The study, which will be published Friday in the journal Science by researchers from Oregon State University, shows that ice sheets can react quite differently depending on the climatic conditions at the time global warming occurs � sometimes actually growing larger and sometimes rapidly disappearing, depending on whether increased snow offsets melting effects, or not.

In this analysis of the Scandinavian Ice Sheet that existed as recently as 10,000 years ago, the study showed that it actually grew for a long period while the climate was warming but still very cold, and then rapidly disintegrated once the climate warmed even further.

OSU experts say those same forces are at work today, and probably mean that in the face of future global warming, the ice in large parts of Antarctica may actually increase, while the massive Greenland ice sheet � which exists in a slightly warmer setting � will almost certainly disappear.

"This study supports what we've been learning about the Greenland ice sheet, which is that it will completely melt within 500 to 1,000 years," said Peter Clark, a professor of geosciences at OSU and an international expert in the study of ancient ice sheets. "Our new analysis of the ancient Scandinavian Ice Sheet, like other studies, is showing how these events unfolded in the past, which will help us better understand what the future will hold."

If there is a net gain in ice in Antarctica, it will probably not be enough to offset the melting of the Greenland ice sheet, scientists say. By itself and without any offsetting mechanisms, a collapse of the Greenland ice sheet would raise sea levels by about 20-25 feet. One prediction is that sea levels should increase by a foot or two by 2100, and up to 25 feet within 500 years. Some of that sea level rise is based on melting of glaciers and major ice sheets, and some is based on thermal expansion of water in the oceans, which increases in size as it gets warmer.

In their study, the OSU researchers used a fairly new and sophisticated technique to date the time that rocks have been exposed at the surface, after they had been dumped in the open by an ice sheet. This technique, which measures isotopes of the element beryllium, allows scientists to determine with much greater accuracy how ice sheets may have moved in the distant past.

"On exposure to cosmic rays from the atmosphere, beryllium isotopes are produced at a known rate and we can use that to understand ice movements at the time," said Ed Brook, a professor of geosciences at OSU and an expert in this dating technique. "It's like a rock getting a very slow suntan, and we can learn a lot from it."

The huge ice sheet the OSU scientists studied once covered much of Northern Europe, including Poland, Estonia, Latvia, Lithuania, Belarus and Finland, where the study was conducted. At its peak it was about 6,000 feet thick and, after the massive ice sheet in North America, was the largest in the Northern Hemisphere. In the most recent Ice Age it existed from 10,000 to about 100,000 years ago, and its melting occurred at the same time as a major sea level rise. But only with this study has its behavior been analyzed in detail.

It appears this ice sheet grew and retreated three different times during this Ice Age. Of particular interest is that it behaved differently depending on whether climate conditions were cold or warmer. Researchers were able to correlate information about climate, largely obtained from ice cores drilled in Greenland, with data about sea level conditions and the ice sheet behavior.

"This study clearly showed that when the climate was warming but still pretty cold, the ice sheet grew due to increased precipitation that fell as snow, and more than made up for any melting," Clark said. "But at some point the warming became more pronounced, did not offset any increases in snow, and the ice sheet disappeared fairly rapidly."

Scientists believe those exact forces may both be displayed as the Earth warms due to the greenhouse effect � there may be some increase in the East Antarctic ice sheet, where the climate is much colder, and rapid loss of ice in Greenland. Of less certainty,

Brook said, is the fate of the West Antarctic ice sheet, whose bottom is actually below sea level and may be at risk of collapse in a warming climate.

Also uncertain, the scientists said, is what effects the rapid melting of Greenland's ice sheet may have on ocean currents. There is significant concern that the rapid injection of large amounts of fresh water into this part of the North Atlantic Ocean may interfere with a major "thermohaline circulation" pattern that is responsible for the currents that warm much of Europe. Lacking those currents, much of Europe's climate would more closely resemble that of central Canada than the comparatively mild climate it currently has. It appears this current pattern has shut down several times in the distant past.

Much of the work on this study was done by Vincent Rinterknecht, an OSU doctoral student in geosciences at the time of the research and now a visiting scientist at the Lamont-Doherty Earth Observatory at Columbia University.

These studies are confirming that the melting of ice sheets and their associated effects can be comparatively abrupt and rapid, the researchers said, once a certain level of warming is reached.

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