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Sandy revives debate over sea-level rise

2:19 PM, Nov 28, 2012   |    comments
An aerial view of a collapsed house along the central Jersey Shore coast.(Photo: Mike Groll, AP)
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Echoes of Superstorm Sandy remain from Manhattan's once-flooded streets to Maryland's battered boardwalks to New Jersey's washed-away beaches.

No surprise. The Eastern Seaboard - or any coastal region - occasionally finds itself in the cross hairs of ferocious ocean storms. But it may have taken Sandy to drive home the added threat that scientists have been warning about for years: a rise in the sea level.

More of the same could lie ahead, suggest ocean scientists such as U.S. Geological Survey oceanographer Asbury Sallenger. The storm triggered the expected arguments about global warming's role, but that debate aside, the new constant for any storm is the increasingly important role likely to be played by sea level. In a study out Tuesday, climate scientists led by Stefan Rahmstorf of Germany's Potsdam Institute report that since 1993 sea level has risen worldwide at a rate 60% higher than predictions. The findings appear in the Environmental Research Letters journal.

"Sea-level rise is accelerating along the East Coast," Sallenger says. "Every inch adds to the kind of inundation we saw in Sandy."

The real question, Sallenger and other ocean experts say, is what effect rising sea levels, which are accelerating along a "hotspot" stretching from Cape Hatteras, N.C., to Maine, will have on storms hitting these places.

In June, Sallenger and colleagues reported in the journal Nature Climate Change that sea-level rise along the U.S. Atlantic Coast has been climbing at a rate three to four times higher than the global average since 1950. About 1.5 inches per decade now, it doesn't sound like a lot. But each inch counts, and New York Harbor's water level is 11 to 16 inches higher than it was a century ago, Sallenger says.

The accelerating sea-level rise springs partly from "subsidence," where groundwater withdrawals to sate thirsty towns and farms along the coast cause the ground to sink, and partly from warming waters in the North Atlantic, the study suggests. Warmer water simply takes up more space than cold water.

Not to be forgotten is that teetering infrastructure poses as much of a problem as global warming, says Alan Weisman, author of The World Without Us. "This is just one storm. People shouldn't lose sight of the fact that cities around the world have been flooding," Weisman says, referring to inundations in Manila, Bangkok and New York over just the past two years. "The politicization of this reality has been disconcerting, and I hope it stops, but I'm not hopeful."

Most climate scientists would see Superstorm Sandy as a largely natural event, not something born as a result of global warming, says Texas Tech climate researcher Katharine Hayhoe. But she acknowledges that the storm, sea-level rise and climate change are hard to disentangle.

A water level that's a few inches higher pales in comparison with a 14-foot storm surge in Lower Manhattan, but those few inches meant the surge was higher than it might have been otherwise.

And was the storm surge stronger because of climate change? Indeed. Warmer ocean temperatures could have provided up to 20% greater power for the storm, so climate change's role isn't necessarily an either-or question, Hayhoe says.

"It is exactly two sides of the coin, and I think we'd both see the same two sides," she says.

Most climate projections that look ahead to the coming century see hurricanes that look stronger, but are fewer in number. Why? Warmer waters strengthen storms but stronger winds above the equatorial oceans wreck the stillness that burgeoning tropical storms need to become ferocious hurricanes.

"While we do expect increased hurricane damage in the U.S. as the climate warms, Sandy is not a pure example of a hurricane," says MIT hurricane expert Kerry Emanuel. It was a hybrid event that started as a tropical storm, grew into a hurricane, and morphed into an intense nor'easter. He says climate science doesn't have enough data to say whether these hybrid storms like Sandy will become more or less frequent.

It also doesn't matter, Emanuel writes in the current Foreign Policy magazine, because the real problem is sea-level rise.

The 2007 Intergovernmental Panel on Climate Change estimated that warming alone, which expands ocean waters, would raise sea levels worldwide by almost two feet over the next century. Add in future melting glaciers in Greenland and elsewhere, and sea level could rise more than 3 feet by then, NASA climate scientist James Hansen and Stanford's Ken Caldeira reported at last year's American Geophysical Union meeting in San Francisco.

"The whole point is that we see it accelerating," Sallenger says. While the "hotspot" study suggests this worsening will continue, he acknowledges other researchers are taking a more "wait-and-see" attitude.

"Geologic records show that sea-level rise happens in big spurts, stops and starts," Sallenger says.

Also worth noting, a 2009 study led by Environmental Protection Agency analyst Jim Titus concluded that 60% of the East Coast's coastal land is zoned for more development, while less than 10% is zoned for wetlands that soak up storms.

"We have to rethink these flood heights to be more like 'Every year your chance of flooding is 1%,' rather thinking it won't happen for another 100 years," says urban flooding expert Jeroen Aerts of Vrije University in Amsterdam. "I'm from Amsterdam, and I walk around New York seeing all this land exposed without a sea wall, and I'm just shocked."

At the same time, dikes, or seawalls, are an unlikely remedy for the entire East Coast, says civil engineer Robert Traver of Villanova (Pa.) University. Of the 25 most-densely populated counties nationwide, 23 are coastal ones. "We can't build barriers around everything," Traver says.

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