Researchers at the University of Colorado at Boulder's National Snow and Ice Data Center and the British Antarctic Survey attribute the retreats to a regional warming trend. The trend has caused the annual melt season to increase by two to three weeks over the last 20 years, they said.

Satellite photos monitored by NSIDC show that the Larsen B ice shelf has continued to crumble after an initial small retreat in spring 1998. In a series of events that began in November 1998, an additional 1,714 square kilometers of shelf area caved away, said Research Associate Ted Scambos of CU-Boulder's NSIDC.

On the opposite side of the peninsula, the Wilkins Ice Shelf retreated nearly 1,100 square kilometers in early March of last year, said Scambos. Scientists looking at weather satellite imagery at that time suspected a breakup was underway and had their suspicions confirmed by radar satellite images.

"The radar images showed a large area of completely shattered ice, indicating an ice front 35 kilometers back from its previous extent," said Scambos. "The sudden appearance of thousands of small icebergs suggests that the shelves are essentially broken up in place and then flushed out by storms or currents afterward."

The British Antarctic Survey scientists had predicted one of these retreats, using computer models to demonstrate that the Larsen B was nearing its stability limit. With the small breakup observed last spring, the shelf had already retreated too far to continue to be supported by adjacent islands and shorelines.

Scientists at both institutes expected the two shelves to fail soon, but the current disintegration is occurring at an even faster rate than earlier breakups gave reason to anticipate.

"We have evidence that the shelves in this area have been in retreat for 50 years, but those losses amounted to only about 7,000 square kilometers," said David Vaughan, a researcher with the Ice and Climate Division of the British Antarctic Survey. "To have retreats totaling 3,000 square kilometers in a single year is clearly an escalation. Within a few years, much of the Wilkins ice shelf will likely be gone."

Ice shelves are floating plates of ice that are still attached to continents and which form when large glaciers flow toward the ocean in polar areas. Where they are supported by islands and sheltering coastline, they can become stable, long-term features, said Scambos.

Surface features on the Larsen B indicate that it has existed for at least 400 years. But as climate inches toward an average summertime temperature just above 0 degrees C -- the melting point of water -- the Larsen and Wilkins ice shelves have begun to disintegrate.

The Larsen B ice shelf is currently about 7,000 square kilometers -- about the size of Delaware. The Wilkins ice shelf is nearly twice that large, Scambos said.

The British researchers, who have monitored the peninsula's climate warming for decades, report an increase in mean annual temperature of about 2.5 degrees C or roughly 4.5 degrees F since the 1940's. Both groups concur that ice shelf breakup is a direct result of local climate warming.

According to Scambos, the recent warming trend has led to greater amounts of ponding melt on the shelf, weakening it. "Melt water at the surface acts to increase the extent of fracturing in the ice," he said. "The weight of the water essentially forces the cracks open, so a relatively small amount of climate warming can destroy a large, centuries-old ice shelf."