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

By / May 15, 2009

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.

AFP PHOTO/HO/ESA

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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?"

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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.

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.

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