Sea-level rise threatens northeastern US cities

Posted by Emma Woollacott

Boulder, Colorado - Melting of the Greenland ice sheet this century may threaten the northeast US and Canada much more than previously thought.


Research led by the National Center for Atmospheric Research (NCAR) indicates that if Greenland's ice melts at moderate to high rates, ocean circulation by 2100 may shift and cause sea levels off the northeast coast of North America to rise by about 12 to 20 inches more than in other coastal areas.


"If the Greenland melt continues to accelerate, we could see significant impacts this century on the northeast US coast from the resulting sea level rise," says NCAR scientist Aixue Hu, the lead author. "Major northeastern cities are directly in the path of the greatest rise."


The northeast coast of North America is especially vulnerable to the effects of Greenland ice melt because of the way the meridional overturning circulation acts like a conveyer belt transporting water through the Atlantic Ocean. The circulation carries warm Atlantic water from the tropics to the north, where it cools and descends to create a dense layer of cold water. As a result, sea level is currently about 28 inches (71 cm) lower in the North Atlantic than the North Pacific, which lacks such a dense layer.


To assess the impact of Greenland ice melt on ocean circulation, Hu and his coauthors used the Community Climate System Model, an NCAR-based computer model that simulates global climate. They considered three scenarios: the melt rate continuing to increase by seven percent per year, as has been the case in recent years, or the melt rate slowing down to an increase of either one or three percent per year.


If Greenland's melt rate slows down to a three percent annual increase, the study team's computer simulations indicate that the runoff from its ice sheet could alter ocean circulation in a way that would direct about a foot of water toward the northeast coast of North America by 2100. This would be on top of the average global sea level rise expected as a result of global warming.
 
If the annual increase in the melt rate dropped to one percent, the runoff would not raise northeastern sea levels by more than eight inches. But if the melt rate continues at its present seven percent increase per year through 2050 and then levels off, the study suggests that the northeast coast could see as much as 20 inches (50 cm) of sea level rise above a global average that could be several feet. However, Hu cautioned that other modeling studies have indicated that the seven percent scenario is unlikely.


In addition to sea level rise, Hu and his co-authors found that if the Greenland melt rate were to defy expectations and continue its seven percent increase, this would drain enough fresh water into the North Atlantic to weaken the oceanic circulation that pumps warm water to the Arctic. Ironically, this weakening of the meridional overturning circulation would help the Arctic avoid some of the impacts of global warming and lead to at least the temporary recovery of Arctic sea ice by the end of the century.


The study will be published tomorrow in Geophysical Research Letters.