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Alan Alda in Scientific American Frontiers

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Rapid Reversal

photo of volcano
Carbon dioxide belched from ancient volcanos helped to rewarm the planet  

Despite the abundance of geologic evidence, scientists remained skeptical of the Snowball Earth hypothesis. How could the snowball ever have melted, they wondered. Modern climate modeling provided an answer. The processes that normally remove carbon dioxide from the atmosphere, such as rain and photosynthesis, would have been suspended during the Snowball years. The gas would have simply built up until the notorious "greenhouse effect" kicked in, trapping energy from the sun and warming the planet.

The Namibian sediments support the idea that the Earth warmed up just a quickly as it cooled. Hundreds of meters of limestone sit directly on top of the glacial deposits. Since limestone forms only in warm water, the juxtaposition of these two sediments implies a global temperature swing from icy to tropical in a very short period of time- probably a few years, according to Schrag.

"Literally, all Hell broke loose," says Schrag. "The global temperature would have averaged higher than 90 degrees Fahrenheit. Today, it's in the 50's."

banded iron formation photo
  This banded iron formation provides evidence of warm temperature immediately following glaciation

Iron formations in the glacial deposits provide still more evidence of this rapid climatic shift. The tropical glacial deposits contain thick layers of iron, called banded iron formations, which only occur when extremely iron-rich seawater is suddenly exposed to oxygen. Normally, atmospheric oxygen prevents iron from building up in the world's oceans. But Hoffman and Schrag suspect that sheets of ice insulated the ocean from the atmosphere for millions of years, allowing iron levels to skyrocket. The banded iron formations found in the glacial deposits mark the geologic moment when the ice melted and the sea met the sky.

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