Gulf Stream water currents in the Atlantic Ocean have slowed to the weakest in as long as 1,000 years, threatening shifts in U.S. and European weather, as well as coastal sea levels including in New York and Boston.

The currents are probably affected by changes in ocean density as fresh water melts from Arctic ice sheets, scientists led by the Potsdam Institute for Climate Impact Research said in a study Monday in the journal Nature Climate Change.

The Gulf Stream, one of the world’s most important flows, pumps warm water north and cold water south and is responsible for the mild climate in northwestern Europe. Recent changes are unprecedented since the year 900, the Potsdam Institute said, adding that man-made climate change appeared to be to blame.

Researchers have for years raised concerns that shifts in the Gulf Stream may change the climate in Europe. One view is that slower circulation may make Europe colder by depriving northern latitudes of warm waters.

“One specific area in the North Atlantic has been cooling in the past hundred years while the rest of the world heats up,” with research suggesting slowing circulation may be to blame, Stefan Rahmstorf, an institute scientist and lead author of its study, said in an e-mailed statement. “If the slowdown of the Atlantic overturning continues, the impacts might be substantial.”

The scientists used sea-surface and atmospheric temperature data from ice-cores, tree-rings, coral, and ocean and lake sediments to document ocean currents that convey heat, tracing temperature swings for more than a millennium.

Climate models should be updated as they underestimate the effects of the temperature swings and ice melt, Pennsylvania State University’s Michael Mann said in the statement.