Scientists have long worried about climate change-induced melting of the huge West Antarctic Ice Sheet. Now they say that not only is the disintegration of the ice already underway, but that it's likely unstoppable.
That means that in the coming centuries, global sea levels will rise by anywhere from 4 to 12 feet. As NPR's Nell Greenfieldboyce reports, that's a larger increase than the United Nations expert panel noted last year. But it would occur over a longer time frame centuries instead of decades.
Ian Joughin, a glaciologist at the University of Washington in Seattle, says people have been speculating that the West Antarctic Ice Sheet is unstable since the 1970s.
"It's what's called a marine ice sheet, which means most of it is on the ocean floor instead of on land above sea level," Joughin says.
The ice sheet's weak points, such as Thwaites Glacier, have been thinning as warm ocean water eats away at it from underneath, he says.
If the glacier were to disintegrate, it would "create a vacuum of ice to which the rest of the ice sheet would ... flow into and largely destabilize much of the rest of the ice sheet," Joughin says. "And that has enough ice to raise sea level by about 10 feet."
Eric Rignot, a researcher at the University of California, Irvine who co-authored the study, scheduled for publication in Geophysical Research Letters, looked at four decades of ground, airplane and satellite data.
What Rignot and others found is that the threshold for a cascading melt has already been reached.
"The system is in sort of a chain reaction that is unstoppable," Rignot says.
He says his team has looked to see if there's anything that would prevent ever more ice from sliding down into the ocean waters.
"But we find no mountains or large hills along the way that could act as a barrier to hold these glaciers back," Rignot says. "It's not like a building collapse that would occur over seconds; it's a collapse that's going to occur over centuries," he says.
How many centuries?
"Our worst-case scenario had the rapid onset of the collapse occurring in just over a couple hundred years," Joughin says.
The best case took more than 900 years.