Climate scientists are about to lose a NASA satellite that's been monitoring the Earth's polar ice caps since 2003. And a replacement won't be in orbit until at least 2015.
The gap could have left scientists without the data they need to determine how fast some of the Earth's largest ice sheets are melting, and how much sea level will rise in the coming years.
So NASA has launched a mission to monitor polar ice by plane.
The mission, called Ice Bridge, took scientists and equipment to the ice sheets of Greenland earlier this year. For the past few weeks, NASA has been flying a DC-8 over glaciers in Antarctica.
One of the people on that plane was Seelye Martin, the chief scientist for Ice Bridge and a researcher at the University of Washington. Just looking down, he says, there is not much sign that the ice is changing.
"But satellite radar observations and satellite altimeter observations show that these [glaciers] are losing a fair bit of mass," Martin says.
That mass of water ends up in the oceans, raising sea level. So far it's only a little, but measurements from the DC-8 will help scientists figure out whether the process is accelerating.
The Ice Bridge mission is designed to continue the work of a dying satellite called ICESat. The satellite has been circling the globe for seven years, even though it was intended to work only for about three years, says Thomas Wagner, a cryosphere program scientist for NASA.
ICESat started out with three lasers that could measure changes in polar ice, but two of the lasers are broken, and the third is just about burned out, Wagner says.
He says data on Antarctica are especially important because the continent has been studied less than Greenland and because it's so big.
"You're talking about something the size of North America covered with miles of ice," Wagner says.
That ice contains enough water to raise sea level dozens of feet. And there's growing evidence that something is happening to several enormous Antarctic glaciers.
"One of these glaciers has lost 9 meters of ice a year," Wagner says. At that rate, he says, it would disappear in a few decades.
Where Land Ice Meets Sea Ice
The Ice Bridge mission is an imperfect substitute for an important satellite, scientists say. An airplane scans only small areas, while the satellite sees big chunks of the globe.
But Martin says the NASA DC-8 carries many more instruments and offers much more detailed information than a satellite. And, he says, ICESat has already shown scientists which regions of ice are changing in ways that could lead to a rapid rise in sea level.
"It seems important to keep an eye on these regions," Martin says. "We wouldn't want to wake up in five or six years and have an oh-my-God moment."
One region of particular interest is called Pine Island, where an enormous glacier is sliding into the Amundsen Sea.
A special radar on the DC-8 allows scientists to study the bottom of the ice sheet, Martin says, which helps them look for signs that the glacier is accelerating.
The scientists are also studying the glacier's ice tongue, which juts out from the coastline. The tongue acts like a cork, Martin says, holding back the ice.
But if enough water gets under the tongue, it could melt and the cork could pop out, he says.
A place like Pine Island can play a big role in sea-level rise, Martin says, because it's where land ice gets added to sea ice.
Sea ice is like the ice cube that's already floating in a glass of water, he says. As it melts, it doesn't raise the water level.
But land ice is like the ice in your freezer, Martin says. When you add a new cube to your glass, the water level rises.
And the glaciers of Antarctica represent a very large ice cube.
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