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http://www.sciencedaily.com/releases/2011/12/111201174225.htm

ScienceDaily (Dec. 1, 2011) — A drop in carbon dioxide appears to be the driving force that led to the Antarctic ice sheet's formation, according to a recent study led by scientists at Yale and Purdue universities of molecules from ancient algae found in deep-sea core samples.

The key role of the greenhouse gas in one of the biggest climate events in Earth's history supports carbon dioxide's importance in past climate change and implicates it as a significant force in present and future climate.

The team pinpointed a threshold for low levels of carbon dioxide below which an ice sheet forms in the South Pole, but how much the greenhouse gas must increase before the ice sheet melts -- which is the relevant question for the future -- remains a mystery.

Matthew Huber, a professor of earth and atmospheric sciences at Purdue, said roughly a 40 percent decrease in carbon dioxide occurred prior to and during the rapid formation of a mile-thick ice sheet over the Antarctic approximately 34 million years ago.

A paper detailing the results was published on Dec. 1 in the journal Science.

"The evidence falls in line with what we would expect if carbon dioxide is the main dial that governs global climate; if we crank it up or down there are dramatic changes," Huber said. "We went from a warm world without ice to a cooler world with an ice sheet overnight, in geologic terms, because of fluctuations in carbon dioxide levels."

For 100 million years prior to the cooling, which occurred at the end of the Eocene epoch, Earth was warm and wet. Mammals and even reptiles and amphibians inhabited the North and South poles, which then had subtropical climates. Then, over a span of about 100,000 years, temperatures fell dramatically, many species of animals became extinct, ice covered Antarctica and sea levels fell as the Oligocene epoch began.

Mark Pagani, the Yale geochemist who led the study, said polar ice sheets and sea ice exert a strong control on modern climate, influencing the global circulation of warm and cold air masses, precipitation patterns and wind strengths, and regulating global and regional temperature variability.

"The onset of Antarctic ice is the mother of all climate 'tipping points,'" he said. "Recognizing the primary role carbon dioxide change played in altering global climate is a fundamentally important observation."

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The collaboration built on Huber's previous work using the National Center for Atmospheric Research Community Climate System Model 3, one of the same models used to predict future climates, and used the UVic Earth System Climate Model developed at the University of Victoria, British Columbia.

"The models got it just about right and provided results that matched the information obtained from the core samples," he said. "This was an important validation of the models. If they are able to produce results that match the past, then we can have more confidence in their ability to predict future scenarios."

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