NASA. Global temperature changes from 1901-2013. While most of the world has warmed, part of the North Atlantic has cooled

The once-mighty ocean current responsible for the relatively mild climate of northern Europe is becoming slower and weaker. The drop-off appears to be the most dramatic in a millennium, and probably even longer than that. The consequences could see some regions experience bitterly colder winters even as the world warms.

Northern Europeans might sometimes crave a little global warming in winter, but they should be careful what they wish for. The reason lies in the global thermohaline circulation, also known as the great conveyor belt. These ocean currents move vast quantities of heat around the planet. If you think this doesn't matter, consider that London is closer to the North Pole than St. John's, Newfoundland, where temperatures seldom get above zero in January and February.

For years, climatologists have debated the effect global warming will have on the thermohaline circulation, and the Gulf Stream in particular. The currents are driven in part by cold, salty water sinking to the bottom of the North Atlantic, where it then flows south.

Credit: NASA. The warming effect of the Gulf Stream can be seen from water temperatures.

Even cold freshwater doesn't sink. The melting of the North American ice sheets at the end of the last Ice Age is considered the most likely explanation for the Younger Dryas event, when temperatures plunged again.

However, it's been less than clear how much melt would create a serious impact. According to Professor Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research, whatever is required, we've had more. In Nature Climate Change, Rahmstorf and colleagues collate evidence that the Gulf Stream has weakened by 15-20%—an extent that is unprecedented in at least the last thousand years and it's probably our fault.

“Maps of temperature trends over the twentieth century show a conspicuous region of cooling in the northern Atlantic,” the paper notes, which is indicative of a loss of warm waters from the south. The authors calculate that more than 400 cubic kilometers (96 cubic miles) of runoff a year from Greenland glacial melt is diluting the once salty waters of the area. They have expressed concern that the trend could get worse.

Although it was not quantified, the Gulf Stream was strong enough to be noticed by Ponce de Leon in 1512, put to use by American merchants in the mid-18th century and mapped by Benjamin Franklin. Evidence from ice cores, tree rings, coral and sediments enabled the authors to track its strength over the last 1,000 years through temperature differentials. They found a 99% probability that what we have witnessed since 1970 is the most rapid decrease in strength in that time.

While a "Day After Tomorrow" scenario of a new ice age is not credible, Rahmstorf warns, “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.” Meanwhile, a sufficient weakening of the stream could make for very cold winters in Britain.