Sea-Level Rise Overstated, but Things Still Grim

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Alexander Colhoun / AP

Good news is relative. A Dow of 10,000 looks awfully sweet right now, for example, but it would've seemed like a disaster back when daily closes were closer to 14,000.

That's the kind of pick-your-perspective choice offered by a new paper published in the journal Science about the catastrophic rise in sea levels we could expect if the West Antarctic Ice Sheet (WAIS) continues to melt away as a result of global warming. According to a study by a team of researchers from the U.K. and the Netherlands, the much feared collapse of the WAIS could cause a 9-ft. rise in the planet's seas and oceans, laying waste to coastal lands and immersing some nations entirely. That's a doomsday scenario by most measures — until you consider that the prevailing theories had put the increase at a staggering 15 ft. to 35 ft. (See pictures of New York going green.)

But the comparative good news becomes a lot less good if you happen to live on either coast of the U.S. The complicated workings of planetary physics would cause seas to rise unevenly, and the Atlantic and Pacific shorelines of North America would be harder hit by an Antarctic thaw than perhaps any other place in the world.

The most dangerous part of the Antarctic ice cap is in the west, where much of the continent lies slightly below sea level. Ice shelves that fringe the land keep the seawater out, but if those should melt, the water would rush in and destabilize the larger sheet, leading to slipping, more melting and the possibility of a catastrophic collapse. Picture New Orleans when the levees overtopped; now picture the flooding going global.

"Once a runaway instability starts, it cannot be stopped until a new stable position is found [for the ice sheet]," writes geophysicist Erik R. Ivins in an editorial accompanying the Science paper. (See pictures of this fragile earth.)

The British-Dutch team, led by polar climatologist Jonathan Bamber of the University of Bristol, long suspected that the old estimates were a little alarmist. For one thing, in previous studies, climatologists had defined the area that would be most susceptible to a collapse too widely, including, for example, the Antarctic Peninsula, which the paper calls "both topographically and glaciologically distinct from the WAIS," mostly because it lies largely above sea level. Its higher elevation would put it out of reach of coastal meltwater, keeping its ice cover primarily intact. What's more, even within the areas of the WAIS that lie below sea level, there are localized spots that poke above it, and these too would be relatively safe. Factoring in these and other mitigators, Bamber's team reran the computer models and came up with their newer, slightly rosier forecast.

So how high would we have to pile the sandbags? It depends where you live, since the ocean would rise higher at some points around the Earth than others. Why? Because adding water to the oceans is not like adding it to a lake or a pond or even a bathtub, where the level rises everywhere uniformly. A lake or a pond or a bathtub is not a 6.6 sextillion–ton sphere of rock and dirt spinning through space. The Earth is, and that makes all the difference.

The ice that would melt into the ocean even in Bamber's updated, less extreme models might be small compared with the overall mass of the Earth, but that redistribution of mass would still cause the planet's gravity field to change slightly, which, in turn, would change the vector of its rotation. Think of the way water sloshes in a bucket, varying by how you swing or carry it. On a vast scale, that's what would happen if the WAIS collapsed, and the direction of the sloshing would hit the U.S. especially hard. Other areas that would take a particularly bad beating would be the coastlines bordering the southern Indian Ocean.

All this makes the need for a deal at the multinational climate summit set to convene in Copenhagen this December all the more pressing. And if you need one more reason to hope we at last get warming under control, consider this: The new study did not even consider the sea-level impact of Greenland, glaciers and other ice-capped lands melting. Add that water to the bucket, and you ought to get things sloshing but good.

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