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ROBERT SIEGEL, HOST:

From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel.

AUDIE CORNISH, HOST:

And I'm Audie Cornish. Greenland's glaciers hold enough water to raise sea level by 20 feet and, as the planet warms, they are melting. But it's been hard to figure out just how fast they're melting and how quickly sea level will rise.

Well, a team of researchers has been trying to do just that and, as NPR's Richard Harris reports, they've released a study that offers some reassuring news.

RICHARD HARRIS, BYLINE: A few years ago, the Jakobshavn glacier in Greenland really caught people's attention. In short order, this slow moving stream of ice suddenly doubled its speed. It starting dumping a whole lot more ice into the Atlantic. Other glaciers also sped up.

For a glacier scientist like Ian Joughin at the University of Washington, the question was whether this was start of a rapid meltdown of Greenland glaciers.

IAN JOUGHIN: Some people fear that, if they could double their speed over two or three years, they could keep doubling and doubling and doubling and reach very fast speeds.

HARRIS: If the world's big glaciers were on their way to a tenfold speedup, that could lead to a staggering six feet of sea level rise by the end of this century, so Joughin and colleagues have been trying to see if that's started. They pored over radar images of 200 Greenland glaciers gathered over the past decade.

Graduate student Twila Moon says some of Greenland's glaciers picked up the pace and starting surging forward more than five miles in a year.

TWILA MOON: It turns out that a glacial pace isn't very slow.

HARRIS: It also turns out that glacial pace isn't consistent. The glaciers like Jakobshavn that started surging haven't kept on picking up speed. In fact, some have slowed down, and glacier speeds vary dramatically. Picture streams of ice that start out as one giant river and then split into two on their way toward the sea.

MOON: We saw cases where one of those might be consistently speeding up while the one right next to it might speed up one year, slow down the next, speed up again.

HARRIS: When they added up what all those glaciers were doing during the last decade, they see a 30 percent increase in speed, overall.

MOON: We're still seeing velocities increase, so we can expect continued sea level rise from the Greenland ice sheet.

HARRIS: But their report in Science magazine says they aren't seeing the runaway meltdown of Greenland that some had feared. That makes sense to Ian Joughin, given the lay of the land.

JOUGHIN: All of the ice flows towards the coast and squeezes through these narrow outlet glaciers, much like toothpaste kind of squeezing out of the nozzle of a toothpaste tube.

HARRIS: That imposes a natural limit on how much ice can flow off the bedrock, but those bottlenecks don't exist in Antarctica, where other glaciers are also surging toward the sea, so that's still an important wild card. But given what's happening in Greenland, Joughin says the worst case scenario for six feet of sea level rise this century is looking very unlikely. His worst case?

JOUGHIN: My guess - and it is a guess - is probably a meter or less, three feet or less.

HARRIS: Joughin isn't breathing a sigh of relief, though. Three feet of sea level rise would still threaten millions of people around the world who live near sea level. And Josh Willis from the Jet Propulsion Laboratory in Pasadena says sea levels won't magically stop rising at the end of the century under any scenario.

JOSH WILLIS: So we're going to see a lot more sea level rise. The question is how fast is it going to come and are we going to be able to adapt in time?

HARRIS: Willis says the problem is, how do you plan if you don't know whether sea level will rise one foot this century, which is the low end estimate, or six feet?

WILLIS: The difference between one foot and six feet is really enormous in terms of cost and adaptation and mitigation measures.

HARRIS: Narrowing that uncertainty requires gathering more observations from satellites, but that's a problem. Those eyes in the sky are going blind. The National Research Council notes with alarm that Earth-observing satellites are wearing out much faster than they're being replaced. With budget troubles, the U.S. could lose most of its Earth-observing capacity in just eight years.

Richard Harris, NPR News.

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