The huge West Antarctic ice sheet is undergoing a glacially slow but unstoppable collapse, new studies show, and scientists say that means a bigger rise in sea level than previously thought.
The worrisome outcomes will not be seen soon - scientists are talking hundreds of years - but over that time the melt that has started could add 4ft to 12ft to current sea levels.
A Nasa study looking at 40 years of ground, aircraft and satellite data of what researchers call "the weak underbelly of West Antarctica" shows the melt is happening faster than scientists had predicted, crossing a critical threshold that has begun a domino-like process.
"It does seem to be happening quickly," said University of Washington glaciologist Ian Joughin, lead author of one study. "We really are witnessing the beginning stages."
It is likely because of man-made global warming and the ozone hole which have changed the Antarctic winds and warmed the water that eats away at the feet of the ice, researchers said at a Nasa news conference.
"The system is in sort of a chain reaction that is unstoppable," said Nasa glaciologist Eric Rignot, chief author of the study in the journal Geophysical Research Letters. "Every process in this reaction is feeding the next one."
Curbing emissions from fossil fuels to slow climate change would probably not halt the melting but could slow the speed of the problem, Mr Rignot said.
Mr Rignot, who also is a scientist at the University of California Irvine, and other scientists said the "grounding line" which could be considered a dam that stops glacier retreat has essentially been breached. The only thing that could stop the retreat in this low-altitude region is a mountain or hill and there is none.
He looked at six glaciers in the region with special concentration on the Thwaites glacier, which is about 235,000 square miles. Thwaites is so connected to the other glaciers that it helps trigger loss elsewhere, said Mr Joughin, whose study was released by the journal Science.
He used computer simulations and concluded that "the early-stage collapse has begun". Mr Rignot, who used data that showed a speed up of melt since the 1990s, said the word "collapse" could imply too fast a loss, it would be more the start of a slow-motion collapse and "we can't stop it".
Several outside experts in Antarctica praised the work and said they too were worried.
"It's bad news. It's a game changer," said Ted Scambos, lead scientist at the National Snow and Ice Data Centre, who was not part of either study. "We thought we had a while to wait and see. We've started down a process that we always said was the biggest worry and biggest risk from West Antarctica."
The Rignot study sees eventually 4ft of sea level rise from the melt, but it could trigger neighbouring ice sheet loss that could mean a total of 10ft to 12ft of sea level rise, the study in Science said, and Mr Rignot agreed.
Recent reports from the Intergovernmental Panel on Climate Change do not include melt from West Antarctic or Greenland in their projections and this would mean far more sea level rise, said Sridhar Anandakrishnan, professor of geosciences at Pennsylvania State University. That means sea level rise by the year 2100 is likely to be about 3ft, he said.
While the West Antarctic ice sheet is melting, the much larger East Antarctic ice sheet seems stable because it is cooler, Mr Scambos said.
Climate change studies show Antarctica is a complicated continent in how it reacts. For example, last month Antarctic sea ice levels - not the ice on the continent - reached a record in how far they extended. That has little or no relation to the larger, more crucial ice sheet, Mr Scambos and other scientists say.