MELISSA BLOCK, HOST:
Antarctica is covered with the biggest mass of ice on earth. The part of the ice sheath that's over West Antarctica is thought to be especially vulnerable to climate change. Scientists now say a slow collapse of the West Antarctic ice sheet is both underway and irreversible. And as NPR's Nell Greenfieldboyce reports, this could eventually raise sea levels more than 10 feet.
NELL GREENFIELDBOYCE, BYLINE: For decades, scientists have worried about the West Antarctic ice sheet.
IAN JOUGHIN: Well, the West Antarctic ice sheet, people have speculated that it's unstable since the '70s.
GREENFIELDBOYCE: Ian Joughin is a glaciologist at the University of Washington's polar science center in Seattle. He says this ice sheet is exposed to the ocean.
JOUGHIN: It's what's called a marine ice sheet, which means much of its sitting on the ocean floor instead of on land above sea level.
GREENFIELDBOYCE: He says there are particular weak points. Take Thwaites glacier. It's been thinning as warm water has eaten away at it from underneath.
JOUGHIN: If glacier were to completely sort of disintegrate, it would kind of create a vacuum of ice to which the rest of the ice sheet would sort of flow into and largely destabilize much of the rest of the ice sheet and that has enough ice to raise sea level by about 10 feet.
GREENFIELDBOYCE: Today, researchers announced it looks like that disintegration has started and it appears to be unstoppable.
ERIC RENAULT: And it's past the point of no return.
GREENFIELDBOYCE: Eric Renault (ph) is a researcher at the University of California Irvine. He's just completed a careful study of 40 years of observations about the West Antarctic glaciers. It will soon appear in the journal, Geophysical Research Letters. He says the breakdown of these glaciers will continue even if the ocean doesn't get any warmer.
RENAULT: The system is in sort of a chain reaction that is unstoppable.
GREENFIELDBOYCE: He says his team has looked to see if there's anything that would prevent more and more ice from sliding down into the ocean waters.
RENAULT: But we find no mountains or large hills along the way that could act as a barrier to all these glaciers back.
GREENFIELDBOYCE: Now, sea levels are already rising. Last year, an expert panel on climate change convened by the United Nations estimated a global sea level rise of up to about three feet by the end of the century. The loss of the West Antarctic ice sheet is going to mean a lot more water, but over a longer time frame. Ian Joughin at the University of Washington has a new study coming out in the journal, Science, that uses computer modeling to make some predictions about when this will happen.
JOUGHIN: It's not like a building collapse that, you know, would occur over seconds. It's a collapse that's going to occur over centuries.
GREENFIELDBOYCE: He can't say how many centuries.
JOUGHIN: Our worst case scenario had the rapid onset of the collapse occurring in just over a couple hundred years.
GREENFIELDBOYCE: The best case took over 900 years. Still, the pace of change in West Antarctica is faster than scientists had expected. That means researchers will have to reconsider how much this is going to contribute to sea level rise in the coming decades. Nell Greenfieldboyce, NPR News. Transcript provided by NPR, Copyright NPR.