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WASHINGTON (Reuters) - New York, Boston and other cities on North America's northeast coast could face a rise in sea level this century that would exceed forecasts for the rest of the planet if Greenland's ice sheet keeps melting as fast as it is now, researchers said on Wednesday.
Sea levels off the northeast coast of North America could rise by 12 to 20 inches more than other coastal areas if the Greenland glacier-melt continues to accelerate at its present pace, the researchers reported.
This is because the current rate of ice-melting in Greenland could send so much fresh water into the salty north Atlantic Ocean that it could change the vast ocean circulation pattern sometimes called the conveyor belt. Scientists call this pattern the meridional overturning circulation.
"If the Greenland melt continues to accelerate, we could see significant impacts this century on the northeast U.S. coast from the resulting sea level rise," said Aixie Hu, lead author of an article on the subject in the journal Geophysical Research Letters.
"Major northeastern cities are directly in the path of the greatest rise," said Hu, a scientist at the National Center for Atmospheric Research in Boulder, Colorado.
This is an even bleaker assessment than an earlier study indicated. A March article in the journal Nature Geoscience said warmer water temperatures could shift ocean currents so as to raise sea levels off the U.S. northeast coast by about 8 inches more than the average global sea level rise.
However, this earlier research did not include the impact of melting Greenland ice, which would speed changes in ocean circulation and send 4 to 12 more inches of water toward northeastern North America, on top of the average global sea level rise.
That could put residents of New York, Boston and Halifax, Nova Scotia, at risk since these cities and others lie close to sea level now, Hu said in answer to e-mailed questions.
Not only would coastal residents be at direct risk from flooding but drainage systems would suffer as salty ocean water would move back into river deltas, changing the biological environment, Hu wrote in an e-mail.
"In a flooding zone, because the higher sea level may impede the function of the drainage system, the future flood may become more severe," he wrote. If cities are prone to subsidence -- where the ground sinks -- higher sea levels would also make that problem worse, according to Hu.
The ice that covers much of Greenland is melting faster now due to global climate change, raising world sea levels. But sea level does not rise evenly around the globe. Sea level in the North Atlantic is now 28 inches lower than in the North Pacific, because the Atlantic has a dense, compact layer of deep, cold water that the Pacific lacks.
Greenland's ice-melt rate has increased by 7 percent a year since 1996 but Hu said it is unlikely to continue. Still, he and his co-authors ran computer simulations that included this fast-paced melting, along with more moderate scenarios with ice-melt increasing by 3 percent or 1 percent annually.
Hu said it was hard to say whether the 7 percent annual increase could go on for the next 50 years but said it was possible since the current rate of increase in climate-warming carbon dioxide is higher than the high end of projections by the U.N. Intergovernmental Panel on Climate Change.
Editing by Bill Trott