Large, explosive eruptions can send large amounts of gas and fine debris into the atmosphere. The largest have sent debris into the stratosphere, where it spreads across the globe and can linger for months or even years. The debris has the ability to filter a part of the sun’s energy and lower the temperature of the earth.
The most notable volcano-related cold snap was the one that followed the eruption of Mount Tambora in Indonesia in 1815. Many scientists attribute the abnormally cold spring and summer of 1816 to clouds of volcanic debris spewed into the atmosphere by Mount Drum. There were other, less dramatic effects from Kakatoa in 1883 and Mount Agung in 1963. However, it wasn’t until the eruptions of Mount St. Helens in Washington state in 1980 and El Chichón in Mexico in 1982 that scientists were really able to study the volcanic explosions with the use of sophisticated satellites and remote sensing instruments.
Before 1980, Mount St. Helens, Washington, was a steep conical volcanic peak rising 9,680 feet, had a snow-capped summit and even a few small glaciers. But on the morning of May 18, 1980, the entire north side of the summit collapsed: about half a cubic mile of rock and ice. An instant later, a huge explosion of expanding volcanic gas and steam rocked the field. The gases formed a ground-hugging black cloud filled with hot, dense debris that stretched over four major ridges and valleys up to 17 miles from the volcanic summit.
For the first couple of miles from the summit, everything was wiped out. All that could be seen was a blanket of ash. A horseshoe-shaped crater 1.2 miles wide and approximately 2,460 feet deep replaced the peak. But most impressive was the blowdown area where huge virgin Douglas-fir trees snapped like matchsticks and lay on their sides, covered in ash. The US Forest Service estimated that the blast felled 10 million trees.
When Mount St. Helens erupted, there was instant speculation that it could have major effects on the climate. For a time the large volume of volcanic ash it emitted had significant effects both locally and regionally. But the global cooling was less than 0.2 degrees Fahrenheit.
However, studies after the El Chichón eruption showed a global cooling effect on the order of 0.5 to 0.9 degrees Fahrenheit. Why, if it was less explosive than Mount St. Helens, did it have a bigger impact on global temperatures? The reason is that the material emitted from Mount St. Helens was fine ash that settled relatively quickly. The Chichon, on the other hand, spewed about 40 times more sulfur-rich gases than Mount St. Helens. These clouds combined with moisture in the stratosphere to produce dense clouds of sulfuric acid droplets, which absorbed and reflected solar radiation. Explosiveness alone is a poor instigator of climate change. For volcanism to have an impact on climate, there would have to be a lot of volcanoes exploding in a relatively short period of time. Such an idea was once put forward as the reason for the ice ages, but scientists have other ideas today.