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After Krakatoa erupted in 1883, global temperatures dropped by about a degree well into the next century. For a long time, scientists figured that such massive quantities of ash in the atmosphere simply blocked out the sun’s rays.
And it’s easy to see why they believed this. An explosive eruption can release millions of tons of ash—tiny jagged particles of rock and glass—that linger in the atmosphere and on the ground for years. Ash can collapse roofs, damage machinery, pose a hazard to aircraft, and cause health problems.
 Global Cooling
But scientists today think the cause of cooling is actually a gas, a sulfur dioxide. High in the atmosphere, sulfur dioxide condenses into fine particles called aerosols that can linger for years and form a kind of screen around the earth. This screen cools the planet by reflecting some sunlight back into space that would otherwise shine on Earth’s surface.
The world witnessed the effect of aerosols on global cooling when the Philippines’s Mt. Pinatubo erupted in 1991. One of the largest eruptions since Krakatoa’s, it released more than 20 million tons of sulfur dioxide, cooling Earth’s temperature by almost a degree for at least two years.
Continue to Yellowstone’s Supervolcano. >>
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