Scientists have found evidence that the Atlantic Ocean current which gives Europe its mild climate is being disrupted.

If it stopped, then the temperatures in western Europe would plunge by five degrees Celsius, creating bitter winters. The culprit for the changes could, ironically, be global warming.

The current, called the North Atlantic Drift, brings warm water northwards from the Gulf Stream. It is being disrupted by a growing amount of freshwater entering the Arctic Ocean, reports New Scientist magazine.

This increase is a result of changes attributed to climate change and possibly global warming: melting ice, increased rainfall and changing wind patterns.

Global conveyor belt

The North Atlantic Drift is part of a global conveyor belt that brings warm, surface water north from the Gulf of Mexico and sends cold, deep water back.

The belt is driven by two "pumps", one in the Greenland Sea and one in the Labrador Sea, where the surface water cools, becomes denser, sinks and then returns south.

A computer model developed by Stefan Rahmstorf of the Potsdam Institute in Germany has already suggested that global warming could turn off the North Atlantic Drift but there has been no evidence that this is really happening.

However, several teams are now reporting changes which fit in with the model's predictions. Bill Turrell, leader of the Ocean Climate Group at the Scottish Executive's Marine Laboratory in Aberdeen, analysed more than 17,000 measurements of seawater salinity between Shetland and the Faroe Islands collected since 1893.

Dr Turrell found that in each of the past two decades the salinity of the deep water flowing south has dropped by 0.01g of salt per kg of seawater. So its density has probably also decreased.

"This is the largest change we have seen in the outflow in the last 100 years," says Dr Turrell. "It is consistent with models showing the stopping of the pump and the conveyor belt."

This contrasts with the situation in the 1950s when the salinity of the outflow was so stable it was used to calibrate equipment.

Warm water

His findings are echoed by work at the Fisheries Laboratory of the Faroes. Monitoring conducted there suggests the deep-water outflow through the channel to the southwest of the islands is getting warmer.

In a study yet to be published, Bogi Hansen of the lab says the level at which water is at ­0.5C sank by 60 metres between 1988 and 1997.

Svein Osterhus of the University of Bergen in Norway has also discovered that a deep-sea current closer to the Arctic has gone into reverse. In 1982 and 1983, deep water flowed southwards from the Greenland Sea into the Norwegian Sea at 10 centimetres per second. But in 1992 and 1993, the water was flowing at one centimetre per second in the opposite direction.

This indicates that the Greenland Sea pump "has been dramatically reduced in power", says Dr Østerhus.

"Any evidence that changes in ocean currents are starting to occur is very important," says computer modeller Dr Rahmstorf. "The freshening and warming of the deep water flowing back into the Atlantic is consistent with global warming but could also have natural causes."