In June 1991, Mount Pinatubo in the Philippines exploded, blasting millions of tons of ash and gas over 20 miles high – deep into the stratosphere, a stable layer of our atmosphere above most of the clouds and weather. Certain gases in the massive plume from this volcano acted like a sunshield by scattering some of the sun’s light, preventing it from reaching the surface and causing average surface temperatures to drop worldwide by an estimated 0.5 degrees Celsius (0.9 degrees Fahrenheit).
“We’ve been trying to better understand how volcanoes alter the climate for about 30 years now,” said Lori Glaze of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “The Mount Saint Helens eruption in 1980 (Washington state) and the El Chichon eruption in 1982 (Mexico) were both similar-sized eruptions. There wasn’t much of a climate effect after Mount Saint Helens, but after El Chichon, there was a big global cooling event for a couple years.”
“We didn’t understand why, so people started looking into that and it turned out that the El Chichon eruption included much more sulfur than Mount Saint Helens,” said Glaze.
The eruptions of El Chichon and Pinatubo were powerful enough to propel their gases into the stratosphere, which gave them the potential to alter short-term climate. “Since the stratosphere is stable, if gas in volcanic plumes gets into the stratosphere, it stays there for a long time – a couple years,” said Glaze. “Although there are many complications, the bottom line is that when these gases produce aerosols in the stratosphere, they scatter some of the sun’s radiation, which warms the stratosphere and causes a net cooling at the surface. The gas in these volcanic plumes (primarily sulfur dioxide, SO2, and hydrogen sulfide, H2S), which doesn’t come out in large amounts, reacts to form a layer of sulfuric acid, H2SO4, in the stratosphere. This layer scatters some of the sun’s infrared radiation.”
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