Britain could become as cold as Moscow

Britain's winter climate could become as cold as Moscow's, according to new evidence that the vital ocean currents of the North Atlantic are beginning to change.

Britain's winter climate could become as cold as Moscow's, according to new evidence that the vital ocean currents of the North Atlantic are beginning to change.

Measurements over the past 50 years have shown that a key deep-sea current running along the ocean floor that separates Scotland from the Faeroe Islands has slowed down by at least 20 per cent. If the discovery is matched at other sites in the North Atlantic it could be the first sign that the warm Gulf Stream, which dominates the British climate, is itself beginning to slow.

One of the greatest fears of global warming is that rising temperatures could upset the Gulf Stream by switching it further south, causing northwestern Europe to descend into the temperature extremes of central Russia, which is at the same latitude. It would mean that a warmer world caused by the man-made emissions of greenhouse gases could, paradoxically, lead to a colder and more uncomfortable climate for Britain.

A team of scientists from Scotland, Norway and the Faeroe Islands publish their findings today in the journal Nature. They show that one of the deep-sea currents running in the opposite direction to the Gulf Stream and many hundreds of metres below it, from the Norwegian Sea into the Atlantic Ocean, has slowed by at least 20 per cent since 1950. Such a substantial change over a relatively short period has surprised the scientists, who believe that global warming may be beginning to exert a profound switch in the warm Atlantic "conveyor belt" that keeps winters mild in Britain and the rest of northwestern Europe.

"I had not expected this but it was predicted that there would be a slowdown due to climate change," said Bogi Hansen, an oceanographer from the Faeroese Fisheries Laboratories at Torshavn in the Faeroe Islands, and the principal author of the study. "Climate change is the most natural explanation for this. It is not the only possible explanation but it is the most likely."

The research centred on measurements taken of water movements in a deep-sea channel that separates the Faeroes from northern Scotland. The current in the Faeroe Bank channel pushes about 2 million cubic metres of water every second into the Atlantic, an amount equivalent to about twice the total flow of all the rivers of the world combined. "It is an awesome waterfall," Dr Hansen said.

The water moves along the seabed many hundreds of metres below the opposing currents caused by the Gulf Stream, or North Atlantic drift. These warmer currents at the surface could not exist without their colder counterparts on the seabed moving in the opposite direction to complete the "conveyor belt".

Scientists fear that if one of the key deep-ocean currents running from the Norwegian Sea into the Atlantic is slowing, then the incoming surface current must also be decelerating Further measurements at another deep channel in the Denmark Strait, which separates Greenland from Iceland, are needed to confirm whether the 20 per cent decrease in flow is matched elsewhere, Dr Hansen said.

"If this reduction we have seen in the Faeroe Bank channel is also seen in the Denmark Strait then we can be sure that the North Atlantic flow has been reduced," he said.

The most likely explanation for the slowdown is the rising amount of freshwater in the Norwegian Sea due to melting sea ice. This not only increases surface sea temperatures but, more importantly, it lowers salinity levels, causing the water to be less dense and therefore less able to sink to the depths necessary to drive seabed currents.

"The seas are warmer and there is more freshwater not just from melting sea ice but from the Siberian rivers. The water has to be salty and dense or it just won't sink," Dr Hansen said.

If sea ice is not forming as fast as it should, then an important "pump" that drives ocean circulation is switched off. Falling salinity levels in the northern oceans have already been recognised as an important change that could affect long-term climate.

Historical studies have shown that the Gulf Stream can flip, causing northern Europe to descend rapidly into an ice age. It is estimated that without the ocean current, Britain's average temperatures would be between 5C and 10C cooler, with much greater extremes in winter and summer.

Some studies have suggested that the Gulf Stream has slowed in more recent history, causing the mini-ice ages of the middle ages and the 17th century, when the Thames frequently froze over.

Tim Osborn, a climate scientist at the University of East Anglia, said: "This ocean circulation helps to keep northern Europe warm in winter, which is why it is so important. Because of the large impact a possible switch in the current may have, it can't be ignored, even though the probabilities may be small," Dr Osborn said.

Next week, more than 100 scientists will meet to discuss the latest findings at an international conference on ocean circulation at the Southampton Oceanography Centre.

John Gould, director of the centre's ocean circulation project, said that any signs of a weakening in the ocean circulation that drives the Gulf Stream could indicate that some long-term change is under way.

"It may be the precursor of something more significant," Dr Gould said. "This is fairly strong and robust evidence and it is another piece in the jigsaw."