Mount Pinatubo eruption and its effect on global climate

The Mount Pinatubo eruption, which occurred on June 15, 1991, is notable for being one of the largest volcanic eruptions of the 20th century. Located in the Philippine Islands, the eruption released approximately 10 billion metric tons of magma and around 20 million metric tons of sulfur dioxide into the atmosphere, creating a significant volcanic ash cloud that affected a vast area. This eruption had profound implications for global climate, as the sulfur dioxide injected into the stratosphere contributed to a temporary cooling of the Earth's surface by about 0.5° Celsius, altering weather patterns and reversing some effects of global warming for several years.

The sulfur dioxide circled the globe in about 22 days and contributed to the formation of sulfuric acid, which remained in the atmosphere for over a year, reducing the heat absorbed from the sun. This cooling also affected ocean temperatures and precipitation patterns, leading to a decline in evaporation and a slowdown in sea level rise. Additionally, the eruption released chlorine species that caused a significant reduction in atmospheric ozone, particularly in tropical regions. These complex interactions underscore the potential for large volcanic eruptions to influence global climate dynamics significantly.

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Definition

Mount Pinatubo is an explosive volcano located near the tropics—at 15.1 degrees north latitude and 120.4 degrees east longitude—in the Philippine Islands in the western Pacific Ocean. The volcano erupted on June 15, 1991, producing the second-largest eruption of volcanic material in the twentieth century. About 10 billion metric tons (11 US tons) of magma—molten rock plus suspended crystals—were brought to the surface, along with abundant volcanic gases, including about 20 million metric tons (22 US tons) of sulfur dioxide. This was the largest amount of sulfur dioxide injected into the atmosphere since the eruption of the Krakatoa (Krakatau) volcano near Java in 1883.

Part of the hot gas, volcanic ash, and larger rock fragments spewed by the eruption tore down the valleys of Mount Pinatubo as pyroclastic flows. Great quantities of volcanic ash and gases rose soon after the eruption to heights of over 34 kilometers (21.1 miles) into the atmosphere. Much of the island of Luzon was completely dark during the day, as the dense ash cloud covered around 125,000 square kilometers (77,671 square miles) around the volcano. Volcanic ash covered everything, and many people died when their roofs collapsed under the weight of the ash. Much of the ash became saturated with water from a nearby typhoon that produced large volcanic mudflows. Many previously abundant organisms, such as foraminifera, were greatly depleted in the nearby oceans after the eruption.

Significance for Climate Change

Mount Pinatubo's eruption gave evidence as to how a big volcanic eruption at equatorial latitudes can change the climate. Larger volcanic eruptions that occurred through geologic time should have had an even more drastic effect on climate than Mount Pinatubo. Based on this eruption, volcanic ash quickly settles to the ground, so its effect on the climate is short-lived. Only large eruptions such as that of Mount Pinatubo can eject volcanic gases—mostly water vapor, carbon dioxide, and sulfur dioxide—into the stratosphere to affect the climate for more than a few days. The sulfur dioxide gas ejected from Mount Pinatubo circled the globe in twenty-two days. Sulfur dioxide gas ejected from erupting volcanoes at higher latitudes generally takes less time to circle the Earth.

Sulfur dioxide injected into the atmosphere by the eruption was rapidly oxidized to sulfuric acid and mixed with water vapor. This reduced the heat absorbed by the atmosphere from the Sun by about 10 percent. Much of the sulfuric acid stayed in the atmosphere for over one year and reduced the average temperature close to the Earth’s surface by about 0.5 degrees Celsius (32.9 degrees Fahrenheit).

This cooling reversed the trend of global warming for several years after the eruption of Mount Pinatubo. For example, the ice sheet in Greenland did not melt as much as usual during this time. This cooling was not uniform, however, as parts of North America, Siberia, and Europe experienced higher-than-normal temperatures during this time. The warming trend in those regions was due to circulation changes in the atmosphere that are not completely understood, although several climate models explaining these changes were successful.

The generally cooling temperature of the lower atmosphere reduced the temperature of the ocean at the surface by about 0.4 degrees Celsius (37.72 degrees Fahrenheit) for several years after the eruption, especially at midlatitudes. This cooling slightly reduced the evaporation rate of the ocean, so there was, on average, less precipitation on the land’s surface. The slow rise in sea level that occurred before the eruption was somewhat reduced as well, presumably because there was less evaporation from the oceans and less melting of the glaciers.

Various species of chlorine (such as Cl, ClO, HCl, HOCl, and ClONO2), bromine, and iodine catalyze the removal of ozone in the stratosphere. The eruption of Mount Pinatubo liberated a large amount of chlorine species. Studies of satellite and ground-based data show that tropical ozone declined by 6 to 8 percent overall, with localized depletion reaching about 20 percent in the stratospheric layer between 24 and 25 kilometers (14.9 and 15.5 miles) in altitude. Globally, ozone levels fell to nearly 5 percent below the long-term average, with volcanic aerosols alone accounting for roughly 2 percent of the decrease.

In Antarctica, the effects were especially dramatic. The combination of volcanic aerosols and naturally occurring polar stratospheric clouds created ideal conditions for chlorine- and bromine-driven reactions that rapidly destroyed ozone. This resulted in the deepest and widest Antarctic ozone hole observed up to that time, with the hole expanding earlier in the season and persisting longer than in previous years. The depletion extended over an area larger than the entire continent of North America, with some regions of the Antarctic stratosphere experiencing near-total loss of ozone. These losses were temporary. As the volcanic particles slowly settled out of the atmosphere over the course of two to three years, the chemical conditions driving the extreme depletion diminished, and by the mid-1990s had returned to previously expected long-term patterns.


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