The West Antarctic ice sheet has long been considered at risk due to global warming, and today two studies report, based on new evidence, an unstoppable retreat has begun. The ice sheet holds enough water to raise sea level by several feet.

The glacier's slow degradation would have a destabilizing effect on the rest of the ice sheet, which holds enough ice to raise global sea level by 10 to 13 feet, according to researchers.

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The "grounding line" between the ice sheet and ocean is retreating inward based on airborne and satellite data, said Eric Rignot, lead author of a study published in Geophysical Research Letters, at a press conference held by NASA.

"Today we present observational evidence that the [ice sheet] has gone into irreversible retreat," Rignot said. "It has reached the point of no return."

The sea-level rise projections from the Intergovernmental Panel on Climate Change will likely need to be revised upward, based on these findings, said Sridhar Anandakrishnan, professor of geosciences at Pennsylvania State University, University Park. Anandakrishnan spoke at the NASA press conference, but was not involved in the study.

"The authors have shown that part of Antarctica is undergoing enormous change," Anandakrishnan said. "That ice has nowhere to go but in the ocean. This results in a rise in sea level around the globe."

In this graphic, the red regions are areas where temperatures have increased the most during the last 50 years, particularly in West Antarctica. The dark blue regions have had a lesser degree of warming. NASA/GSFC Scientific Visualization Studio

Researchers from the University of Washington, using computer modeling, also report that the West Antarctic ice sheet is collapsing.

"Our simulations provide strong evidence that the process of marine ice-sheet destabilization is already under way on Thwaites Glacier, largely in response to high subshelf melt rates," the authors wrote in the study, to be published May 16 in Science. "Similar behavior also may be under way on neighboring Pine Island Glacier."

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If there is good news, the scientists say, it's that the collapse could take anywhere from 200-900 years. However, once started, it's unlikely to stop.

"One potential way in which the retreat of these glaciers could be stabilized would be if during its retreat the grounding line -- the boundary between floating and grounded ice -- were to reach a region of the ice-sheet bed where the bed slopes towards the ocean," said glaciologist Jonathan Kingslake of the British Antarctic Survey, who wasn't involved in either study. "The authors show that almost no regions of such stabilizing bedrock exist in this region behind the current grounding line. Hence, they conclude, the retreat is likely to continue unstably for decades to come."

The idea that the glacier's retreat once started could not be stopped has been discussed since the 70s, Anandakrishnan said. "We've crossed a critical threshold. We finally have enough observations to put it all together and say 'We're finally in this state.'"