Gulf Stream water currents in the Atlantic Ocean have slowed to the weakest in as long as 1,000 years, threatening shifts in US 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.

Arctic melt is affecting the Gulf Stream.

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.