The ocean "engine" that helps to drive the warm waters of the Gulf Stream and keeps Britain relatively mild in winter has begun to slow down, say scientists.
Measurements of ocean currents in the North Atlantic reveal that they have weakened by about 30 per cent since 1992. The findings, published in the journal Nature, fit computer predictions of what would happen when Greenland glaciers begin to melt because of global warming. The models suggest that extra freshwater released into the North Atlantic could weaken ocean currents and even shut down the Gulf Stream.
Britain benefits from the enormous amounts of heat - equivalent to the output of a million power stations - carried from the Caribbean by the Gulf Stream and the North Atlantic Drift, and a tailing off in these currents could have a major impact on the country's climate.
Scientists estimate that the detected 30 per cent weakening of the Atlantic currents could lead to a fall of about 1C in Britain's average temperatures over the next 20 years.
They also warn that the weakening could be the first signs of an accelerating trend that could eventually lead to a more drastic change, including a complete shutdown of the currents. If this were to happen, average temperatures in Britain could fall by between 4C and 6C, leading to winter temperatures similar to Newfoundland in Canada, which is on the same latitude as the UK but does not benefit from the Gulf Stream.
Professor Harry Bryden, of the National Oceanography Centre at the University of Southampton, said the ocean currents of the North Atlantic acted as a conveyor belt that carried warm water at the surface in one direction and transported cold, deep-water currents in the other.
"It is a massive system that includes the Gulf Stream and it carries heat northward out of the tropics into the northern Atlantic, warming the atmosphere and helping to provide northern Europe with a moderate climate," Professor Bryden said.
For the past 50 years, oceanographers have measured the strength of these currents along a stretch of the North Atlantic situated at a latitude of 25 degrees north of the equator, from Florida in the west to the African coast in the east.
When they analysed that rate of ocean flow - measured in Sverdrups (Sv), or a million tons of water flowing per second - they found that in 1992 it was about 20Sv, but in 2004 it had fallen to 14Sv. "In previous studies over the past 50 years, the overturning circulation and heat transport across 25 degrees north were reasonably constant. We were surprised that the circulation in 2004 was so different from previous estimates," Professor Bryden said.
The study used data from an array of instruments anchored at 22 moorings, nine of which are positioned east of the Bahamas, four in the mid-Atlantic and nine across the continental slope of east Africa. Each mooring is anchored to the seabed on wires 5,000 metres (16,400ft) long, and holds instruments that continuously record salinity, temperature, pressure and current flow. Stuart Cunningham of the National Oceanography Centre said: "Continuous monitoring could alert us to potential rapid climate change."