MORE ON VOLCANOES

"Volcanoes erupt all the time, all over the world, but truly 'big-ass' ones are rare. If they weren't - well, we probably wouldn't be around to worry about global warming. The anthropologist Stanley Ambrose has argued that a supervolcanic explosion at Lake Toba on Sumatra, roughly seventy thousand years ago, blocked the sun so badly that it triggered an ice age that nearly wiped out Homo sapiens. What distinguishes a big-ass volcano isn't just how much stuff it ejaculates, but where the ejaculate goes. The typical volcano sends sulfur dioxide into the troposphere, the atmospheric layer closest to the earth's surface. This is similar to what a coal-burning power plant does with its sulfur emissions. In both cases, the gas stays in the sky only a week or so before falling back to the ground as acid rain, generally within a few hundred miles of its origin. But a big volcano shoots sulfur dioxide far higher, into the stratosphere. That's the layer that begins at about seven miles above the earth's surface, or six miles at the poles. Above that threshold altitude, there is a drastic change in a variety of atmospheric phenomena. The sulfur dioxide, rather than quickly returning to the earth's surface, absorbs stratospheric water vapor and forms an aerosol cloud that circulates rapidly, blanketing most of the globe. In the stratosphere, sulfur dioxide can linger for a year or more, and will thereby affect the global climate.” – Steven D. Levitt and Stephen J. Dubner – Superfreakonomics

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