ANCIENT continents ripping apart might have turned the world into a giant ice sheet, according to a new climate model. The finding provides a simple mechanism for global cooling that would explain the controversial 鈥淪nowball Earth鈥� theory, which proposes that the oceans froze over at least twice between 550 and 800 million years ago.
The supercontinents from which the present continents arose had extremely dry interiors. Yannick Donnadieu at the French National Centre for Scientific Research (CNRS) in Gif-sur-Yvette and his colleagues found that the break-up of one such giant land mass 750 million years ago would have triggered extra rainfall in interior regions, causing the rocks to weather much faster.
According to their calculations, chemical reactions as the rocks weathered would have sucked massive amounts of carbon dioxide from the atmosphere, lowering its concentration in the air from 1830 parts per million to less than 250 ppm (Nature, vol 428, p 305). CO2 is a greenhouse gas that helps to trap heat in the atmosphere, and the removal of so much of it would have created a trigger for 鈥渋cehouse鈥� climate conditions.
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The researchers say their analysis is more accurate than past estimates of the effects of continental break-up. 鈥淯nlike previous models, we had nice tools to include variation for latitude and longitude, water cycle and weathering,鈥� says Donnadieu.
The new finding is a surprise, says Paul Hoffman of Harvard University, who proposed the Snowball Earth theory in 1999 with his colleague Daniel Schrag. He says weathering was thought to be a contributing factor in the proposed cooling, but considered to be self-limiting. 鈥淭he colder it gets, the less weathering you expect,鈥� he says. 鈥淏ut this model shows that, in the tropics, enough weathering continues to get a snowball from weathering alone.鈥�
But Nicholas Christie-Blick from Lamont-Doherty Earth Observatory in New York, a critic of the Snowball Earth idea, says that although the weathering theory seems to work well for the first snowball episode, it cannot explain the second one because there were no continents breaking up at that time. 鈥淢y inclination is to seek a phenomenon that applies to both events,鈥� he says.