New sea level alarm not to be ignored

From National Geographic

There are days when even a born optimist starts to waver in his conviction. The release of a new study projecting that sea level could rise between five and six feet by 2100—when many children born today will still be alive and have been forced to move inland—made Thursday one of those days.

There have been lots of other studies, you might say. True: The last sea-level alarm (in what seems an endless series) came just a month ago. That analysis showed that in the 20th century, sea level rose faster than at any time in the past 2,800 years, and that our fossil-fuel emissions were very likely responsible.

Climate has changed naturally even within human history, that study said, and sea level has changed with it—but not as fast as we’re changing it now.

If you’ve been following climate science for a while, though, that wasn’t terribly surprising.  And if you’re an optimist, that study didn’t knock you off your stride.

In fact there was something almost reassuring about it: Extrapolating out to 2100, it projected a sea level rise of just three to four feet—in line with the most recent and reliably conservative report of the Intergovernmental Panel on Climate Change (IPCC). What’s more, the paper was co-signed by Stefan Rahmstorf, who until then had been a prominent exponent of higher sea-level-rise forecasts. So it was only slightly perverse to say that things were actually looking up.

The worst wasn’t going to happen. With three feet of sea level rise, the United States would “only” be looking at a loss of a land area the size of Massachusetts.

Or at spending many hundreds of billions of dollars to defend the coasts.

Or, just maybe, at cutting carbon emissions in time to keep things from getting so bad.

The Physics of Ice

Comes now the study published in Nature Thursday by Robert Deconto of the University of Massachusetts and David Pollard of Penn State. It’s different from other alarms, and here’s why.

Deconto and Pollard aren’t projecting the future based only on the experience of the past few millennia. They’re projecting it with a computer model of the West Antarctic Ice Sheet and of the Antarctic climate—that is, from the laws of physics.

Just a model, you might say, and translating those laws into an accurate model of an ice sheet is hard. True again: the problem has stumped scientists for decades. They’ve known that ice melts, and that if climate warms enough, the ice sheet will collapse at some point, dumping lot of water into the sea. But they’ve had trouble saying how much warmth is enough and how fast the collapse might proceed. No one has ever watched it happen.

The geologic record offers some test cases. Some 125,000 years ago, for instance, Earth was in an interglacial period, like the one we’re in now, a warm interlude between 100,000-year-long ice ages. The temperature then was about the same as it is today, a degree or two warmer at most. But the best evidence indicates sea level was at least 20 feet higher—which in itself is disconcerting, suggesting as it does that we might be poised on the brink of something big.

Where did 20 feet of water come from? The Greenland ice sheet contains more than enough, but it sits on land and can’t easily fall into the sea.

The West Antarctic Ice Sheet contains enough water to raise sea level 15 feet. And if you could strip away the ice and look at the bedrock, as scientists have done with airborne radar, you’d see how vulnerable it is: Most of the ice sits not on land but on the seabed. It’s a big dome of ice rising out of a seafloor basin, like a soufflé out of a bowl. Beyond the submarine ridges that form the sloping sides of the basin, floating ice shelves extend out to sea. They act like buttresses, propping up the ice dome and keeping it from collapsing and washing away.

Read the full story at National Geographic