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First, the bad news that we've been hearing for years. The global sea level could rise by 15 to 20 feet, flooding the world's coastal cities if a giant Antarctic ice sheet melts away. But maybe this will make you feel a bit better: we will all be long, long gone before it could happen. A new study published in Nature magazine predicts the conditions that could trigger that ice sheet to melt. And here's NPR's Richard Harris with the story.

RICHARD HARRIS: You could call the West Antarctic ice sheet the planet's Achilles' heel. It holds a vast amount of water, locked up as ice, and it's sitting below sea level, so it's inherently unstable.

David Pollard at Penn State University says there's been intense research recently to figure out how the ice sheet has behaved over the past five million years.

Dr. DAVID POLLARD (Geosciences, Penn State University): Before there was only a vague idea of how the West Antarctic ice sheet grew and decayed over those time scales.

HARRIS: Now, a scientific drilling project has brought back sediment samples taken from underneath the ice. That allows scientists to study the mud layers, like so many tree rings, to show what ice there has done over history.

Dr. POLLARD: It's really exciting. They've shown that it really has collapsed and re-grown, multiple times.

HARRIS: Pollard and a colleague have taken that detailed information and asked what it portends for the future of the West Antarctic ice sheet.

Dr. POLLARD: The main reason that it collapsed in the past is the ocean has got warmer around the periphery of Antarctica, increasing the rate of melting of these floating ice shelves which fringe West Antarctica.

HARRIS: These floating ice shelves act like buttresses to keep the much larger ice sheet pinned back. And whenever the shelves melt away, the ice behind them flows into the sea and sea levels rise.

Warming the ocean water around Antarctica by maybe two to five degrees Celsius could trigger that chain of events again, Pollard says. That degree of warming's not specifically forecast for this century, but at the rate we're heating up the planet, it seems inevitable at some point.

The only bright spot in Pollard's study is that the west Antarctic ice sheet won't melt away too rapidly.

Dr. POLLARD: It takes about 2,000 to 3,000 years to melt most of it.

HARRIS: So, do you feel more reassured or more nervous after having done this analysis?

Dr. POLLARD: I'd say I feel more nervous.

HARRIS: That's because there's now a clear history showing that this massive ice sheet has melted before, under conditions that the Earth may soon again experience. And while the full effect may not unfold for thousands of years, it would transform the planet into a place we would not recognize today.

Stefan Rahmstorf at Potsdam University in Germany says there are still so many unknowns about how Antarctic ice behaves that Pollard's study is surely not the final word on this subject.

Dr. STEFAN RAHMSTORF (Potsdam University): We certainly don't need a collapse of the ice sheet to cause major problems with sea level rise.

HARRIS: Even if Antarctica contributes little water to the oceans this century, Rahmstorf says, that as the seas warm ocean water will expand and there's a growing consensus that seas could rise by at least two or three feet — and quite possibly more — before the end of this century.

Dr. RAHMSTORF: Unless of course we stop the global warming fairly soon.

HARRIS: Rahmstorf says Europe's global warming policy at the moment is based on a goal to limit global warming to a maximum increase of two degrees Celsius, about four degrees Fahrenheit.

Dr. RAHMSTORF: Just like we have this temperature limit, we should also have a sea level limit.

HARRIS: He advocates setting that at one meter, about three feet, of sea level rise and no higher even in the coming centuries. Even if that can be accomplished, many vulnerable low-lying places on earth would be swamped, Rahmstorf acknowledges. But it's hard to imagine doing any better. And, as the unstable ice of west Antarctica reminds us, we could easily do much, much worse.

Richard Harris, NPR News.

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