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MICHELE NORRIS, Host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Michele Norris.

Scientists project that sea level is likely to rise a foot or two this century. That's because of climate change. But there's an important asterisk here. Researchers don't really know how the Greenland ice sheet will respond to global warming. Greenland is the largest island in the world and the ice sheet at its center is two miles thick. It's possible it can melt a lot faster than it is now - that would drive up the global sea level. But scientists now have a bit of reassuring news on that score.

NPR's Richard Harris reports.

RICHARD HARRIS: When you fly over Greenland, it seems like one huge chunk of ice, frozen solid. But if you're lucky enough to set foot on that ice during the summertime, the picture is very different. Some of Greenland gets warm enough for the surface to melt, and that water creates streams across the ice.

Last summer, Sarah Das followed one of those down to where it spilled into a whole lake in dramatic fashion.

SARAH DAS: I thought when we're walking over and I saw the ripples, I thought it was just the wind pushing the water against the shore.

HARRIS: They are really bonafide rapids.

DAS: Those are real rapids.

HARRIS: Das is from the Woods Hole Oceanographic Institution. She helicoptered up to this seasonal lake as part of a project to learn the fate of this meltwater. Ultimately, if it ends up underneath the ice, it can lubricate the ice sheet and make it flow faster to the coast and ultimately out to sea raising sea level.

Das and her research partner, Ian Joughin from the University of Washington, walked around the lake. At the other end, they find an even more impressive torrent of water. It's flowing out of the lake through a canyon that it is carved through the ice.

IAN JOUGHIN: It's amazing. This feature was probably not here two years ago, and now there's a 60-foot feet channel filled with a roaring waterfall.

HARRIS: Joughin says the water is probably plunging down 3,000 feet to the bedrock far below.

JOUGHIN: So that's one heck of a big waterfall. We are, sort of, tentatively creeping up on this thing like a monster.

(SOUNDBITE OF RIVER RUSHING THROUGH)

HARRIS: The raging river has come to a wall of ice. The water takes its nose dive straight down. This is called a moulin.

DAS: It is quite an exciting discovery and it's amazing to be standing here right at the edge of it. You can - you really can actually feel the energy of the water diving down underneath you. It's amazing.

HARRIS: What they really want to know, though, is how all this meltwater is affecting the flow of the broader ice sheet once it gets to the bedrock below.

DAS: It's kind of hard to say what the impact one individual lake would have on the velocity of a large portion of the ice sheet. But, in general, the observation seem to be that throughout the summertime the ice sheet is speeding up quite a bit.

HARRIS: To figure out just how much faster the ice is flowing, Das and Joughin set up instruments to measure the movement of the ice. Now, nine months later, they're getting some answers. The results are published online in Science Magazine. One of the most dramatic findings is these temporary lakes can be really temporary.

JOUGHIN: One of the lakes we're monitoring was about two miles wide and about 40 feet deep, and it drained in the space of about 90 minutes, with flow more than the flow over Niagara Falls.

HARRIS: Here's what it sounded like in a recording made by a seismometer that was placed in the lake to measure ice quakes.

(SOUNDBITE OF ICE QUAKE)

HARRIS: So much water flowed under the ice, it actually lifted up the glacier by three feet and sent the whole glacier slip-sliding a bit toward the coast.

JOUGHIN: And we saw a brief speed up, but then a couple of days later really things didn't look all that much differently than, say, they had been a week before.

HARRIS: Satellite measurements showed the same thing was happening over hundreds of miles of western Greenland. So the bottom line, all that meltwater does lubricate the ice, but the effect on global sea level is less than a quarter of an inch per decade. Joughin says this rate is likely to increase a bit as Greenland heats up.

JOUGHIN: But it's not going to be a catastrophic effect, I don't think. It's not likely to have major impacts over the next century. It may have a little impact but not a lot.

HARRIS: There are still big unknowns about Greenland's ice. Most notably, scientists don't know just how fast the big glaciers that ride on the coast will tumble into the sea. So, Joughin and Das are feeling a bit more comfortable about the fate of the Greenland ice sheet in the coming century. But that's not to say they are worry-free.

JOUGHIN: Even a foot or two of sea level rise has huge economic consequences, and it's definitely something we need to have a much better understanding of.

HARRIS: To that end, Ian Joughin and Sarah Das planned to head back to Greenland again this summer to learn more about the melting ice as it slips towards the sea.

Richard Harris, NPR News.

NORRIS: And you can see a video of the raging torrents created by Greenland's summer thaw at npr.org.

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