If W. Antarctic Ice Sheet melts, how high will sea levels rise?

All this made the time ripe for a more rigorous look at the impact that losing much of the ice sheet would have on sea level. Bamber's team doesn't assume complete loss of the sheet because in some areas in West Antarctica, the relief of the underlying bedrock would prevent the ice from traveling.

Researchers silent on when collapse might occur

The team has nothing to say in this study about time-scales for a collapse of much of the WAIS. Many of the models glaciologists use to project the behavior of the WAIS and of Greenland's ice sheet have yielded reaction times measured in thousands of years.

Based on evidence from the past four or five ice ages, "we know you can get rid of ice sheets very quickly, in a couple of thousand years," Bamber explains. "But it takes much, much longer to grow them back. That's why we're concerned about tipping points in the climate system."

Sixty-six feet in 500 years

Even more startling is evidence since the peak of the last ice age, he continues. At one point, the sea level rose 20 meters in 500 years. "That has to be from the ice sheets," he says. "That shows they can do something really pretty spectacular."

Today, it would take a meltdown of all of West Antarctica's ice sheet, all of Greenland's, and a significant chunk of East Antarctica's to push sea levels that high.

Which brings him back around to the WAIS today. Work that he and his colleagues have been conducting as they try to track changes in the mass budget for the WAIS, particularly the sections of ice sheet that empty into the Bellinghausen and Amundsen Seas – a region of the WAIS that he says is particularly unstable, given its underlying topography.

"All the losses, and they are big losses, are taking place along that coast," he says.

Melting has accelerated dramatically in 10 years

"The take-home message is that the loss has been accelerating really quite dramatically in the last 10 years," he says. The same holds true for Greenland, even with the uncertainties that attend the measurements.

What's worrisome, he says, is the gap between the range of responses climate models show for Greenland and Antarctica's ice compared with what scientists are observing.

Models don't match observations

For instance, scientists at the Hadley Center for Climate Prediction and Research in Britain factored the results of warming scenarios used in the last set of UN reports on global warming into ice-sheet models. In all scenarios, Greenland loses mass to ice melt, while Antarctica's ice gains mass overall.

"But this is not what we observe," he says. "East Antarctica isn't gaining mass, and West Antarctica is losing the same as Greenland. This makes us think there's something seriously wrong with the state of the art" for predictions of what could happen with the ice caps at the top and bottom of the world.

Closing that gap, he concludes, requires more reconstructions of past changes in ice extent and pace of movement. These provide a reality check on the models. And the community needs to design models that do a better job of reproducing the ice activity scientists are seeing today.

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