Greenland ice melts from the bottom up.

When it comes to predicting future sea levels, scientists are adrift in an ocean of uncertainty. Climate researchers expect the seas to swell over the coming century as global climate warms, but they can't tell whether the rising tides will prove relatively modest or drenchingly severe.

The two biggest question marks lie at opposite ends of the world. Glaciologists know so little about Greenland and Antarctica that they can't even say whether the two regions are currently gaining or losing ice. Now, a team of scientists reports that the issue is even more complex than previously thought.

In the past, scientists had monitored the ice loss from Greenland principally by measuring the icebergs that break off into the sea. Satellite measurements in northern Greenland, however, reveal that glaciers there lose more ice by melting from beneath than by forming icebergs, according to a report in the May 9 Science.

The satellite method represents a new way to survey glaciers, says Eric J. Rignot, leader of the team and a researcher at the Jet Propulsion Laboratory in Pasadena, Calif. "We were able to find some very new things about the glaciers of Greenland without ever stepping foot in Greenland," he says.

Rignot and his colleagues used the two European ERS satellites to study the northern glaciers of Greenland. Both satellites carry onboard radars that can survey the entire globe.

The scientists gauged the speed of 14 glaciers in northern Greenland by combining radar pictures of the same glacier taken by the two satellites only a day apart--a technique known as satellite interferometry. In a second part of the study, Rignot and his coworkers used interferometry to locate each glacier's grounding line--the place where ice flows down to the sea and forms a floating glacial shelf.

When ice flows past the grounding line, it either melts as it encounters the ocean or eventually breaks off as an iceberg. In the past, glaciologists had assumed that iceberg formation was the more significant cause of ice loss. Rignot's group, however, found that only 30 percent of the ice disappeared through iceberg formation, with the rest lost by melting, principally at the base. Taken with other data, the new results suggest that the ice sheet of northern Greenland is thinning, raising the global sea level.

The story for the rest of Greenland is not as clear, says H. Jay Zwally of NASA's Goddard Space Flight Center in Greenbelt, Md. Zwally's studies of southwest Greenland hint that ice there is thickening, which would take water away from the ocean.

Zwally calls the new radar method an "amazing technique." Scientists cannot acquire more images, however, because one of the ERS satellites is no longer working. New interferometry studies must await another satellite radar mission , as yet unplanned. In the meantime, NASA expects to launch a laser mission in 2001 that can precisely track changes in the height of the Greenland and Antarctic ice caps.