Satellite radar keeps tabs on glacial flow.

In all the concern over greenhouse warming, one doomsday scenario stands out as the most chilling: the melting of Earth's huge ice sheets. In this catastrophic vision, rising temperatures erode the glacial caps of Antarctica or Greenland, raising sea levels worldwide and inundating New York, Tokyo, and other major coastal cities.

While most scientists believe nothing so dramatic will happen, at least in the next century, they remain concerned because glaciologists lack all but the most basic knowledge about the workings of the vast ice sheets. Two research groups now report a means of revealing these glacial secrets. In separate studies announced this week, they show how different radar measurements made from satellites can detect changes within the icy mantles of Antarctica and Greenland.

In the Dec. 3 SCIENCE, Richard M. Goldstein of NASA's Jet Propulsion Laboratory in Pasadena, Calif., and his colleagues describe using a technique called satellite radar interferometry to study the Antarctic ice sheet. The satellite collecting the information was the Earth Remote Sensing Satellite-1 (ERS-1), operated by the European Space Agency. The craft carries a radar that emits radio waves and senses the rays as they bounce off Earth's surface.

"Interferometry is certainly a very exciting technique with a lot of promise;' comments H. Jay Zwally at NASA's Goddard Space Flight Center in Greenbelt, Md.

The technique of interferometry involves comparing the phases of two different waves, somewhat like matching the sound from a tuning fork with that of a guitar. When the two notes are close but not identical, they oscillate in and out of sync, producing a "beating" pattern that guitar players use to tune a string.

The same rule applies to radar waves. To measure ice motion with interferometry, Goldstein's group compared two nearly identical images of the same location, taken six days apart. The ice's movement during that interval altered the phase of the radar waves returning to the satellite, creating an interference pattern of light and dark bands that are similar to the beats used by guitar players. By counting the bands, the researchers can judge how far out of sync the images are, which provides a measure of how far the ice has moved.

Goldstein and his co-workers used radar interferometry to clock the speed of a feature in Antarctica called the Rutford Ice Stream, which flows into Ronne Ice Shelf. Unlike glaciers, which move through mountain valleys, ice streams are wide rivers of fast-moving ice that flow through regions of much slower ice.

The radar study showed that the Rutford Ice Stream cruises along at a speed of 390 meters per year, roughly 100 times faster than the ice bordering it. By repeating such measurements, scientists can track changes in the ice's speed.

Glaciologists are focusing attention on the Rutford and other Antarctic ice streams because these pipelines carry ice from the stable continental interior toward the ocean. There they form floating ice shelves that break apart into icebergs. Scientists have posited that a dramatic acceleration of the ice streams could cause a major section of Antarctica's ice sheet to collapse into the ocean (SN: 2/13/93, p. 104).

Aside from providing a means of monitoring speed, satellite interferometry can also measure the position of an ice stream's grounding line, the boundary where the ice goes afloat. If the grounding line were to retreat rapidly upstream, that could hasten an ice sheet's demise, according to some researchers.

R. Keith Raney of the Canada Centre for Remote Sensing in Ottawa says that satellite interferometry complements other radar techniques. In the same issue of SCIENCE, Goddard's Mark Fahnestock and his colleagues report making high-resolution images of Greenland's ice sheet, also with radar data from the ERS-1 satellite.

They found what appears to be an ice stream in the northeast corner of the island. Some 550 kilometers long, this feature resembles the ice streams that previously had been identified only in Antarctica. By comparing images taken several months or years apart, the researchers say they can track the behavior of ice sheets.