The researchers found that warming ocean waters have been melting one ice shelf, called Larsen-C, from below, and said the same process probably melted two other shelves - Larsen-A and Larsen-B.

"This has a several implications," said Andrew Shepherd of the Scott Polar Research Institute at Cambridge University in Britain, who led the study.

"Rapid melting in a warm ocean explains why sections of the ice shelf have successively disintegrated," Shepherd said in an interview conducted by e-mail.

"The fact that warm water is able to reach the Antarctic Ice Sheet means that other regions may become equally susceptible," he added. "The release of cold melt-water into the oceans could disturb stable patterns of global ocean circulation."

The Larsen Ice Shelf is on the eastern half of the part of the Antarctic that sticks up toward Argentina.

Scientists knew it was melting, but it started a sudden and severe collapse in 1999.

They were fairly certain that warming ocean waters were to blame, but were not able to conclusively prove it.

Writing in Science, Shepherd and colleagues said they had the satellite data to show the process underway in the last remaining solid part of the ice shelf - Larsen-C.

They estimated that over the next century Larsen-C will thin enough to collapse, too.

The effects will not be immediate. Ice shelves are floating on the ocean, so they do not cause sea levels to rise when they break up and melt. But Shepherd and other experts have said the glaciers behind them could melt faster if the protective ice shelves disappear. That could not only help raise ocean levels but could help shift ocean circulation and weather patterns.

"In fact, recent work has shown that the glaciers that fed the now disintegrated Larsen-A and B have accelerated, but it is difficult to quantify the net sea level rise so soon," Shepherd said.

"In total, the ice behind the Larsen Ice Shelf is equivalent to about 0.3 m (about a foot) of global sea level rise."

Other chunks are breaking off the Antarctic as well, and experts argue about the causes.

For example, the West Antarctic Ice Sheet (WAIS) has been steadily melting since the end of the last ice age. But human-induced global warming can speed the process.

In September scientists reported the process is also underway in the Arctic.

Canadian and U.S. researchers said the 3,000-year-old Ward Hunt Ice Shelf, the largest ice shelf in the Arctic, had collapsed.