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Unformatted text preview: 68 Scientific American January 2000 Snowball Earth O ur human ancestors had it rough. Saber-toothed cats and woolly mammoths may have been day- to-day concerns, but harsh climate was a consum- ing long-term challenge. During the past million years, they faced one ice age after another. At the height of the last icy episode, 20,000 years ago, glaciers more than two kilometers thick gripped much of North America and Europe. The chill delivered ice as far south as New York City. Dramatic as it may seem, this extreme climate change pales in comparison to the catastrophic events that some of our ear- liest microscopic ancestors endured around 600 million years ago. Just before the appearance of recognizable animal life, in a time period known as the Neoproterozoic, an ice age pre- vailed with such intensity that even the tropics froze over. Imagine the earth hurtling through space like a cosmic snow- ball for 10 million years or more. Heat escaping from the molten core prevents the oceans from freezing to the bottom, but ice grows a kilometer thick in the 50 degree Celsius cold. All but a tiny fraction of the planets primitive organisms die. Aside from grinding glaciers and groaning sea ice, the only stir comes from a smattering of volcanoes forcing their hot heads above the frigid surface. Although it seems the planet might never wake from its cryogenic slumber, the volcanoes slowly manufacture an escape from the chill: carbon dioxide. With the chemical cycles that normally consume carbon dioxide halted by the frost, the gas accumulates to record lev- els. The heat-trapping capacity of carbon dioxidea green- house gaswarms the planet and begins to melt the ice. The thaw takes only a few hundred years, but a new problem arises in the meantime: a brutal greenhouse effect. Any crea- tures that survived the icehouse must now endure a hothouse. As improbable as it may sound, we see clear evidence that this striking climate reversalthe most extreme imaginable on this planethappened as many as four times between 750 million and 580 million years ago. Scientists long presumed that the earths climate was never so severe; such intense cli- mate change has been more widely accepted for other planets such as Venus [see Global Climate Change on Venus, by Snowball Earth Snowball Earth Ice entombed our planet hundreds of millions of years ago, and complex animals evolved in the greenhouse heat wave that followed by Paul F. Hoffman and Daniel P. Schrag by Paul F. Hoffman and Daniel P. Schrag Ice entombed our planet hundreds of millions of years ago, and complex animals evolved in the greenhouse heat wave that followed Copyright 1999 Scientific American, Inc. Mark A. Bullock and David H. Grinspoon; Scientific American, March 1999]. Hints of a harsh past on the earth began cropping up in the early 1960s, but we and our col- leagues have found new evidence in the past eight years that has helped us weave a more explicit tale that is capturing the...
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This note was uploaded on 02/08/2011 for the course EAS 1601 taught by Professor Lynch during the Spring '08 term at Georgia Institute of Technology.
- Spring '08