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Hurricane Sandy sparked a lot of interest in rising sea levels, as it swept across the Northeast. Over the next century, a lot of that extra water could come from melting ice in Antarctica and Greenland. Scientists have now developed a much clearer view of how quickly that ice has been melting over the past two decades.

And as NPR's Richard Harris reports, that's given them a better sense of how quickly sea level will rise in the years to come.

RICHARD HARRIS, BYLINE: There's enough ice sitting on Greenland and Antarctica to drive up sea level catastrophically, by more than 200 feet. Thankfully, nobody expects it all to melt. But over the past two decades, scientists have been struggling to understand just what is happening to all that ice.

Andy Shepherd is at the University of Leeds.

ANDY SHEPHERD: There are three different techniques. The oldest is called altimetry. It's the same sort of instrument that an airplane carries to tell how high it is above the ground.

HARRIS: Satellites also measure the flow of giant glaciers, as they stream toward the sea. And a third technique measures tiny changes in gravity, which is a way to measure the mass of ice sheets. But these three different techniques didn't provide one clear answer. So Shepherd brought together all the world's experts on this topic to resolve many of the discrepancies.

And it's a tricky business because ice isn't simply behaving as a monolith. For example, ice in some parts of Antarctica is growing because warmer air holds more moisture and leads to more snowfall. But other parts are melting, as warm sea water eats away at the edges.

SHEPHERD: It wasn't clear because of the uncertainty of the data whether the ice sheet was growing or shrinking. And now we're able to say with confidence, over all that time period, that it was shrinking.

HARRIS: Greenland's ice has also been shrinking. And when they put it all together, they found melt water from these huge ice sheets has increased sea level by a very modest half an inch over the past 20 years. They published the results in Science magazine. If you want to let out a sigh of relief, Shepherd says go ahead.

SHEPHERD: My job is to tell people good and bad news. And if the ice sheets have not made a huge contribution to sea level, then I think people ought to be happy about that.

HARRIS: But don't relax too much. Ian Joughin, a co-author of the paper at the University of Washington, says the melting of these giant ice sheets has accelerated rapidly in recent years, as the Earth as warmed.

IAN JOUGHIN: And that's a big cause for concern. What we really don't know is how much they're going to continue to go up.

HARRIS: Scientists are now feeling more confident about being able to measure change as it happens. But figuring out what the ice is going to do is tricky. The biggest wildcard is what will happen to some huge glaciers in Greenland and Antarctica, which in just a few years, have picked up a lot of speed as they have flowed toward the sea.

JOUGHIN: Unfortunately there's a lot we still we don't understand about the various processes that are controlling the fast ice flow, and how rapid the icebergs are discharged to the ocean.

HARRIS: These surges appear to be driven by ever-warming seawater, which is flowing under the edge of the ice sheets and eating away at them from below.

JOUGHIN: And in Antarctica, this is really most of the melting that goes on. And as climate warms and more warm water comes in contact with these ice shelves, it tends to thin them and potentially break them up.

HARRIS: The big question is how quickly this will happen in the coming decades. Scientists at one point thought the ice could melt catastrophically, adding more than ten feet to sea level in the course of a century or so.

Now, the experts in this field are thinking that sea level is much more likely to rise two, or maybe three feet, by the end of the century. That includes additional sea-level rise from melting mountain glaciers, and from the ocean itself, which expands as water heats up. And three feet, or a meter, is a lot.

Stefan Rahmstorf is at the Potsdam Institute for Climate Impact Research near Berlin.

STEFAN RAHMSTORF: What today is a once-in-a-century storm surge event in New York City would happen every three years if you had a one-meter higher sea level. So a one-meter sea level rise is huge.

HARRIS: And by no means the end of the story. As long as the Earth keeps warming, the seas will keep on rising.

Richard Harris, NPR News.

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