Current changes could cool UK

The flow of cold water from the Arctic appears to be falling sharply

By BBC News Online's environment correspondent Alex Kirby

Scientists say they have found further evidence that the currents in the North Atlantic are changing.

They say the amount of cold water flowing south from the Arctic has fallen significantly in the last 50 years.

This could affect the Gulf Stream, which helps to give the UK and north west Europe their temperate climate.

One estimate suggests that without the Gulf Stream winter temperatures in the UK could fall by an average of 11 degrees C, giving parts of the country the same temperatures as Svalbard, the Norwegian archipelago 750 miles from the North Pole.

The scientists believe it could be another effect of the changing climate.

The scientists, from the Faroe Islands, Norway and the UK, report their findings in the magazine Nature.

They measured the flow of cold, dense Arctic water across the Faroe Bank channel, which lies between the Faroes and Shetland, the island group north of the Scottish mainland.

Accelerating decline

They estimated that the flow had fallen by 20% in the last half century, with most of the decrease in the last 30 years.

North west Europe would freeze without the Gulf Stream

And they say the rate of decline has accelerated within the last five years.

The cold water is dense because it is salty. It flows in the ocean depths, and when it reaches tropical latitudes it drives a convection process that carries warm water back towards the Arctic.

The Gulf Stream is part of this process.

It brings to the UK 27,000 times more heat than all the country's power stations can supply, and is estimated to keep Britain five degrees Celsius warmer than it would otherwise be.

The researchers' leader, Dr Bogi Hansen, of the Faroese Fisheries Laboratory, said the heat pump provided by the south-flowing cold current would not work in the same way if there were more fresh water.

"When you add fresh water you reduce pumping efficiency", he said.

He thought the shrinking of the Arctic ice sheet or melting along the Siberian coastline could be causing more fresh water to enter the Arctic Ocean and dilute the current.

But Dr Hansen said it was possible the reduction his team had measured was a natural fluctuation, though he saw no grounds for assuming that it was natural.

Global implications

About one third of the cold water flows through the Faroe Bank channel, and Dr Hansen said it was not clear whether the other channels were coping with larger flows in compensation.

The UK could be as cold as Svalbard

But if the total flow had fallen, he said, there could be significant consequences, both for the Gulf Stream, and globally.

"If you look at the deep water of the world's oceans, they are fed by only two sources", he said.

"One of them is in the Antarctic and the other is up here. So if you reduce one of these sources, you reduce the whole circulation of one of the world's oceans."

Positive feedback

Other studies have suggested that the Arctic sea ice has thinned by about 40% in three decades, and that the north polar ice is shrinking annually by an area the size of the Netherlands.

The possibility that the Gulf Stream could be seriously affected has been regarded as remote. But some scientists say it could happen, and would provide an example of positive feedback - climate change inducing effects which would intensify it.