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A NASA satellite that's been monitoring the earth's polar ice caps is about to die. That satellite has helped show that some of the earth's largest ice sheets are melting, which could lead to a dramatic rise in sea level. But scientists won't know for sure without more data. A replacement for the dying satellite is years away. So NASA has begun monitoring polar ice using airplanes. NPR's Jon Hamilton has more.

JON HAMILTON: Punta Arenas, Chile is as close as you can get to Antarctica and still find an airport for a big jet. So for the past couple of weeks, a NASA DC-8 has been taking off from that airport and heading south.

Professor SEELYE MARTIN (University of Washington; Chief Scientist, NASA's Ice Bridge Mission): There's a lot of ice down there.

(Soundbite of laughter)

HAMILTON: Seelye Martin of the University of Washington is the chief scientist for NASA's Ice Bridge mission. He spoke from his hotel in Punta Arenas after an 11-hour flight. Martin says that from the plane, you can't see the glaciers changing.

Prof. MARTIN: But satellite radar observations and satellite altimeter observations show that these are losing a fair bit of mass. Now it's not a lot of mass. We're talking about a millimeter of sea level rise per year, but it's something to be concerned about.

HAMILTON: And measurements from the DC-8 will help scientists figure out whether the process is accelerating. The NASA plane already spent several weeks taking similar measurements in Greenland during the Northern Hemisphere's summer. NASA's Thomas Wagner says Ice Bridge is an effort to continue the work of the dying satellite called ICESat.

Mr. THOMAS WAGNER (NASA Scientist): The satellite's now seven years old. It was only planned to really work for about three years.

HAMILTON: The lasers that it uses to measure polar ice are just about burned out. Wagner says data on Antarctica are especially important because it's been studied less than Greenland and because it's so big.

Mr. WAGNER: You're talking about something the size of North America covered with miles of ice, okay? It's like over 90 percent of the world's fresh water. You're flying over one of the tallest mountain ranges in the world, and it's just little peaks, in some cases, peaking up, you know, from the ice.

HAMILTON: There's enough frozen water to raise sea level dozens of feet if it melted. And Wagner says there's growing evidence that something is happening to the ice in several enormous Antarctic glaciers.

Mr. WAGNER: One of these glaciers has lost nine meters of ice a year. So, the math works out so that these glaciers wouldn't be around if they continue to lose ice at this rate.

HAMILTON: That suggests the melting started pretty recently, perhaps because of global warming. Scientists say the Ice Bridge mission is an imperfect fix for a lost satellite. An airplane scans only a small area, while the satellite sees big chunks of the globe. But Seelye Martin says the DC-8 carries instruments that offer much more detail about what's going on than a satellite could. And he says ICESat has already identified the regions that could trigger a rapid rise in sea level.

Prof. MARTIN: It seemed important to keep an eye on these regions that are sensitive and subject to rapid change. So we wouldn't wake up in five or six years and have an oh-my-God moment - you know, what happened?

HAMILTON: One of these sensitive regions is called Pine Island, though it lacks either pine or an island. It's where a massive glacier is sliding into the Amundsen Sea. Martin says a special radar on the DC-8 is allowing scientists to study the bottom of the ice, which helps them look for signs that the glacier is accelerating.

Prof. MARTIN: We're also interested in the ice tongue, which is sort of the cork in a champagne bottle. It sort of serves to - sort of to help hold back the ice.

HAMILTON: The tongue sticks out into a bay. Martin says if scientists can figure out how much liquid water is under the frozen tongue, they'll have a better idea whether this particular cork is about to pop. Martin says it's places like Pine Island where ice moves from land to sea that cause oceans to rise. He says that's because sea ice is like ice in a glass of water.

Prof. MARTIN: As the ice melts, the level of the water in a glass isn't going to change.

HAMILTON: But it's a different story with land ice. When that goes into the sea, it's like someone dropping a new ice cube into your glass.

Prof. MARTIN: So the floating ice isn't going to really change the level of the water in a glass, but the addition of ice to the glass is.

HAMILTON: And the miles of ice piled up on Antarctica represent a very large ice cube.

Jon Hamilton, NPR News.

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