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.

Scientists from 17 research institutes in 10 European countries have collaborated as a consortium known as CLAMER in documenting the build-up of fresher water in the Arctic. One of the techniques involves sending salinity gauges down to different depths of the sea from the RV Polarstern ice-breaker, a German research ship.

The scientists have collated about 5,000 salinity profiles of the Arctic and estimated a 20 per cent increase in freshwater over the entire region since the early 1990s, when measurements of this type began. This corresponds to a rise of some 8,400 cubic kilometres of freshwater over this period, which is roughly equivalent to the entire volume of freshwater and ice that is released each year from the Arctic into lower latitudes.

Laura de Steur, an oceanographer from the Royal Netherlands Institute for Sea Research, said that for the past 12 years a mostly clockwise pattern of winds has kept a huge volume of this fresher water within the Canada Basin. This body of relatively salt-free water has probably resulted from increased river run-off and melting ice, she said. "The volume of water discharged into the Arctic Ocean, largely from Canadian and Siberian rivers, is higher than usual due to warmer temperatures in the north causing ice to melt. Sea ice is also melting quickly, adding even more freshwater to the relatively calm Arctic Ocean," Dr de Steur said.

"In addition, sea ice that is thinner is more mobile and could exit the Arctic faster. In the worst case, these Arctic outflow surges can significantly change the densities of marine surface waters in the extreme North Atlantic. What happens then is hard to predict," she added.

John Toole, of the Woods Hole Oceanographic Institution in Massachusetts, said that when seawater gets as cold as it does in the Arctic region, salinity becomes a crucial factor in determining whether water floats near the surface or sinks deeper. This is critical in terms of what drives the pump of the thermohaline ocean circulation.

"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.