Hansen, Mt. Pinatubo's eruption last June has become a personal challenge. It has provided what he calls "an acid test" of his ability to simulate climate with a computer.
His comment has an environmentally important double meaning. Pinatubo has thrown a sulfuric-acid mist around the earth that is forcing scientists to test their theories of the atmospheric impact of volcanoes with an ongoing natural "experiment."
The eruption spewed some 20 million to 30 million tons of sulfur-dioxide gas into the stratosphere. This has produced a veil of sulfuric-acid droplets. They absorb outgoing infrared (heat) radiation and warm the stratosphere. They reflect sunlight and cool the earth's surface.
At least that's what modern climate theory says should happen. Dr. Hansen and his colleagues at the National Aeronautics and Space Administration (NASA) Goddard Institute for Space Studies in New York built that theory into one of their advanced mathematical climate models - the kind of model they use to study the prospects for man-made global warming. Its ability to predict Pinatubo's climatic effect over the next few years will show how good such computer simulations really are.
Earth scientists haven't fully understood the climatic influence of volcanoes. Richard Turco, an atmospheric scientist at the University of California, Los Angeles notes that scientists have long known that "volcanism helped create Earth's atmosphere and continues to modify it." Eruptions release vast quantities of water vapor, carbon dioxide, and other gases.
But, Dr. Turco adds, "not until recently have we been able to ascertain how the climate is affected by a volcanic eruption." The massive outpourings of dust and gas that characterize explosive eruptions had distracted attention from the real climate-changing factor.
Volcanologist Stephen N. Carey at the University of Rhode Island at Kingston says scientists now realize that dust and ash fall out of the air fairly quickly. "The agent which causes climate change is sulfur gas ... that forms a layer of sulfuric-acid droplets that persist in the stratosphere," he says.
Dr. Carey is especially interested in the eruption of Tambora in Indonesia in 1815. The next year - known as "the year without a summer" - saw widespread cooling and associated crop failures. "I think the evidence is pretty overwhelming, showing a link between the eruption of Tambora and the strange things that happened a year later," he says.
Droplets in a volcanic sulfuric-acid veil are typically around 0.1 micrometers (millionths of a meter) in diameter. They make up a fine mist that is about 75 percent acid and 25 percent water. Carey notes that such tiny particles stay aloft for long periods. He says that is why Tambora's effects took a year to show up.
Tambora's eruption was larger than that of Pinatubo. It sent an estimated 150 million tons of acidic aerosol into the stratosphere - some five times more than Pinatubo's veil.
Yet both events are puny compared with super-eruptions in the geologically recent past. Studies of ash and lava deposits indicate eruptions hundreds of times greater than Tambora within the past 100,000 years. Carey says that "potentially, we're still at risk" from such massive events.
There may also have been an unknown number of climatically disastrous eruptions within historic times. For example, volcanologists refer to the "mystery" eruption of AD 536. They have yet to find geological evidence for the eruption. But the circumstantial evidence is strong. Byzantine historian Procopius recorded that "during this year ... the sun gave forth its light without brightness ... for the beams it shed were not clear." Famines were widespread in subsequent years.
Carey says this looks very much like a volcanic cooling with associated crop failures. But he explains that one of the problems in identifying the responsible eruption is that there were a lot of active volcanoes in that period. "There were probably lots of eruptions that we don't know about" in historic times, he says.
Meanwhile, volcanologists have Pinatubo's climatic impact to study. M. Patrick McCormick, an atmospheric physicist at the NASA Langley Research Center in Hampton, Va., says, "It could be the biggest geophysical event of the century."
Dr. McCormick organized a special section of reports of early Pinatubo research in last January's issue of the journal Geophysical Research Letters. His enthusiasm for this research opportunity rests not just on the fact that the eruption was the largest in a populated area so far this century. He also notes that it was an intensively observed eruption - a fully known initial event - whose consequences scientists now can monitor in detail.
This challenges climate modelers such as James Hansen. It will take many years to see if their forecasts of a man-made global warming are correct. But the day of reckoning for predictions of Pinatubo's cooling is just a year or two away.
The prediction is complicated. Volcanically induced cooling may be offset by warming due to buildup of such heat-trapping "greenhouse" gases as carbon dioxide from burning fossil fuels. Also the anomalous warming of the eastern tropical Pacific Ocean called El Nino is under way. This has global weather effects that also tend to warm the planet.
As Hansen told a Senate hearing last month: "The El Nino and greenhouse gases are tugging the climate toward higher temperatures, while the volcanic aerosols try to cool the world. We expect aerosol cooling to dominate the next two years, but in any case we should learn things which improve our understanding and the models." In spite of such uncertainties, scientists have gained enough new insight into volcanism and climate change for the American Geophysical Union (AGU) - an earth-science professional s ociety - to issue a special report for the public last month.
This warns that we should not take the volcano-climate connection lightly. It notes: "Recent major eruptions have caused the average surface temperature to fall globally by 0.2 to 0.3 degrees C for one to three years. Even such a relatively minor drop could, however, translate into more severe local or regional temperature changes. Historically, such downturns in temperature have had catastrophic consequences for world food supply."
The report also notes that early research on the Pinatubo aerosol veil supports the suggestion of some atmospheric chemists that such veils enhance ozone destruction. Traveling in the same region as the protective ozone layer, the droplets provide sites for chemical reactions that release more of the ozone destroying chlorine from the man-made chlorofluorocarbons already present in the stratosphere. The report notes that, "although still needing confirmation," these preliminary studies "suggest that ozon e is being destroyed in conjunction with volcanic aerosols and at lower latitudes, not just near the poles."
Carey notes that, to assess the long term risk of volcanic-induced climate change and ozone destruction, scientists need a better knowledge of eruption rates. He adds that the starting point would be a comprehensive worldwide study of present volcanoes and ancient volcanic deposits. Such geological "history" has immediate environmental relevance.