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The collapse of massive portions of the Antarctic ice sheet now appears inevitable and could trigger far more sea-level rise than once projected, according to a major new University of Washington study and another from NASA researchers.

It may be too late to stop the entire West Antarctic ice sheet from eventually sliding into the ocean — no matter how the U.S. and other nations respond to climate change, the studies concluded.

But taking major steps to combat warming temperatures soon could determine whether the initial collapse ultimately takes a geologically speedy 200 years — or a thousand or more.

And that could determine whether a sea-level rise of a dozen feet or more happens at a pace beyond what anyone has thus far foreseen.

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“A large sector of the West Antarctic ice sheet has gone into a state of irreversible retreat — it has passed the state of no return,” said Eric Rignot, with NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

That move by itself could increase sea levels by about 4 feet, Rignot said. But it also will help set in motion other changes that could cause the ice sheet’s contribution to sea-level rise to triple to 12 feet or more.

“As this sector collapses, it will entrain the other sectors to collapse as well,” Rignot said in an interview Monday.

Said Ian Joughin, a glaciologist with the UW’s Applied Physics Laboratory and lead author of the other new study, “You can’t just vaporize all that ice and assume it’s not going to have other consequences. Given all that, it’s hard to imagine the whole western ice sheet not collapsing.”

Rignot’s work, based on an analysis of 40 years of observations, dovetails with computer modeling by Joughin and colleagues, who were able to examine how one particular glacier, a linchpin for the ice sheet, would respond to varying amounts of melting in the future.

“Eric looked at all the thinning and the retreat and said, ‘We don’t see anything on the bed topography that suggests it’s going to stop,’” said Joughin. “What we did is take a model and run it forward.”

And what the UW team found was that no matter how they adjusted the amount of melting, which would be influenced by changes in global temperatures in coming decades, that glacier still disappeared.

“Whether we turn the dial up or down, that part of the ice sheet goes away,” Joughin said. “It’s too late. But the longer it gets drawn out, the more time people will have to move inland.”

And that timing is something researchers from both teams said they believed humans could still influence.

The UW study, published Monday in Science, and Rignot’s work, posted at the same time in the journal Geophysical Research Letters, both attempted to understand the complex shifts taking place beneath the ice sheet in the Amundsen Sea.

There, warm water is eroding the underside of Thwaites Glacier, a moving ice field roughly the size of South Dakota.

The existence of the warm water may be part of the natural system, but scientists from both teams said climate change is clearly a contributing factor in bringing the warm water in contact with the ice. That is causing Thwaites to melt and thin, which spills more fresh water and icebergs into the ocean.

“Thwaites is one of the biggest, fastest-moving glaciers coming out of Antarctica,” said Benjamin Smith, an associate professor of earth and space sciences at the UW, who worked on the research with Joughin. “If it starts going faster, we could see it losing more and more ice to the ocean over time, and once it starts speeding up it’s going to keep speeding up until it’s all gone.”

And Thwaites serves as a barrier of sorts; its disappearance would expose far more stable portions of the ice sheet to rapid melt.

“If you run this model far enough into the future, most of the West Antarctic ice sheet ends up in the ocean,” Smith said.

That would add a dozen feet or more to sea-level rise, over and above any contribution from Greenland. The East Antarctic ice sheet is expected to remain stable for the foreseeable future.

None of this would happen soon. The UW team projected that Thwaites’ melting would contribute little more than a quarter-millimeter to sea-level rise each year over the next century. But the changes then begin to escalate, dramatically increasing the pace and extent of melting.

Precisely how fast the change takes place will depend on whether or not human beings dramatically dial back greenhouse-gas emissions, scientists said. It’s also not yet clear how fast changes to fossil-fuel emissions would translate to reductions in the pace of ice-sheet melting.

“The most important message is that warming the ocean, probably by not very much, can make ocean level rise by a whole lot,” Smith said. “While we may not be able to prevent it from rising in the long run, we could make it a whole lot worse.”

Craig Welch: 206-464-2093 or On Twitter @craigawelch

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