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Britain could be heading for a "big freeze" if global warming switches off an important ocean current in the Atlantic, some scientists say.

Britain is kept relatively mild in the winter by the warm air blanket brought to us from the tropics by a branch of the Gulf Stream.

But if global warming continues to melt major ice sheets, that supply of warm air could come to an abrupt end, according to a number of experts.

The Gulf Stream relies on a sensitive "conveyer belt" action, which could be "switched off" - quite suddenly - if it becomes diluted by fresh water from the melting ice-sheets, they claim.

Dr Terry Joyce, an oceanographer from Woods Hole Oceanographic Institute, US, believes there is a 50% chance of a sudden climate change happening in the next 100 years.

"It will be quick," he says. "Suddenly one decade we're warm, and the next decade we're in the coldest winter we've experienced in the last 100 years, but we're in it for a 100 years."

The possibility of much harsher winters in the UK is reported in the Horizon programme on BBC Two.

Ice melt

It is the Gulf Stream that allows us to live the way we do. But now scientists have found evidence that the current that carries the protective Gulf Stream is slowing down - and may even stop.

Dr Bill Turrell, from the Marine Laboratory, Aberdeen, has measured a drop in the salinity, the first warning sign that the current might collapse.

"These changes are fundamental. They are substantial. They are going to impact our climate and the climate our children have to live in," he tells Horizon.

The US space agency (Nasa) has measured big increases in the speed of some of Greenland's largest glaciers, and melt water on the Greenland ice sheet in 2001 was twice that recorded 10 years ago.

A white sea in front of the White Cliffs?
Scientists also predict that with an increase in global temperatures will come an increase in rain at northern latitudes.

Huge Siberian rivers are discharging more water into the North Atlantic than ever before, and are predicted to increase their discharge by up to 50% in the next 100 years.

These factors combined could lead to a large amount of fresh water making its way into the North Atlantic.

Climate switch

This particular geographical region of the North Atlantic is vital because it is the point at which the Gulf Stream current sinks and overturns to join the Atlantic Conveyer, a vast rotating belt that takes cold water back to the tropics on the floor of the ocean.

Sinking - the process vital for powering the conveyer - relies on a change in the density of water. As sea-ice forms at high northern latitudes, it leads to an increase in the salinity of the cold, dense salty water underneath, which sinks down into the depths.

The one thing that can stop the sinking is fresh water.

Fresh water effectively dilutes the salty seawater to the point at which it cannot sink - and the conveyer shuts down. With no conveyer, there is no Gulf Stream, and our benign winters come to an end.

Most ocean scientists believe the conveyer has a crucial freshwater threshold level, at which it will shut off - like a light bulb.

The trouble is no one really knows where that threshold level is.

Past precedent

Dr Joyce says: "The likelihood of having an abrupt change is increasing - global warming is moving us closer and closer to the brink.

"We don't know where it is, but we know one thing: we're moving closer to the edge."

And once the light bulb is turned off, no one is sure how to turn it back on.

The conveyer remained switched off for over 1,000 years during the Younger Dryas period, the most significant shutdown since the last ice age.

Professor Richard Alley, a climate scientist from Pennsylvania State University, tells Horizon: "I don't think that an abrupt, sudden trip and fall down the stairs is the most likely outcome. But I think that the probability of that is high enough that we should really think about it."