By Richard Black
Environment correspondent, BBC News website
Changes to ocean currents in the Atlantic may cool European weather within a few decades, scientists say.
Researchers from the UK's National Oceanography Centre say currents derived from the Gulf Stream are weakening, bringing less heat north.
Their conclusions, reported in the scientific journal Nature, are based on 50 years of Atlantic observations.
They say that European political leaders need to plan for a future which may be cooler rather than warmer.
The findings come from a British research project called Rapid, which aims to gather evidence relating to potentially fast climatic change in Europe.
The key is the Gulf Stream. After it emerges from the Caribbean, it splits in two, with one part heading north-east to Europe and the other circulating back through the tropical Atlantic.
As the north-eastern branch flows, it gives off heat to the atmosphere, which in turn warms European land.
"It's like a radiator giving its heat to the atmosphere," said Harry Bryden from the National Oceanography Centre (NOC) at Britain's Southampton University.
"The heat it gives off is roughly equivalent to the output of a million power stations," he told reporters.
By the time it reaches the northern latitudes around Greenland and Iceland, the water has cooled so much that it sinks towards the ocean floor, a process known as "overturning".
This cooler water heads south, forming the return stream of a conveyor belt. The complete cycle sees warm water coming northwards on the ocean's surface, and the cold water returning hundreds or thousands of metres underwater.
Florida-based scientists monitor the northwards-flowing Gulf Stream, and have found it has remained roughly constant over the last 50 years.
The Rapid team monitored at roughly 25 degrees north
The NOC researchers concentrated on the colder water flowing south; and they found that over the last half century, these currents have changed markedly.
"We saw a 30% decline in the southwards flow of deep cold water," said Harry Bryden.
"And so the summary is that in 2004, we have a larger circulating current [in the tropical Atlantic] and less overturning."
And less heat then delivered to European shores.
Computer models of climate have regularly predicted that the North Atlantic conveyor may well reduce in intensity or even turn off altogether, a concept that was pushed beyond credence in the Hollywood blockbuster The Day After Tomorrow.
What happens is that as Arctic ice melts and Arctic rivers flow faster - trends which have both been documented - the northern oceans become less saline.
Less salinity means a lower density; the waters then cannot sink, so the conveyor weakens.
Computer models have predicted that if it turned off completely, Europe would cool by perhaps four to six degrees Celsius.
Commenting in Nature, Detlef Quadfasel from the University of Hamburg writes that the NOC experiments provide "...the first observational evidence that such a decrease of the oceanic overturning circulation is well underway."
The NOC researchers admit that the case is not yet proven.
The analysis involves only five sets of measurements, made in 1957, 1981, 1992 and 1998 from ships, and in 2004 from a line of research buoys tethered to the ocean floor.
Even if the trend is confirmed by further data, it could be down to natural variability rather than human-induced global temperature change.
"This issue of variability is very important," said Harry Bryden, "and we do not have any good grasp of it.
"Models can predict it, but we think we ought to go out and measure it."
In 2004 buoys were deployed from ships onto tethers
Michael Schlesinger from the University of Illinois at Urbana-Champaign, a leading expert in models of climate and ocean circulation, believes that even with these caveats, the NOC team has probably come up with a link to human-induced climate change.
"The variability question is the right one to ask," he told the BBC News website, "but the phasing is wrong."
A decade ago Professor Schlesinger showed that the North Atlantic conveyor undergoes a natural 70-year cycle of strengthening and weakening.
"The Bryden measurements are out of phase with this cycle," he said.
"The natural cycle had a northern cooling until the mid-1970s and a warming afterwards, and here we see an apparent cooling."
He is also convinced by other details of the NOC measurements showing that the changes in the southerly underwater flow have occurred at great depths.
"The slowing down of the southward return occurs between 3,000 and 5,000m; and this more or less constitutes a 'smoking gun'," he said.
So what does all this mean for European weather? Will it necessarily get colder - or will the apparent recent trend of warmer summers continue?
"If this trend persists," said Harry Bryden, "we will see a temperature change in northern latitudes, perhaps of a degree Celsius over a couple of decades."
But climate is a complex phenomenon; other factors could conspire, even so, to produce a net warming.
"The UK government is looking, in terms of mitigating climate change and adapting to it, at a warming scenario," said Phil Newton of the UK's Natural Environment Research Council, which funds the Rapid investigators.
"You might now be asking what sort of mitigation and adaptation they should be looking for."
To answer this question, the Rapid team plans to continue its measurements in the next few years.
Its buoys remain in place, and ships can go to gather their data as often as finance allows.
The findings will have resonance beyond the shores of the UK and Europe, as extra heat left circulating around the tropical Atlantic could have major impacts on weather systems in Africa, the Caribbean and Central America.