(CNN) -- The complete melting of a major section of west Antarctica's ice sheet appears inevitable, and the process could lead to higher end-of-century global sea levels than previously anticipated, researchers said Monday.
Warm ocean currents and geographic peculiarities helped kick off a chain reaction at the Amundsen Sea-area glaciers, melting them faster than previously realized and pushing them "past the point of no return," NASA glaciologist Eric Rignot told reporters by phone Monday.
The glacial retreat there "appears unstoppable," said Rignot, lead author of a joint NASA-University of California Irvine paper that used 40 years of satellite data and aircraft studies.
NASA says the region has enough ice to raise global sea levels by 4 feet. Estimated conservatively, it still could take several centuries for that portion to melt into the ocean, Rignot said.
But the melting could have an impact this century, said Sridhar Anandakrishnan, geosciences professor at Pennsylvania State University.
The United Nations' most recent climate change report estimates sea levels could rise from about 1 foot to 3 feet by 2100, levels that could displace tens of millions of people from coastal areas around the world.
Yet that estimate largely didn't take into account melting from west Antarctica, because few studies for that area had been completed, Anandakrishnan said.
"So as this paper and others come out, the (U.N.) numbers for 2100 will almost certainly" lean closer to 3 feet, he said.
Why scientists think it's unstoppable
The rate at which the area's ice is melting has increased 77% since 1973, and the reasons are several, researchers said.
The ice sheet there, unlike that in much of east Antarctica, is attached to a bed below sea level. That means ocean currents can deliver warm water at glaciers' base, or grounding lines -- places where the ice attaches to the bed, NASA said.
The heat makes the grounding line retreat inland, leaving a less massive ice shelf above. When ice shelves lose mass, they can't hold back inland glaciers from flowing toward the sea. Glaciers then flow faster and become thin as a result, and this thinning is conducive to more grounding-line retreat, NASA said.
"The system (becomes) a chain reaction that is unstoppable, (with) every process of retreat feeding the next one," Rignot said.
A hill or a mountain behind the grounding line would slow this retreat. But the beds behind nearly all the Amundsen Sea glaciers slope downward, researchers said.
Rignot said he believes global warming and a depletion of the Earth's ozone layer is partly to blame, saying they have changed the winds in the area, causing more warm water to be invected toward the glaciers.
The findings don't clash with news that Antarctic sea ice recently hit record levels, Rignot and Anandakrishnan said. They said sea ice forms and melts quickly, while glaciers are subject to longer-term changes. And the same winds that stir subsurface heat toward the base of Antarctic ice shelf also can expand sea ice cover, Rignot said.
Not a first
Such a melting would be uncommon, but not necessarily unprecedented, Anandakrishnan said. Evidence shows that west Antarctica retained an ice sheet during the last few 100,000-year cycles of glacial formation and retreat, he said.
But evidence also suggests the entire west Antarctica ice sheet might have melted 500,000 to 600,000 years ago, Anandakrishnan said.
The six Amundsen Sea glaciers are just a portion of the entire west Antarctic ice sheet. Though much of the other west Antarctic sections are grounded below sea level, the Amundsen Sea area is more vulnerable, in part because it has fewer hills behind the leading edges and because the shape of the sea floor helps usher more warm water to the base, NASA said.
Anandakrishnan said it was possible that the melting in the Amundsen Sea area could destabilize other ice sheets. The entire west Antarctic ice sheet has enough ice to raise the global sea level by about 16 feet, NASA said.