The Atlantic “overturning” is a current which brings warm water north and cold water south. This ocean current affects weather and sea levels, warms cities on both sides of the atlantic, drives Gulf streams and delivers nutrients from the sea floor that provides for North Atlantic ecosystems and fisheries.

However, that current is currently moving more slowly than it has at any point in the last thousand years. According to new research, “the gradual but accelerating melting of the Greenland ice-sheet, caused by man-made global warming, is a possible major contributor to the slowdown.”

Further weakening of the system, could cause flooding, alter weather patterns disrupt ecosystems and threaten fisheries in Europe and North America.

“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. Now we have detected strong evidence that the global conveyor has indeed been weakening in the past hundred years, particularly since 1970,” said Stefan Rahmstorf in a statement.

Rahmstorf works with the Potsdam Institute for Climate Impact Research and is lead author of the new study, soon to be published in the journal Nature Climate Change.

Obviously, the researchers do not have detailed circulation data for the region going back into prehistory. However, because the ocean current is a lead cause of temperature variation, weather, sea levels and other elements of the subpolar north Atlantic climate, the researchers were able to gather proxy-data.

Proxy-data is gathered by testing coral, tree rings, ocean sediments, coral and other stable elements of the region to determine climate patterns and work backward from there to determine the flow of the Atlantic overturning.

According to the proxy-data, the last time the current was it its current slow level was around 900 AD.

The Atlantic overturning is driven by variations in water density. Warmer, less dense, water flows north from tropical regions and rises to the surface. Simultaneously, denser, cold water from northern regions sinks toward the bottom and flows south. However, the introduction of large amounts of fresh water disrupts this process. The freshwater dilutes ocean salt water, making it less dense which prevents the overturning cycle from functioning properly.

“Now freshwater coming off the melting Greenland ice sheet is likely disturbing the circulation. So the human-caused mass loss of the Greenland ice sheet appears to be slowing down the Atlantic overturning – and this effect might increase if temperatures are allowed to rise further,” said Jason Box of the Geological Survey of Denmark and Greenland.

The consistent cooling of the North Atlantic, immediately south of Greenland, while everything around that area warms is unusual. It is not what current climate models predict.

“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. 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,” says Michael Mann of Pennsylvania State University

According to the researchers, the melting of the Greenland ice sheet will only slightly reduce heating but it will not be significant enough to offset global warming. There is also no risk at all of a ‘The Day After Tomorrow’ ice age. It could be significant enough however to have negative effects for the region.

“If the slowdown of the Atlantic overturning continues, the impacts might be substantial. 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,” said Rahmstorf.

If there is too much cooling it could even cause the Atlantic overturning cycle to break down completely within the century, causing rapid and difficult to reverse changes to the region.