Ice in Antarctica suddenly appeared — in geologic terms — about 35 million years ago. For the previous 100 million years the continent had been essentially ice-free.

The question for science has been, why? What triggered glaciers to form at the South Pole?

Matthew Huber, assistant professor of earth and atmospheric sciences at Purdue University, says no evidence of global cooling during the period had been found.

"Previous evidence points paradoxically to a stable climate at the same time this event, one of the biggest climate events in Earth's history, was happening," Huber says.

However, in a paper published this week in the journal Science, a team of researchers found evidence of widespread cooling. Additional computer modeling of the cooling suggests that the cooling was caused by a reduction of greenhouse gases in the atmosphere.

Even after the continent of Antarctica had drifted to near its present location, its climate was subtropical. Then, 35.5 million years ago, ice formed on Antarctica in about 100,000 years, which is an "overnight" shift in geological terms.

"Our studies show that just over thirty-five million years ago, 'poof,' there was an ice sheet where there had been subtropical temperatures before," Huber says. "Until now we haven't had much scientific information about what happened."

Before the cooling occurred at the end of the Eocene epoch, the Earth was warm and wet, and even the north and south poles experienced subtropical climates. The dinosaurs were long gone from the planet, but there were mammals and many reptiles and amphibians. Then, as the scientists say, poof, this warm wet world, which had existed for millions of years, dramatically changed. Temperatures fell dramatically, many species of mammals as well as most reptiles and amphibians became extinct, and Antarctica was covered in ice and sea levels fell.

History records this as the beginning of the Oligocene epoch, but the cause of the cooling has been the subject of scientific discussion and debate for many years.

The research team found before the event ocean surface temperatures near present-day Antarctica averaged 77 degrees Fahrenheit (25 degrees Celsius).

Mark Pagani, professor of geology and geophysics at Yale University, says the research found that air and ocean surface temperatures dropped as much as 18 degrees Fahrenheit during the transition.

"Previous reconstructions gave no evidence of high-latitude cooling," Pagani says. "Our data demonstrate a clear temperature drop in both hemispheres during this time."

The research team determined the temperatures of the Earth millions of years ago by using temperature "proxies," or clues. In this case, the geologic detectives looked for the presence of biochemical molecules, which were present in plankton that only lived at certain temperatures. The researchers looked for the temperature proxies in seabed cores collected by drilling in deep-ocean sediments and crusts from around the world.

"Before this work we knew little about the climate during the time when this ice sheet was forming," Huber says.

Once the team identified the global cooling, the next step was to find what caused it.

To find the result, Huber used modern climate modeling tools to look at the prehistoric climate. The models were run on a cluster-type supercomputer on Purdue's campus.

"That's what climate models are good for. They can give you plausible reasons for such an event," Huber says. "We found that the likely culprit was a major drop in greenhouse gases in the atmosphere, especially CO2. From the temperature data and existing proxy records indicating a sharp drop in CO2 near the Eocene-Oligocene boundary, we are establishing a link between the sea surface temperatures and the glaciation of Antarctica."

Huber says the modeling required an unusually large computing effort. Staff at Information Technology at Purdue assisted in the computing runs.

"My simulations produced 50 terabytes of data, which is about the amount of data you could store in 100 desktop computers. This represented 8,000 years of climate simulation," Huber says.

The computation required nearly 2 million computing hours over two years on Pete, Purdue's 664-CPU Linux cluster.

"This required running these simulations for a long time, which would not have been allowed at a national supercomputing center," Huber says. "Fortunately, we had the resources here on campus, and I was able to use Purdue's Pete to do the simulation."

Additional members of the research team included David Zinniker at Yale; Robert DeConto and Mark Leckie at the University of Massachusetts, Amherst; Henk Brinkhuis at Utrecht University (Netherlands); and Sunita R. Shah and Ann Pearson at Harvard University. Zhonghui Liu, an assistant professor at the University of Hong Kong and a former postdoctoral fellow of Pagani's at Yale, was the study's lead author.