Pinatubo lives – volcano blamed for global temperature drop, ozone depletion – 1993 – The Year in Science
SOMETHING STRANGE WAS IN TH AIR IN 1993. IN JANUARY THE AVERAGE WORLDWIDE LEVEL OF OZONE IN THE STRATOSPHERE WAS IN A RECORD-BREAKING SLUMP; IN THE SUMMER NORTHERN HEMISPHERE levels reached new regional lows; in September the notorious ozone hole over Antarctica returned, nearly twice as deep as ever before. And although carbon dioxide continued to accumulate in th atmosphere, aggravating the greenhouse effect, temperatures were actually cooler than normal in the summer, particularly in the interiors of continents–which may or may not have helped precipitate the Midwest’s disastrous floods, depending on whom you talk to.
What’s going on here? Many researchers strongly suspect that the record ozone losss and the surprising coolness share a common cause: the eruption of Mount Pinatubo in the Philippines in June 1991.
Mount Pinatubo was the largest eruption of the century, launching more than 30 trillion pounds of rock, dust liquid, and gas into the air. Its majestic plume disappeared in a matter of days, and the beautiful sunsets its dust created were gone in a matter of months. But Pinatubo also left behind 40 billion pounds of sulfur dioxide gas, which rose to the strastosphere immediately after the explosion. There it reacted with oxygen and water vapor to form a tenacious haze of sulfuric acid droplets. Although invisible to the naked eye, 30 percent of this mist was still hanging in the air this past fall.
Volcanic clouds of sulfuric acid act as a planetary shade: they bounce sunlight back into space and thereby cool the planet. Just how much they cool it has been hard to determine; unless an eruption is truly titanic, its effect gets lost in the constant fluctuations of Earth’s weather patterns. Pinatubo, however, had the necessary might. And that was a boon for climatologists, who were able to test their predictions of global warming by seeing how well the same climate models could predict the global cooling induced by the volcano.
James Hansen and his colleagues at NASA’s Goddard Institute for Space Studies in New York published their forecast six monts after the eruption. The sulfuric acid cloud was so big, they said, that the shadow it cast would do more than just block out any warming created by humans–it would caus a worldwide slide in temperature of about one degree by the winter of 1992. They said that 1993 would see a slow warming, and by the mid-nineties the man-made greenhouse effect would be back in full swing.
With two years of real temperature readings to look at, Hansen has reason to be proud of his divination. The planet did indeed reach its coolest point late in 1992, and last year saw a gradual recovery. “It was a nice check on the climate’s response,” says Hansen. The model also accurately forecast that summers would be noticeably cool in the interior portions of the continents–and that’s what happened in both 1992 and 1993, particularly in the American Midwest. The interior heats up quickly under the summer sun, being so far from the moderating influence of the oceans, and Hansen predicted that it would also cool down quickly under Pinatubo’s veil.
Could that cooling have had anything to do with the summer’s tremendous Mississippi floods? “It’s very plausible that you’d have a relation,” says Hansen. The cooling may have helped create a succession of cold fronts that blocked the normal west-to-east movement of rainstorms last year, dumping all the rain in the Midwest and leaving the East with a drought. Hansen, however, doesn’t trust his current model enough to speculate further. “It’ll require a year or so before we can say to what degree Pinatubo was involved,” he says.
Pinatubo’s effect on the ozone layer is equally hazy. Indeed, the relationship between volcanos and ozone in general is complicated. Among the main ozone destroyers, we’ve been told, are man-made chlorofluorocarbons (CFCs) that are carried to the stratosphere. There the chlorine is released, and in the presence of ultraviolet sunlight it reacts with the three oxygen atoms that make up a molecule of ozone, breaking the bonds between the atoms and grabbing onto them so they can’t reunite. yet CFCs aren’t the only source of chlorine: Pinatubo released 4 billion pounds when it erupted. This fact has caused some people to decry the CFC-ozone line as a hoax: if it were true, they argue, volcanic eruptions should long ago have wiped out the ozone layr.
