The eruption of Mount Pinatubo resulted in the deaths of more than seven hundred people locally, and aerosols injected into the stratosphere affected both the earth’s ozone layer and global temperatures.
Mount Pinatubo lies 100 kilometers northwest of Manila in the Philippine Islands. Pinatubo erupted in about 1356, sending flows of hot ash down its slopes and growing to a height of 1,743 meters. By the time the Spanish conquistadores arrived in the islands, the volcano had been inactive for two centuries. To these early explorers, Mount Pinatubo was but one of two hundred dormant volcanos that dotted the land. Then, some 635 years after its last eruption, Mount Pinatubo began rumbling as lava rose beneath it.
Scientists set up a monitoring network and watched as the volcano’s internal pressures mounted. They used tiltmeters to detect bulging of the mountainside, seismometers to measure the small earthquakes caused by magma (molten rock) thrusting its way toward the surface, and gas detectors to measure the types and amounts of volcanic gases being released. Pinatubo thus became an important instructional tool concerning events that precede a major eruption. Further measurements made during and after the volcano’s eruption in 1991 have played an important role in the continuing debate over mathematical climate models, which predict greenhouse warming.
On April 2, 1991, an explosion within Mount Pinatubo blew open a geothermal area leaving a one-kilometer-long line of new fumaroles—holes that emit volcanic fumes and steam. Various minor quakes and explosions occurred with growing intensity and frequency, causing the local authorities to evacuate ever-larger regions. The U.S. government evacuated the dependents of military personnel from Clark Air Force Base, which lies approximately 20 kilometers east of Pinatubo’s summit.
More than two thousand minor quakes were recorded on June 7, as magma worked its way to the surface. Two days later, Pinatubo erupted, spewing forth lava and volcanic gases. By June 11, a large lava dome appeared near the summit, generally a sign that a large explosion is imminent. Three tremendous explosions rocked the volcano the next day. The first occurred early in the morning and lasted for fifteen minutes. The second, more powerful, blast occurred later that morning and sent a tephra (volcanic ash) column thundering 20 kilometers into the sky. The third blast occurred slightly before noon. Grapefruit-sized pumice stones peppered the countryside, including the Clark air base. Ash fell everywhere; it so thick in the air that day turned to night, and people had to cover their faces with cloth or plastic to keep the ash from getting into their eyes and noses.
Pyroclastic flows
Another major eruption occurred at Pinatubo on June 13, and by June 14, fifty-five thousand people had been evacuated from the villages within a 20-kilometer radius of the volcano. The volcanic activity then appeared to have subsided, but that afternoon the quiet was shattered by the largest eruption yet. A heavy rain was easily brushed aside by the ash plume, which shot upward at twice the speed of sound to a height of 30 kilometers. Pyroclastic flows again snaked down the mountain’s flanks, destroying villages and vegetation.
The next morning brought Pinatubo’s biggest eruption. On June 15, 1991, a gigantic explosion ripped the top from the mountain and thrust a plume of ash an astounding 40 kilometers into the sky. Ash darkened the sky in regions as far away as Manila, where it fell so thickly that drivers had to stop from time to time to brush it from their windshields. Earthquakes rocked the region as subterranean caverns, newly emptied of lava, collapsed. The eruption thundered on for fifteen hours. The situation was judged to be so dangerous at the Clark air base that an additional fifteen thousand Americans were evacuated. Ash collected on roofs and became heavy with water from the monsoon rains. Unable to bear the weight, buildings collapsed.
Pinatubo erupted intermittently throughout June and into July. Lahars
The region immediately surrounding Mount Pinatubo became a vast wasteland. Damage estimates were in the billions of dollars. Hundreds of square kilometers of cropland were destroyed. More than 108,000 houses were partially damaged or destroyed, leaving more than 200,000 people homeless. At least 722 people were killed: 281 in the initial eruption, 83 in lahars, and 358 as a result of exposure to diseases such as measles and pneumonia in the evacuation camps. Clark Air Force Base was ruined. Ironically, the U.S. and Philippine governments were engaged in negotiations over the future of the air base when the eruption of Pinatubo intervened and left little to negotiate. For those living near Mount Pinatubo, the eruption was a tragedy of the first magnitude.
The eruption of Pinatubo also had global physical consequences. Every eruption injected tremendous amounts of dust, water vapor, carbon dioxide, sulfur dioxide, and hydrochloric acid into the atmosphere. Each of these substances might be expected to affect climate locally or globally. The modern-day study of the effects of volcanoes on climate goes back to Benjamin Franklin, who described a peculiar fog that hung over northern Europe in 1783 and made the sun’s rays so faint that the summer was cooler than normal. Franklin suggested that this fog might be attributable to the volcanic eruption at Laki, Iceland. Franklin had no way to verify his conjecture, but Pinatubo’s investigators were able to monitor the dust cloud produced by the volcano from space. Dust can remain in the stratosphere for up to two years, and, by tracing its progress, scientists gained a better understanding of the dynamics of the stratosphere.
One of the most important challenges in environmental science is to predict the effects that human beings have on climate. In order to do this, scientists use mathematical models—that is, sets of physical laws expressed as mathematical equations. Herein lies the most important impact of Pinatubo’s eruption: It provided scientists with a way to see if any of the existing climate models were good enough to predict what actually happened.
