The collapse of the giant West Antarctica ice sheet is underway, two groups of scientists said Monday. They described the melting as an unstoppable event that will cause global sea levels to rise higher than projected earlier.

Scientists said the rise in sea level, up to 12 feet, will take centuries to reach its peak and cannot be reversed. But they said a decrease in greenhouse gas emissions could slow the melt, while an increase could speed it slightly.

Warm, naturally occurring ocean water flowing under the glaciers is causing the melt. “We feel it is at the point that it is . . . a chain reaction that’s unstoppable,” regardless of any future cooling or warming of the global climate, said Eric Rignot, a professor of Earth science at the University of California at Irvine. He was the lead author of a NASA-funded study that was one of the two studies released Monday.

The only thing that might have stopped the ice from escaping into the ocean and filling it with more water “is a large hill or mountains,” Rignot said. But “there are no such hills that can slow down this retreat,” he added.

The peer-reviewed NASA study has been accepted by the journal Geophysical Research Letters and is expected to be published within days.

The NASA announcement coincided with the release of a University of Washington study that contained similar findings. It will be published Friday in the journal Science.

Both studies observed ice retreating from four massive glaciers in West Antarctica — Pine Island, Thwaites, Smith and Kohler.

The Thwaites glacier alone holds enough water to increase sea level by two feet, the University of Washington study said. Together, the glaciers hold enough water to raise it by several feet.

Sea levels will not rise suddenly, in spite of what the word “collapse” implies, said a statement by the university announcing its report. “The fastest scenario is 200 years, and the longest is more than 1,000 years.”

The statement said university scientists used detailed maps and computer models to reach their conclusion “that a collapse appears to have already begun.”

“Scientists have been warning of its collapse, based on theories, but with few firm predictions or timelines,” the statement said.

The new projections of sea-level rise by both studies are higher and potentially more devastating than earlier projections by international scientists who authored an Intergovernmental Panel on Climate Change report last year and U.S. scientists who wrote the federal government’s National Climate Assessment, which was issued this month.

The findings probably will force the IPCC to increase its current estimate of up to three feet of sea-level rise by 2100, said Sridhar Anandakrishnan, a professor of geosciences at Pennsylvania State University.

The IPCC bases its results on reviews of earlier studies, and the recent observations on polar ice “are only now starting to come together,” said Anandakrishnan, who was not involved in the NASA study.

Tom Wagner, cryosphere program scientist at NASA’s Earth Science Division in Washington, said this is not the first time scientists have said West Antarctica ice will collapse.

“That idea that this is unstoppable has been around since the 1970s,” Wagner said. “We’ve finally hit this point where we have enough observation to put this together” and say it is happening.

Earlier projections of a collapse are one reason scientists criticized some IPCC projections as overly conservative.

In the National Climate Assessment, released last week, scientists already predicted a harsh scenario for the Chesapeake Bay. “As sea levels rise,” they said, “the Chesapeake Bay region is expected to experience an increase in coastal flooding and drowning of . . . wetlands” that protect against storm surge.

Sea-level rise would be made worse because the land is sinking in the lower bay region because of ancient geological forces.