ROBERT SIEGEL, host:
From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel.
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And I'm Michele Norris.
Scientists are worried about west Antarctica. If the ice sheet there melts, sea levels could rise dramatically. One particular glacier seems to be disappearing with alarming speed. So scientists decided to check it out with a robotic submarine that can roam beneath the floating ice.
NPR's Nell Greenfieldboyce reports on the sub's risky voyage and its surprising discovery.
NELL GREENFIELDBOYCE: Last year, a team of researchers took a ship from the southern end of Chile and spent about two weeks traveling to reach Pine Island Glacier. This glacier flows down off the West Antarctic Ice Sheet into the sea. It's been flowing down faster over the last few decades, which means more ice is melting, and scientists are not sure why.
Pierre Dutrieux is with the British Antarctic Survey. He says where the glacier hits the sea, there's a vast wall of ice.
Mr. PIERRE DUTRIEUX (British Antarctic Survey): So the face of the glacier that is apparent to us when we're on the boat is 50 meters high.
GREENFIELDBOYCE: So like 150 feet.
Mr. DUTRIEUX: Right.
GREENFIELDBOYCE: Below the water is more ice. It's ice for about a quarter of a mile down. Under all this is where the scientists plan to send a robot.
Mr. DUTRIEUX: It does look like a little yellow submarine, I would say actually.
GREENFIELDBOYCE: Dutrieux says this yellow submarine is a little over 20 feet long. No people sit inside. There's just sensors to do things like map the ice above and the sea floor below. When it's under the ice, the sub can't communicate with the boat.
Mr. DUTRIEUX: You just give it its mission and send it on its way. Then you wait something like 24 hours, depending on the length of the mission, and during that time, you have nothing else to do but to stress, really.
GREENFIELDBOYCE: He says a few years ago, during the first attempts to send a robot sub under Antarctic ice, that sub just disappeared and never returned. This time around, the yellow submarine did almost get trapped on one of its missions, but it always managed to come back. All in all, the robot roamed for more than 300 miles beneath the huge shelf of floating ice.
And it discovered something: A small mountain range coming up from the sea floor.
Mr. DUTRIEUX: So we were very surprised to discover this ridge. It was very exciting, actually.
GREENFIELDBOYCE: Dutrieux says he and his colleagues had expected the sea floor to be flat. They realized that this ridge could help explain why Pine Island Glacier is now melting so quickly. In the past, the glacier actually sat on top of this ridge. The ridge acted as a speed bump to slow the slide of ice into the sea.
But in recent decades, the ice melted enough that it lifted off the ridge. Warm ocean water then crept in and started eating away at the underside of the glacier, making it melt even faster. These results are reported in the journal Nature Geoscience.
Mr. RICHARD ALLEY (Glaciologist, Pennsylvania State University): This is a really important step forward.
GREENFIELDBOYCE: Richard Alley is a glacier researcher at Penn State University. He says the robot sub's work is a technological tour de force, and he says this glacier is a test case for understanding what could happen to other west Antarctic glaciers in the future. He says when it comes to rising sea levels, Pine Island Glacier itself isn't a huge danger.
Mr. ALLEY: The fact that it's gone over this bump is not the end of the world there, probably.
GREENFIELDBOYCE: But, he says, neighboring glaciers pose a bigger threat.
Mr. ALLEY: The biggest worry is probably the next one over, which is Thwaites. If all of Thwaites were to go, it would make a notable difference to sea level rise.
GREENFIELDBOYCE: He says Thwaites Glacier is currently sitting on top of its own big speed bump on the seafloor. So this robot sub's discoveries should help scientists understand what could happen if that ever changes.
Nell Greenfieldboyce, NPR News.
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