updated 10/30/2003 2:45:19 PM ET 2003-10-30T19:45:19

Two sections of Antarctica’s Larsen ice shelf have collapsed over the past decade and another portion could be headed for the same fate as warming ocean waters undermine the ice, researchers say.

CURRENTS OF WATER deep beneath the surface are melting the floating ice shelf from below, said Andrew Shepherd of the University of Cambridge in England.

Large sections of the shelf collapsed and broke into icebergs in 1995 and 2002, and the major section could be weak enough to fail within a century, a research team led by Shepherd reports in Friday’s issue of the journal Science.

Changes in the surface level of the ice shelf, which rests along the Antarctic Peninsula extending toward South America, were studied using nine years of measurements from the European Remote Sensing satellite.

Between 1992 and 2001 the surface level of a 12,000-square-mile (31,000-square-kilometer) region of the shelf lowered between 3 inches and 6.6 inches (7.5 to 16.7 centimeters) per year, they found, with the decline more pronounced at the northern end of the shelf. It was in the northern part of the shelf that the earlier collapses occurred.

Scientists have attributed the ice thinning to surface melting, with meltwater running into crevices and weakening the shelf.

But Shepherd’s team concluded that surface melting and increasing ice density were lesser factors, leading them to suggest that melting at the bottom of the ice caused by water currents was the main cause of ice shelf decline.

SCIENTIFIC DEBATE CONTINUES

Eugene W. Domack, a professor at Hamilton College in Clinton, N.Y., called the paper “exciting,” but said the existing theory, focusing on surface melting from warming conditions, is very attractive.

“We could see the process in action, we could see waterfalls cascading off the front of the ice shelf, so that is a major factor,” he said in a telephone interview.

Thinning does affect how long the shelf can endure warming conditions, and bottom melting can occur in addition to surface melting, he said, “so I like Shepherds’ addition to the story.”

Christine Hulbe, a professor at Portland State University, said the paper provides a “great measurement” of changes under way.

However, she disagreed with the conclusion that ice thinning is related to the fracturing and collapse of the shelves, saying thinning can reduce the stress on the ice.

The section that disintegrated in 2002 was an area where there were ponds of meltwater on the surface, she said. Areas equally thin but without meltwater ponds didn’t collapse.

GONE IN A CENTURY?

While not positive about the melting rate at the ice bottom, Shepherd’s team estimated the average annual thinning of the shelf at about 30 inches (76 centimeters), with melting of about 6 to 9 feet (2 to 3 meters) per year measured in some locations.

The main part of the ice shelf is about 975 feet (300 meters) thick, he said, while the sections that collapsed previously had thinned to about 650 feet (200 meters) thick.

That means, at the estimated melting rate, the main shelf could fail within a century, sooner if the water warms more, Shepherd said in a telephone interview.

If the current undermining the Larsen ice shelf is the same one observed in the nearby Weddell Sea, which has been warming, the collapse could come sooner, he added. However, it has not been confirmed that it is the same current, Shepherd added. The deep water in the Weddell Sea has warmed by about 0.6 degrees Fahrenheit (0.3 degrees Celsius) since 1972.

Because ice shelves float, their collapse doesn’t threaten any increase in sea levels, but the breakup of such giant sheets can affect ocean circulation in ways not well understood.

And researchers say the Larsen shelf could signal similar problems with other Antarctic features including the massive Ross ice shelf, a field of floating ice the size of France.

© 2003 Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.


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