a sudden change in Atlantic currents

Scientists have identified a massive volume of freshwater floating like a cap on the Arctic Ocean, which they believe could suddenly flow into the North Atlantic and affect the warm Gulf Stream that keeps Britain mild in winter and cool in summer.

Studies have shown that a surface layer of fresher water in the Arctic Ocean has increased in volume by about 20 per cent over the past two decades. Scientists believe this is the result of melting sea ice and an increased flow of rivers from Siberia and Canada carrying large volumes of meltwater from permafrost regions.

In the Canada Basin area of the Arctic to the west of Greenland, for instance, scientists are monitoring a huge pool of icy meltwater more than 7,500 cubic kilometres in size, which is about twice the volume of Africa's Lake Victoria. This pool of cold freshwater, which is less dense than the saltier seawater on which it floats, is being kept in its place by circulating winds.

The scientists are concerned that a sudden change in wind patterns might send this fresher water south via the Labrador Current into the North Atlantic where it could interfere with the complex "thermohaline" ocean circulation. These currents, which keep the warm Gulf Stream flowing towards Britain and the rest of north-west Europe, flow between the sea surface and the seabed and are controlled by the relative saltiness of seawater.

"In the past we know that a sudden change in Atlantic currents has happened because of a relatively sudden release of freshwater into the North Atlantic. We can imagine that it could happen again," said Benjamin Rabe, of Germany's Alfred Wegener Institute for Polar and Marine Research in Bremerhaven.

"If that amount of freshwater is going to be suddenly released it would influence ocean currents, for instance the thermohaline circulation of the North Atlantic. I think we should definitely look at this further. The thermohaline ocean current has only been monitored for a few years," he added.

"There is a suspicion that if the Arctic continues to store up low-salinity surface water, it's just a matter of time before it ships it further south and into the North Atlantic where it may impact the intensity of the thermohaline circulation. This is what happens when we simulate this in computer models," Dr Toole said.

"What we are seeing is a build-up of fresher water in the Arctic and that this build-up of liquid freshwater is bigger than we've seen over the instrumental records which extends over several decades. It's difficult to say what the consequences of that may be," he added.