The nightmare scenario of a shutdown in the meridional ocean current that drives the Gulf Stream was dramatically portrayed in disaster film The Day After Tomorrow. That scenario had Europe and North America plunged into a new ice age virtually overnight. Although no scientist thinks the switch-off could happen that fast, they do agree that even a weakening over a few decades would have profound consequences.

The Gulf Stream originates in the Gulf of Mexico, flows up the US east coast, then crosses the Atlantic, where it splits in two, with one branch crossing to West Africa. The other branch, the North Atlantic Drift, extends towards Europe. The warm water it brings to Western Europe's shores raises the temperature by as much as 10 degrees in some places and without it the continent would be much colder and drier. Researchers are unsure what to make of the 10-day hiatus in the current in 2004. "We'd never seen anything like that before and we don't understand it. We didn't know it could happen," said Harry Bryden, of Britain's National Oceanography Centre, who presented the findings to a conference in Birmingham on rapid climate change.

Is it the first sign that the current is stuttering to a halt? "I want to know more before I say that," Professor Bryden said.

Lloyd Keigwin, of the Woods Hole Oceanographic Institution in the US, said the 2004 shutdown was "the most abrupt change in the whole [climate] record". "Suppose it lasted 30 or 60 days, when do you ring up the prime minister and say let's start stockpiling fuel? … How can we rule out a longer one next year?" he said. Professor Bryden's group stunned climate researchers last year with data suggesting that the flow rate of the Atlantic circulation had dropped by about 6 million tonnes of water a second from 1957 to 1998. If the current remained that weak, he predicted, it would lead to a one-degree drop in temperature in Britain in the next decade. A complete shutdown would lead to a four- to six-degree cooling over 20 years.

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