NASA satellite to keep tabs on globe's ice sheets

Sunday December 15, 2002

By ANDREW BRIDGES
AP Science Writer

LOS ANGELES (AP) NASA scientists will be turning a warm eye to the cold Earth this week with the launch of a laser-equipped satellite designed to keep tabs on the waxing and waning of the planet's largest ice sheets.

The Ice, Cloud and Land Elevation Satellite, or Icesat, is designed to spend a minimum of three years making nonstop measurements of the elevation of the ice, two miles thick in places, that blankets Greenland and Antarctica.

Doing so will help answer the question of whether those mammoth ice sheets, which contain an estimated 8 million cubic miles of fresh water, are growing or shrinking.

``Very simply, we do not know,'' said Jay Zwally, the mission's project scientist at NASA's Goddard Space Flight Center in Greenbelt, MD. ``Not only do we not know what is happening today, we don't know what is going to happen in the future.''

Icesat is scheduled for launch aboard a Delta II rocket Thursday afternoon from Vandenberg Air Force Base on the central California coast.

The question the mission is designed to answer is important: if more ice melts off the sheets than piles up as snow, the loss would contribute to the already measurable rise in global sea levels. Scientists fear that rise could swamp coastal regions of the world and upset the ocean circulation patterns that play an important role in determining climate conditions.

Sea levels are currently rising about .8 of an inch every decade. About half of that rise is attributable to the melting of small glaciers and the warming of the oceans, which swell as temperatures rise. The cause of the other half is unknown, although ice sheet melting is suspected.

The Intergovernmental Panel on Climate Change estimates sea levels could rise 18 inches over the next century give or take 15 inches. Icesat's determination of whether the ice sheets are on the wane should shrink that uncertainty, Zwally said.

Today, spot measurements of the ice sheets give a mixed and incomplete picture of what's occurring.

At a recent meeting of the American Geophysical Union, scientists reported that melting of the fringes of the Greenland ice sheet and of ice on the Arctic Ocean had reached levels unseen for decades. In Antarctica, however, the area covered by sea ice during the winter is growing.

Once in orbit and at work, Icesat will bounce a laser beam 40 times a second off the Earth below as it whips overhead at 16,000 mph. Should someone chance to look up as the satellite passes, the pulse from the 330-watt laser would appear as a green star, scientists said. By the time the pulse reaches Earth, it will have less energy than a camera flash and will pose no danger.

How long it takes for the pulse to travel down and bounce back up varies depending on the satellite's distance from the surface; any change in the elevation of the ground surface, including the melting of glaciers, would alter that distance.

Scientists liken the method to measuring changes in the level of water in a bucket from overhead.

Over the course of a year's worth of repeated measurements, the satellite should detect changes in ice sheet elevation as small as .4 inches. Should that amount of ice melt, it would raise sea levels by mere hundredths of an inch.

The $282 million mission is also designed to measure sea ice at both poles, as well as ice sheets in Peru and the Canadian Arctic. It can also look at land elevations and measure the height profiles of clouds.

Scientists will attempt to use Icesat to look at Alaska's glaciers as well, but they may prove too small to monitor from space.

Scientists hope to combine data from Icesat with that acquired by the Gravity Recovery and Climate Experiment, or Grace, mission. That twin-satellite mission measures changes in the Earth's gravity field, including those resulting from the shifts in mass caused by large-scale movements of water.

Combining data from the two missions should allow scientists to estimate both the mass and volume of the ice sheets, and track changes in both, said Michael Watkins, the Grace project scientist at NASA's Jet Propulsion Laboratory.

Accompany Icesat into space will be a second payload, the $16 million Cosmic Hot Interstellar Plasma Spectrometer satellite, or Chipsat. That satellite will look into space at the glow of the interstellar medium, the gas that fills the space between the stars. ^ =

On the Net:

http://icesat.gsfc.nasa.gov/

http://www.ipcc.ch/

(Copyright 2002 by The Associated Press. All Rights Reserved.)