New York, Boston "Directly in Path" of Sea Level Rise

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But if the current 7 percent rate were to persist, up to 20 inches (50 centimeters) of extra water would inundate cities such as New York and Halifax, Nova Scotia.

Still, Waleed Abdalati, a glaciologist at the University of Colorado, Boulder, said that the 7 percent scenario is really a worst case, particularly because "melt rate" is sometimes a misnomer, he said.

When glaciers break off into the ocean, they don't immediately melt, so scientists can't really say that all of Greenland's ice loss is due to melting, he explained.

But the finding is still important, he added, because "a few inches [of sea-level rise], depending on the time frame on which this occurs, makes a significant amount of difference," especially when it impacts heavily populated coastal areas.

What's more, previous studies had not taken into account how Greenland's melt might interact with an oceanic "conveyor belt" in the Atlantic Ocean, which transports water north from the tropics.

Normally in the belt tropical water gets cooler and becomes a deep layer of dense, cold water in the North Atlantic.

But the freshwater flow from Greenland slows down this conveyor belt and prevents the deep, dense water from accumulating. This would make deep water warmer and less dense, causing surface waters to expand throughout the North Atlantic.

It's for this reason that the northeast coast of North America is particularly vulnerable to rising seas, said Hu, whose research will appear May 29 in the journal Geophysical Research Letters.

Uncertainties

Such predictions, and the study of ice sheets in general, is often plagued with uncertainties.

For instance, scientists still don't understand ice sheet dynamics, such as how fast an unstable ice sheet will melt.

And data from the Intergovernmental Panel on Climate Change (IPCC)—which many researchers depend on as the gold standard in climate change predictions—can become easily outdated.

The 2007 IPCC assessment, for example, projected a sea-level rise of up to 23 inches (59 centimeters) this century. But the rapid decline of the world's ice sheets has led many researchers to believe the rise will be even greater.

"The more we know," said the University of Colorado's Abdalati, "the more we're finding things are more severe than we thought."

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