The collapse of the West Antarctic Ice Sheet, which holds enough ice to raise global seas by up to 15 feet (about 4.6 meters), is already underway and appears to be "unstoppable," two separate U.S. studies said Monday.

"There's been a lot of speculation about the stability of marine ice sheets, and many scientists suspected that this kind of behavior is under way," Ian Joughin, a glaciologist at the University of Washington, said in a statement about one of the studies. "This study provides a more quantitative idea of the rates at which the collapse could take place."

Joughin and colleagues reported in the U.S. journal Science that they used detailed topography maps and computer modeling to investigate a particularly unstable member of the West Antarctic Ice Sheet known as the Thwaites Glacier.

They found that in some places this glacier has been losing tens of feet, or several meters, of elevation per year. Further research showed that the Thwaites Glacier could disappear in an inevitable way in the next 200 to 1,000 years, raising sea levels by nearly 2 feet (about 60 centimeters).

The glacier also acts as a linchpin for the rest of the Ice Sheet, which could cause another 10 to 13 feet (3 to 4 meters) of global sea level rise if the glacier melts away, according to the researchers.

A second study, published in the U.S. journal Geophysical Research Letters, examined the retreat in the grounding lines, the point where the ice from the glacier reaches the ocean and goes afloat, of Thwaites and five other massive glaciers in the Amundsen Sea sector of West Antarctica, using radar observations captured between 1992 and 2011 by the European Earth Remote Sensing satellites, ERS-1 and ERS-2.

The glaciers "have passed the point of no return" and "will be a major contributor to sea level rise in the decades and centuries to come," said lead author of the study, Eric Rignot, a professor at the University of California at Irvine who is also with NASA's Jet Propulsion Laboratory.

"A conservative estimate is it could take several centuries for all of the ice to flow into the sea," Rignot said.

According to Rignot and colleagues, the six glaciers already contribute significantly to sea level rise, releasing almost as much ice into the ocean annually as the entire Greenland Ice Sheet.

The West Antarctic Ice Sheet has long been thought unstable, likely to lose ice and contribute to sea level rise at very high rates. Rignot said the fact that the retreat in glaciers' grounding lines is happening simultaneously over a large sector suggests it was triggered by a common cause, such as an increase in the amount of ocean heat beneath the floating sections of the glaciers. (Xinhua)