A new study by researchers at NASA and the University of California at Irvine finds a rapidly melting section of the West Antarctic ice sheet appears to be in an irreversible state of decline, with nothing to stop the glaciers in this area from melting into the sea. (NASA)

A new study by researchers at NASA and the University of California at Irvine finds a rapidly melting section of the West Antarctic ice sheet appears to be in an irreversible state of decline, with nothing to stop the glaciers in this area from melting into the sea. (NASA)

For two decades, scientists have kept a close watch on a vast, icebound corner of West Antarctica that is undergoing a historic thaw. Climate experts have predicted that, centuries from now, the region’s mile-thick ice sheet could collapse and raise sea levels as much as 11 feet.

Now, new evidence is causing concern that the collapse could happen faster than anyone thought. New scientific studies this week have shed light on the speed and the mechanics of West Antarctic melting, documenting an acceleration that, if it continues, could have major effects on coastal cities worldwide.

Twin papers this week show that the rate of ice loss from West Antarctica is increasing — with the acceleration particularly pronounced in the past decade — and also why this is happening: Warmer ocean waters are pushing up from below and bathing the base of the ice sheet.

The findings add to a growing body of evidence suggesting that the effects of climate change are outpacing scientific predictions, driven in part, scientists say, by soaring levels of greenhouse gases in the atmosphere.

It often has been speculated that West Antarctica may be the most unstable of the world’s great ice sheets, a group that also includes the still-larger Greenland and the massive East Antarctica. And research published in May suggested that for the oceanfront glaciers of West Antarctica, held in place by moorings at the seafloor, a point of no return already may have been reached.

View Graphic Warm sea currents are undermining glaciers in Antarctica.

Now, researchers at the University of California at Irvine, NASA’s Jet Propulsion Laboratory and three other institutions have reconciled several measuring methods, including those based on satellite and radar measurements, to determine just how much ice mass West Antarctica has lost to the oceans in the past two decades.

The researchers found that the ice sheet contributed about 4.5 millimeters, or 0.18 inches, to global sea-level rise from 1992 to 2013, with more than 70 percent of the loss occurring in the second half of that time period — meaning the rate of loss is accelerating.

“For long-term stability and small sea-level rise, accelerating mass loss is not reassuring,” said Pennsylvania State University glaciologist Richard Alley, commenting on the paper, which was published Tuesday in Geophysical Research Letters.

A second study, published Thursday in the journal Science, explains why this is occurring. It turns out that in the Amundsen Sea off the West Antarctica glaciers, warmer deep ocean water is “shoaling,” or rising from below, and lapping at the base of the glaciers. The surface ocean waters around Antarctica are generally quite cold because of snow and runoff from the glaciers, but these warmer waters are managing to push up to the ice shelf.

“We now show that the ocean is the major contributor of heat” to West Antarctica, said lead study author and oceanographer Sunke Schmidtko of the University of East Anglia in Britain. “And it’s not just the shelf itself — it’s something that happens offshore in the global ocean.”

This could ultimately prove to be one of the most important geophysical processes on the planet, for the simple reason that the ice sheet of West Antarctica would, if it collapsed entirely, contribute about 3.3 meters, or nearly 11 feet, to global sea-level rise, Alley said.

“There are strong reasons to believe that if the thinning goes too far, it might cross a threshold and then accelerate much more rapidly,” he said.

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The great ice sheets of the world, like West Antarctica, are so massive that, at present, they exert a gravitational pull on the surrounding ocean, which slopes upward toward them. However, the loss of West Antarctica would lead to less gravitational pull and more water spreading out across the ocean — a secondary effect that would further contribute to sea-level rise worldwide. And the Northern Hemisphere — including the United States, a nation that has contributed more than most to the current global-warming trend — could get a bit extra, Alley said.

The research is just the latest suggestion of the possibly worsening effects of climate change. On Wednesday, the World Meteorological Organization said that 2014 is on track to be one of the warmest years — and perhaps the warmest — on record. Ocean surface temperatures have been unusually warm, particularly in a year in which the El Niño weather phenomenon did not materialize.

The findings from West Antarctica could call into question one principal finding from the latest report of the United Nations’ Intergovernmental Panel on Climate Change (IPCC), considered to be the world authority on global warming. In 2013, the panel put its high-end projection for likely global sea-level rise, by the year 2100, at a little more than three feet.

But the researchers studying West Antarctica are not so sure. “The upper bound defined by the IPCC, they may underestimate some of the components, particularly the ice sheets,” said ­UC-Irvine’s Isabella Velicogna, an author of the paper estimating the rate of ice loss from West Antarctica’s glaciers.

So how fast could the loss of West Antarctica unfold? Velicogna’s co-author, Eric Rignot of UC-Irvine, suggested that in his view, within 100 to 200 years, one-third of West Antarctica could be gone.

Rignot noted that the scientific community “still balks at this” — particularly the 100-year projection — but said he thinks observational studies are showing that ice sheets can melt at a faster pace than model-based projections take into account.

The consequences of such an amount of sea-level rise for the United States — or for any other coastal region — are staggering to contemplate.

Benjamin Strauss of Climate Central, whose Surging Seas project tracks the possible effects of sea-level rise and who was not involved in either study, said he estimates that “12.8 million Americans live on land less than 10 feet above their local high-tide line.” Of course, by the time West Antarctica may have begun contributing more significantly to sea-level rise, these numbers will presumably have increased.

Strauss also estimated that $2.4 trillion worth of property is occupying this land (excluding Alaska and Hawaii). The cities that would be most affected include Miami, New Orleans and New York. The amount of sea-level rise contemplated here is quite similar to the storm surge seen in New York during Hurricane Sandy — a surge of 9.4 feet over the normal tide level was recorded at the Battery — only it would be permanent.

Other scientists urged caution in interpreting the findings, saying it is not clear whether the recent accelerated melting is an anomaly or a persistent phenomenon that will continue into the future. Ocean circulation patterns in the south polar region are still not fully understood, and it is possible that the migration of warmer water into the Amundsen Sea is unrelated to the overall climate warming trend, said Olga Sergienko, a glaciologist at Princeton University’s Cooperative Institute for Climate Science who was not involved in the studies.

“This represents only about 20 years of observation, and on the time scale of ice sheets that’s just a blink,” said Sergienko, who also is with the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory in Princeton, N.J.

Understanding the role ocean currents play is important because air temperatures in this region of Antarctica are too low to contribute significantly to the loss of surface ice, said Michael Oppenheimer, a geosciences professor at Princeton who was not part of the new studies. But he added that the rate of melting in the future depends on complex interactions that require additional study to fully comprehend.

“The warm water appears to be gradually melting away the ice shelves and interacting with ice on land,” Oppenheimer said. “One of the things we don’t know is how much of that warm water is sitting there because of global warming and how much is sitting there because of some natural process.

“It is suspicious,” he added, “that we’re seeing this acceleration at the same time that the world is warming.”

The Antarctic, isolated from Earth’s other land masses and influenced by patterns of wind and ocean circulation that are unique to the South Pole, has been slower to show signs of warming than other parts of the planet.

Many climate-change skeptics have noted that winter sea ice around the continent has expanded and thickened in recent seasons, even as the Arctic continues to lose ice cover. But climate scientists say seasonal changes in Antarctica sea ice do not contradict the overall warming pattern seen in the rest of the world.

“The land ice is clearly losing mass,” Oppenheimer said. “Counter­intuitively, as ice is being lost from Antarctica in various places, there is additional fresh water coming to the surface of the ocean. And fresher surface water freezes more easily.”

Terrence McCoy contributed to this report.