Antartic glacier thins and speeds up.

One of the largest glaciers in Antartica is thinning according to satellite measurements. The finding spurs concerns that changes in the glacier's ice shelf along the Antartic coast may increase the amount of ice that drains from the interior of the continent and floats out to sea.

Each year, the 200-kilometer-long, 25km-wide Pine Island Glacier depletes the Weat Antartic Ice Sheet of about 69 cubic kilometers of ice. Eventually the glacier flows into the sea, where it forms a floating ice shelf about 40km wide, says Andrew Shepherd, a physicist at University College London. When he and his colleagues analyzed satellite measurement taken between 1992 and 1999, they found that large portions of the Pine Island Glacier thinned during that period. The British scientist report their findings in the Feb. 2 SCIENCE.

At the point called the grounding line, where the glacier reaches the ocean and the ice lifts off the bedrock and floats, the 700-meter-thick ice thinned by about 1.6 m annually during the period studied. As distance inland increased, the glacier showed less thinning.

Much of the Pine Island Glacier rests on bedrock that lies more than 1 km below sea level, says Shephered. If the glacier continues to thin at the current rate, large parts of it would be afloat within 600 years--a development that could accelerate the flow of ice off the continent.

Because the bedrock under the Pipe Island Glacier slopes inward toward the center of the continent, the ice stream has to march uphill quite a distance before it again flow downhill at the coast. The resulting resistance to the glacier's flow will disappear if the ice stream floats free of the bottom, Shepherd notes.

Although the satellite data showed thinning ice over a large area, most of the change was confined to the glacier's fastest-moving portions, which flow about 2.5km per year. This observation bolsters the team's contention that changes in ice flow, not melting or decreased precipitation, cause the thinning.

Because the rate didn't change significantly during the 8 years of observation, the scientists don't attribute the thinning to a short-term phenomenon such as a glacier surge. Instead, the researchers say any increase in flow could be the response to the earlier break-up of a more extensive offshore ice shelf that had slowed the glacier's flow--an idea bolstered by computer models of glaciers.

If the current ice shelf disappears, the speed of the glacier at the grounding line would increase about 30 percent, says Marjorie Schmeltz of NASA's Jet Propulsion. Laboratory in Pasadena, Calf. She presented the results of her digital simulations at the American Geophysical Union's annual meeting last December. Similarly, if the current ice shelf had been larger in the past, its shrinkage could have led to the higher flow rates and the thinning that Shepherd and his colleagues have observed, she says.