The ocean currents of the Atlantic Ocean's Gulf Stream are the weakest they've been in a thousand years, which could change weather in the U.S. and Europe, scientists say.
The changes could also affect cities like Boston and New York by creating higher sea levels, they say.
The Gulf Stream, a huge, powerful and warm current, transports more water than "all the world's rivers combined," notes the National Oceanic and Atmospheric Administration.
The weakening of the current is likely due to changes in the density of the ocean waters as more and more fresh water is melted from Arctic ice sheets and Greenland's glaciers, researchers suggest in a study published in the journal Nature Climate Change.
That massive influx of fresh water has now begun to slow the ocean's circulating currents, report researchers from the Potsdam Institute for Climate Impact Research, the Geological Survey of Denmark and Greenland, Penn State University and other institutions.
Normally, as the Gulf Stream moves into northern latitudes, its salty, dense water sinks and then travels back toward the south at depth. Salt water is denser and slightly heavier than fresh water and so sinks more easily.
The phenomenon, known as "overturning circulation," has a major impact on climate as the warm water moving north keeps Europe's climate warm, while it balances out climate elsewhere by moving cold water back toward the tropics.
However, since the fresh water from melting ice sheets and glaciers is less dense than saltwater, it resists the normal sinking motion at the northern edge of the Gulf Stream and slows it down, with a ripple effect across all the globe's oceans.
The researchers say their study suggests the current circulation of the Gulf Stream has been reduced by about 15 percent to 20 percent, more than at any time in the last thousand years.
Of particular concern for the U.S. is the possibility of an impact on sea levels with further weakening of the current.
Another recent study identified a sudden, unexpected sea level rise of 4 inches in 2009 and 2010 that researchers attributed to a slowing of the Atlantic's overturning circulation.
With bigger changes in circulation could come even bigger increases, says Stefan Rahmstorf of the Potsdam Institute.
"For a big breakdown of the circulation, [sea level rise] could amount to one meter, in addition to the global sea level rise that we're expecting from global warming," he says. A meter is about 3.2 feet, or 39 inches.
It's a trend that has not gone unnoticed, he adds.
"We have detected strong evidence that the global conveyor has indeed been weakening in the past hundred years, [and] particularly since 1970," he says.