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

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    The Wilkins Ice Shelf, on the West Antarctic Peninsula, has become the latest frosty poster child for global warming's effect on ice in West Antarctica. The disintegration of an ice bridge, shown here as an outline after the bridge broke up at the end of April, means the entire shelf is now vulnerable to break-up. Wilkins has become the southernmost shelf along the peninsula to disintegrate.
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Jonathan Bamber scans his audience – a mix of young scientists-in-training and graybeards – and asks: "If I melted the West Antarctic Ice Sheet tomorrow, how much would sea level rise?"

The answer he typically gets, he continues, ranges from 5 to 7 meters (16 to 23 feet). After all, this has become a kind of canonical range well-grounded in the scientific literature, right?

Not so much, it turns out. And therein lies some of the backstory to a study by Dr. Bamber and his Dutch and British colleagues that appears in Friday's issue of the journal Science.

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Monitor colleague Moises Velasquez-Manoff has summarized the results here. But if you want to give your web browser a rest, here are the bullet points:

• If the West Antarctic Ice Sheet (WAIS) melts, global average sea levels would rise by 3.3 meters. That's down significantly from the typical estimates. But it still represents an immense creeping disaster, direct and indirect, for more than than 3.2 billion people worldwide who live within 200 miles of a coastline.

• The East and West Coasts of North America would see increases 25 percent higher than the global average, Bamber told an audience in March at the Woods Hole Oceanographic Institution. With a wry grin, he asked the group: "Do you believe in karma?" (India's coasts would see such higher-than-average effects as well.)

Here's a bit of the backstory

When estimating what would happen if the WAIS vanished into the ocean, scientists have been using a figure for sea-level rise that first appeared in a peer-reviewed science journal 30 years ago. But the estimate itself had originated 10 years earlier in a paper that never appeared in a peer-reviewed journal, Bamber explained during his March talk. Both were written by the same scientist.

That doesn't necessarily mean the original calculations were wrong. But clearing peer review – as messy a process as it can be –  provides a level of scrutiny that the original calculations apparently didn't undergo.

"The numbers are 40 years old," Bamber says. "And they're based on what? It's almost impossible to tell."

Better ways to track icecaps, now

Meanwhile, the tools used to study the planet's great ice caps have improved dramatically.

In particular, satellites have substantially bolstered scientists' ability to trace changes in the amount of ice and its movements. NASA's ICESAT uses a laser altimeter to measure changes in the surface features of ice. GRACE, a pair of satellites that orbit in tandem, measure subtle changes in the gravity field as ice gains or loses mass. And synthetic-aperture radar can measure changes in the speed of glacial ice as it heads seaward.

Moreover, researchers have reconstructed the WAIS's past behavior though sediment samples taken from the ocean floor off the continent.  And they've gotten a better feel for what the ground under the ice sheet looks like.

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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