Satellite Laser To Take The Pulse Of West Antarctic Ice Sheet

February 23, 1999

MADISON - By shining a laser from space onto the Antarctic and Greenland, scientists may soon peel away some of the mystery surrounding the fate of the massive ice sheets that, through natural fluctuation or human-induced climate change, could drastically alter the levels of the world's oceans.

If all goes according to plan, in 2001 a small satellite with a powerful ability to precisely measure changes in the height of the vast sheets of polar ice will sweep into near-polar orbit on a three- to five-year mission to monitor change at the Earth's poles and elsewhere.

ICESAT, as the satellite is named, will be equipped with a sensitive laser altimeter. The device, aboard one of a constellation of satellites that will make up NASA's Earth Observing System, will help scientists measure, at unprecedented scales, change in the mass balances of the ice sheets that together contain 77 percent of the Earth's fresh water and 99 percent of its glacier ice.

The altimeter, known as GLAS for Geoscience Laser Altimeter System, will allow scientists like Charles Bentley to finally get a precise look at the dynamics of the West Antarctic Ice Sheet, one of the least understood and most critical variables in the global climate change equation.

Bentley, a University of Wisconsin-Madison glaciologist and a 40-year veteran of Antarctic ice studies, is one of the world's preeminent authorities on the ice sheet and a member of the GLAS science team. He has been making trips to the Antarctic since 1957 to study the West Antarctic Ice Sheet and the rapidly moving ice streams that carry ice from the interior of Antarctica to the edge of the continent.

"My focus would be on using the laser altimeter to see what's happening on the West Antarctic Ice Sheet, to look for evidence of rapid shrinking," said Bentley. "We're more interested in how it changes with time than how high it is."

From its orbit 420 miles above the Earth, ICESAT will emit pulses of infrared laser light at a rate of 40 times per second, each pulse illuminating a "footprint" 70 meters in diameter. The laser light will be reflected back into space and collected by a telescope aboard the satellite. The distance from the satellite to a reflecting surface -- be it a cloud, an ice sheet or a swelling volcano -- will be determined by measuring the time taken for the laser pulse to make the round trip.

The satellite and its laser altimeter represent a potentially powerful new way to precisely measure change over the vast desert of ice.

Interest in the West Antarctic Ice Sheet, which contains 3 million cubic kilometers of ice, is keen. It is a dynamic -- some say unstable -- system that each year dumps huge amounts of ice into the ocean, sometimes spawning icebergs the size of Rhode Island or larger.

It is also at the center of scientific controversy over its long-term stability. At the margins of the ice sheet, marine ice shelves that protrude over the ocean may be susceptible to a warming sea. Some scientists think that if the ice shelves were thinned enough, an important brake on the ice streams that shuttle ice from the interior of Antarctica would be removed, leading to an accelerated discharge of ice into the ocean and an attendant rise in sea level.

It is also the only ice sheet on the planet that sits atop slippery marine sediments well below sea level, setting up what some scientists think may be a scenario where the ice sheet slowly collapses into the sea. Bentley said scientists disagree about how long such a slow-motion cataclysm might take. Estimates range from a few hundred to a few thousand years or longer.

But the laser system aboard the $200 million ICESAT, said Bentley, will help scientists keep tabs on the ice sheet and changes in its mass balance, the difference between ice shed into the ocean and the accumulation on the ice sheet of ice and snow that falls as precipitation.

Noticing change, however, may take longer than the projected mission of ICESAT, said Bentley. He explained that while the satellite can measure new accumulations of precipitated ice and snow, annual fluctuations in the amount of new ice and snow could mask any small but telling changes in the ice sheet's mass balance. A relatively short mission such as the one ICESAT will conduct, he said, may not be long enough to pin down trends in annual precipitation in the Antarctic.

"An important problem is the fact that you don't get the same amount of snowfall each year. Like Madison, Wis., the West Antarctic Ice Sheet experiences a lot of interannual variability in the amount of precipitation it gets. That is by far the most serious limiting problem in determining changes in the mass of the West Antarctic Ice Sheet."

ICESAT, which is designed to also measure clouds and other parts of the Earth's land surface, is being built at NASA's Goddard Space Flight Center in Maryland. GLAS, the satellite's laser altimeter system, is being developed under the direction of Bob Schutz at the University of Texas at Austin.

University of Wisconsin-Madison

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