The reason this hasn’t happened, according to Azadeh Tabazadeh and Richard Turco, both atmospheric scientists at UCLA, is because volcanoes as a rule barely raise the level of chlorine in the stratosphere. Volcanoes, they explained in a paper published this past year, release chlorine in the form of hydrochloric acid–and hydrochloric acid, unlike CFCs, is soluble in water. The inside of the typical volcanic plume is filled with condensing drops of water. Tabazadeh and Turco constructed a chemical model showing that these drops quickly absorb the hydrochloric acid and fall to the ground. The part of the plume that reaches the stratosphere is thus scrubbed clean of chlorine.
YET VOLCANOES ARE NOT Innocent bystanders in the ozone depletion plot. Though they may not be putting a significant amount of chlorine into the stratosphere, they may be helping to create conditions similar to those found over the South Pole, where man-made CFCs are most effective at destroying ozone. For reasons that are still not well understood, the reactions that liberate chlorine from the CFC molecules happen extremely quickly on the surfaces of ice crystals that form in polar stratospheric clouds. But in 1989 David Hofmann and Susan Solomon of the National Oceanic and Atmospheric Administration showed that the same speedup of chlorine liberation that occurs on the ice cystals can theoretically occur too on the surface of sulfuric acid droplets–such as the ones that have been shading the planet since Pinatubo erupted. And while the ice crystals are largely confined to the poles, the sulfuric acid droplets can help set chlorine free at lower latitudes, destroying ozone and exposing much more populated regions of the Earth to higher levels of ultraviolet radiation.
After Pinatubo’s eruption, ozone concentrations in the stratospher did drop at an unusually fast clip. In the summer the United States experienced an ozone layer 10 percent thinner than normal, which translates into a 20 to 30 percent increase in the type of ultraviolet radiation that causes skin cancer. Over parts of Russia, ozone concentrations were 25 percent below normal. Solomon and Hofmann say satellite data show that the sulfuric acid reactions they predicted were indeed happening–but something else was happening, too. Because their scenario works best at cold temperatures, they thought it would occur mostly at the higher latitudes (although still far from the poles). Yet in 1993 ozone levels declined over most of the globe. “It’s clear that chemistry is contributing to what’s going on,” says Solomon, but it may not be the only factor.
It’s also possible, she says, that Pinatubo is affecting ozone levels in some places by changing the circulation of the stratosphere. Ozone is produced primarily at lower latitudes, and normally it gets distributed around the planet on stratospheric winds: warm air rises in the tropics and flows toward the poles, cooling and sinking to the lower stratosphere as it goes. Though Pinatubo’s haze reflects sunlight away from Earth, it also absorbs heat rising from its surface, warming the air around it in the stratosphere. Air that might normally sink as it cooled would stay at higher altitudes for longer periods of time, and the whole stratospheric circulation would slow down. As a result, less ozone would be carried to the middle and high latitudes.
What looks like ozone destruction in those regions, then, could be in part just a distribution failure. “It’s a little like waking up in the morning and the milk isn’t thee,” says Solomon. “Did somebody take the milk away? Or did the milkman fail to deliver the milk?” She and other researchers will be watching carefully as Pinatubo’s veil continues to thin next year; the speed at which the ozone layer recovers will tell a lot about which theory of ozone loss is more correct, or how the two mechanisms might operate in concert.
Though Pinatubo’s effects will soon be gone, atmospheric researchers hope they aren’t forgotten. Hansen believes that Pinatubo provided compelling evidence that predictions of global warming have to be taken seriously, since they’re based on the same models that so accurately foretold Pinatubo’s cooling effects. And as long as there are abnormal levels of man-made chlorine in the atmosphere–which will be for 50 years or more, given current international efforts to curb CFCs–volcanoes will continue to pose a threat to the ozone layer. “Pinatubo may have been the volcano of the century, but it was nothing like the volcano of the millennium,” says Solomon. Tambora, an Indonesian volcano, erupted in 1815, releasing a plume roughly ten times the size of Pinatubo’s. “If we’re getting 10 or 15 percent ozone depletion over the Northern Hemisphere now, what would have happened if we’d had a Tambora? And there’s no guarantee that we’re not going to have Tambora tomorrow.”
COPYRIGHT 1994 Discover
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