This study is more than a matter of mere academic interest because of its significance in the understanding of the greenhouse effect.
James E. Hansen, the director of the Goddard Institute for Space Studies, galvanized the scientific and political community when he testified before the U.S. Congress in 1988 that global warming
What has been learned from the Pinatubo eruption that might clarify the issue of global warming? Although some results are encouraging, climate models are not yet good enough to forecast climate changes accurately. John R. Christy and his colleagues with the Earth System Science Laboratory at the University of Alabama began using satellites to monitor microwave emissions from the earth. With this information, they were able to obtain a truly global average of the temperature of the lower few kilometers of the atmosphere. Some climate models predicted that Pinatubo’s eruption would cool the earth by about 0.5 degrees Celsius for one or two years and that increased greenhouse gases would warm the earth about 0.4 degrees Celsius over ten years. Christy and his group found that Pinatubo cooled the earth by 0.65 degrees Celsius, in reasonable agreement with the prediction. Using fifteen years of data, however, they found little or no evidence for any increased greenhouse warming.
Even though Pinatubo released large amounts of carbon dioxide, the amount of carbon dioxide in the air actually decreased after the eruption, according to Charles D. Keeling, who operated the gas analyzer at Mauna Loa in Hawaii. It was speculated that the slight cooling of the earth caused by Pinatubo allowed more carbon dioxide to dissolve in the ocean.
Satellite observation showed that Pinatubo released twenty times more sulfur dioxide than would have been expected based on an analysis of the cooled lava using traditional techniques. The sulfur dioxide was ejected high into the stratosphere along with water vapor, where the two combined to form tiny droplets of sulfuric acid. A suspension of tiny particles in a gas is called an aerosol. The estimated 30 megatons, or 30 million metric tons, of sulfuric acid aerosol produced by Pinatubo played three important roles. First, it formed a haze in the stratosphere, which joined the volcanic dust in blocking sunlight to produce global cooling. Second, it eventually drifted down to the upper troposphere, where the sulfuric acid droplets acted as nucleation sites around which water droplets formed. This produced high cirrus clouds that again blocked sunlight and contributed to global cooling. Third, the sulfuric acid made the chlorine already in the stratosphere more active in destroying ozone. It is estimated that this caused an additional 20 percent loss in ozone over the poles. Apparently, the four megatons of hydrochloric acid produced by the volcano itself did not rise to the stratosphere but instead were quickly washed from the volcanic cloud by rain.
Scientists have learned a great deal by carefully monitoring the effects of the eruption of Mount Pinatubo. The better understanding of nature that such studies can achieve is expected to lead in turn to a better understanding of human effects on the environment.
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Abrahamson, Dean Edwin, ed. The Challenge of Global Warming. Washington, D.C.: Island Press, 1989. Collection of essays written for the layperson provides an overview of global warming, its causes, and its effects. Contributors are scientists writing in their areas of expertise. Includes an interesting chapter about possible actions to take and an extensive recommended reading list. -
Anderegg, C. R. The Ash Warriors. Washington, D.C.: U.S. Government Printing Office, 2005. Presents an account of Mount Pinatubo’s eruption and the heroic evacuation of Clark Air Force Base. -
Castro, Eddee Rh. Pinatubo: The Eruption of the Century. Quezon City, Philippines: Phoenix, 1991. Provides brief, accurate treatments of the eruption and its global effects as well as its local effects on the land and people. Includes more than one hundred photographs. -
Leggett, Jeremy. “Global Warming: The Worst Case.” Bulletin of the Atomic Scientists 48 (June, 1992): 28-33. Presents an overview of the problem and the worst-case consequences. Argues against taking speculative action. -
Seitz, Stefan. The Aeta at the Mt. Pinatubo, Philippines: A Minority Group Coping with Disaster. Translated by Michael Bletzer. Quezon City, Philippines: New Day, 2004. Details the evacuation of the aboriginal people of Mount Pinatubo, the group hardest hit by the eruption. -
Shimizu, Hiromu. The Orphans of Pinatubo. Manila: Solidaridad, 2001. Presents stories from the Aeta of the eruption and life following the event. -
Simkin, Tom, and Richard S. Fiske, eds. Krakatau 1883: The Volcanic Eruption and Its Effects. Washington, D.C.: Smithsonian Institution Press, 1983. Contains eyewitness accounts and various studies of the eruption and its climatic and other effects. Features Benjamin Franklin’s comments on the climatic effects of an Icelandic volcano. Includes pictures, maps, figures, bibliography, and index. -
Singer, S. Fred. “Warming Theories Need Warning Label.” Bulletin of the Atomic Scientists 48 (June, 1992): 34-39. Discusses the great uncertainties involved in predictions of global warming and recommends further study of the issue. -
_______, ed. Global Climate Change: Human and Natural Influences. New York: Paragon House, 1989. Collection of essays by twenty-one scientists addresses many topics related to climate change, including the atmosphere, the oceans, nuclear winter, asteroid impacts, and volcanoes. More cautious and optimistic than many other works about global warming. Intended for readers with at least some background in the physical sciences.
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