New research suggests the observed slowdown in major Atlantic Ocean currents is linked to the Greenland ice sheet's accelerated melting.

The rapid melting of the ice sheet is believed to be a result of man-made global warming, and its impacts could have a major impact on marine ecosystems, sea levels, and weather systems across Europe and the United States, the Potsdam Institute for Climate Impact Research (PIK) reported.

"It is conspicuous that one specific area in the North Atlantic has been cooling in the past hundred years while the rest of the world heats up," said Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research, lead author of the study to be published in Nature Climate Change. "Now we have detected strong evidence that the global conveyor has indeed been weakening in the past hundred years, particularly since 1970."

Data from ice cores, tree rings, coral, and sediments suggest the recent changes are unprecedented since about 900 A.D., which means they are most likely related to man-made global warming.

 The slowdown in the Atlantic meridional overturning circulation and other ocean currents is believed to be driven by differences in the density of the ocean water. Warm, light water flows from the south towards the north, while cold, dense northern water flows southwards.  Freshwater from the melting Greenland ice sheet is most likely disrupting this natural circulation by diluting ocean water (freshwater is less dense that ocean water with high salinity). The researchers believe the cooling of the North Atlantic is more dramatic than what computer climate simulations have generally determined.

"Common climate models are underestimating the change we're facing, either because the Atlantic overturning is too stable in the models or because they don't properly account for Greenland ice sheet melt, or both," said Michael Mann of Pennsylvania State University. "That is another example where observations suggest that climate model predictions are in some respects still overly conservative when it comes to the pace at which certain aspects of climate change are proceeding."

The scientists do not believe northern Atlantic cooling will prompt a new ice age, but even a gradual change in ocean currents is likely to have major negative effects.  It could even lead to a "tipping point" in the Earth system, triggering rapid and irreversible changes. The Intergovernmental Panel on Climate Change (IPCC) has predicted there is a one-in-10 chance of this happening within a century, but expert surveys have suggested the risk could be even higher.

"If the slowdown of the Atlantic overturning continues, the impacts might be substantial," said Rahmstorf. "Disturbing the circulation will likely have a negative effect on the ocean ecosystem, and thereby fisheries and the associated livelihoods of many people in coastal areas. A slowdown also adds to the regional sea-level rise affecting cities like New York and Boston. Finally, temperature changes in that region can also influence weather systems on both sides of the Atlantic, in North America as well as Europe."