About 33 million years ago, the Earth abruptly went from being warm and wet to having Antarctic ice cover.  Only 23 million years after the Paleocene-Eocene Thermal Maximum, a time of some of the warmest temperatures on Earth, ice covered the surface.  What happened?

According to a recent study by scientists at Yale and Purdue universities, the carbon dioxide level dropped. Carbon dioxide is a greenhouse gas that is contributing to the increased global temperatures on Earth today.

The scientists pinpointed the threshold for low levels of carbon dioxide below which an ice sheet forms at the South Pole. 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.

“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.”

Having an ice-covered South Pole appears to be the tipping point for cooling the rest of the planet.  The team found that the threshold level of carbon dioxide necessary for ice formation is about 600 parts per million.  For reference, today’s carbon dioxide level is approximately 390 parts per million.  This is why ice sheets still remain on Earth today.

With carbon dioxide levels forecast to rise to 550-1,000 parts per million in the next 100 years, when will the ice sheets completely melt away?  Because the melting of an ice sheet is different than starting an ice sheet, and because the process is not linear, scientists can’t say for sure.  But it’s clear that once the carbon dioxide levels rise high enough, the Earth will have reached a tipping point in the warming direction and the ice sheets will melt away.
Huber next plans to investigate the impact of an ice sheet on climate.

“It seems that the polar ice sheet shaped our modern climate, but we don’t have much hard data on the specifics of how,” he said. “It is important to know by how much it cools the planet and how much warmer the planet would get without an ice sheet.”

So how warm will Earth be in the future?  What’s the cooling impact of the ice?  Will greenhouse gases continue to rise?  Will increased cloud cover compensate for the lack of ice?

Explore how greenhouse gases and ice affect Earth’s temperature and learn more about feedback and tipping points in the High-Adventure Science climate investigation.

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