Ice? Ice? Maybe.

NASA scientists hope to get new insight into the future of global ocean levels with the launch this week of a laser-equipped satellite designed to measure the waxing and waning of the planet's largest ice sheets.

The Ice, Cloud and Land Elevation Satellite, or Icesat, is intended to spend a minimum of three years making nonstop measurements of the elevation of the ice sheets that blanket Greenland and Antarctica.

That will help answer the question of whether those layers of ice, which are up to two miles thick in places and 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 from Vandenberg Air Force Base on the central California coast. Joining it atop the Delta II is the $16 million Cosmic Hot Interstellar Plasma Spectrometer satellite, or Chipsat, which will look at the glow of the interstellar medium, the gas that fills the space between the stars.

The question that the $282 million Icesat project is designed to answer is important: if more ice melts off the sheets than piles up as snow, the water would contribute to the already measurable rise in global sea levels. Scientists fear that rise could flood coastal regions and upset the ocean circulation patterns that play an important role in determining climate conditions.

Sea levels currently are rising about 0.8 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 expand 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 measurements should shrink that uncertainty, Zwally said.

At a recent meeting of the American Geophysical Union, scientists said the fringes of the Greenland ice sheet and ice on the Arctic Ocean are melting at rates unseen for decades. In Antarctica, however, the area covered by sea ice during the winter is growing.

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

Over a year's worth of repeated measurements, the satellite should detect changes in ice sheet elevation as small as 0.4 of an inch. Should that amount of ice melt, it would raise sea levels by mere hundredths of an inch.

Icesat also will measure ice sheets in Peru and the Canadian Arctic, land elevations and the height of clouds. Alaska's glaciers may be too small to monitor from space.

However, the satellite can answer only one question in the debate over global warming.

Even if it confirms that the world's ice is melting, some scientists dispute the widely held view that carbon dioxide is responsible for raising global temperatures.

Even if global warming is taken as a given, there is vast disagreement on whether it is necessary to resist those changes, because some argue the earth can adapt to virtually any environmental change.

And even among those who agree that humans have to change their ways, governments are divided over how to do it. Some favor a compulsory approach such as the Kyoto Protocol, which mandates reductions in CO2 emissions. The Bush administration, however, usually favors more market-based mechanisms, and has rejected Kyoto.

A debate within that debate concerns who should make reductions if they are mandated: whether to hit industry or consumers, developing countries or industrial nations.