Here's a possible scenario of global warming: if the West Antarctic Ice Sheet collapsed, it would raise sea levels substantially, and coastal cities around the world would gradually find themselves underwater. So scientists are understandably concerned about what's going to happen to Antarctic ice in the coming decades.

A new study takes a fresh look at the ultimate fate of this ice sheet. NPR's Richard Harris reports.

RICHARD HARRIS: Scientists have been worried about the West Antarctic Ice Sheet for decades, because a lot of it is resting on ground below sea level. If warm ocean water gets under it, the ice could start flowing off the continent and into the sea. And that ice, and ultimately water, would increase sea levels by a lot.

How much? Well, a landmark research paper published 30 years ago concluded that the vulnerable ice sheet contains enough water to raise global sea level by 20 feet.

Professor JONATHAN BAMBER (University of Bristol): The strange thing about that study is that nobody has really reevaluated the number since then.

HARRIS: Enter Jonathan Bamber from the University of Bristol in the U.K. He and his colleagues realize that there's a lot more information about Antarctica's geology today which could help them refine how much of that ice sheet is actually vulnerable to a runaway collapse. Using radar and gravity data, they've a much better idea of the shape of the ice cap and the shape of the rock below.

Prof. BAMBER: And what we found was that it's not the whole of West Antarctica. It's about 70 percent of the area that would actually melt away into the ocean, according to this hypothesis.

HARRIS: So Bamber's group now concludes in Science Magazine that if West Antarctica does collapse catastrophically, global sea level will eventually rise by about 11 feet, not 20.

And here's something odd: sea level doesn't go up the same everywhere. That's because the West Antarctic Ice Sheet weighs a lot, and that mass creates a gravitational pull that currently piles up ocean water in the Southern Ocean. If the ice sheet melts, the earth's gravity field will shift and water will flow north. So Bamber says sea level will rise more on some coastlines, less on others.

Prof. BAMBER: Global sea level rise is 11 feet. But in some places, it's about 25 percent more than that. And unfortunately, it looks like the Pacific and Atlantic seaboards, of North America at least, are areas where the relative change in sea level is going to be greater.

HARRIS: So in the long run, United States shorelines could retreat more than other coastlines elsewhere on the planet. But that's the long run. How much of that could show up in the coming decades?

Prof. BAMBER: That is the $64 million question.

HARRIS: And Bamber has no answer. Neither does any other glaciologist, including Robert Bindschadler at NASA's Goddard Space Flight Center. But what Bindschadler does say is he doesn't want the new research on the long-term fate of the ice sheet to distract people from the more immediate worries.

Dr. ROBERT BINDSCHADLER (Glaciologist, NASA Goddard Space Flight Center): Because it really doesn't change the big picture. Decision-makers, the public, they have to worry about this century, or maybe if they're really visionary, next century. And this paper doesn't change that picture one iota.

HARRIS: If the West Antarctic Ice Sheet does collapse, it's likely to take hundreds and hundreds of years. And Bindschadler says the global changes in gravity, which would redistribute ocean water around the world, could take even longer.

Mr. BINDSCHADLER: Those won't come into play for many centuries, if not millennia down the road.

HARRIS: And even then, Bindschadler can think of scenarios in which the West Antarctic Ice Sheet could still melt away entirely and still raise global sea level by 20 feet. Twenty feet, 11 feet - Jonathan Bamber agrees that in either case, we can't breathe easy.

Prof. BAMBER: Seventeen million people in Bangladesh alone would be displaced for about a four-foot sea level rise. So, yeah, I mean, 11 feet is just unthinkable.

HARRIS: And there are signs that we could be heading gradually in that direction. Two large streams of ice that flow from the Antarctic continent into the ocean have been speeding up for the past few years. And scientists are wondering if this could be the first signs of an ice sheet collapse.

Richard Harris, NPR News.

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