The powerful ocean current that bathes northern Europe in warm waters from the tropics has wea-kened dramatically in recent years, a consequence of global warming that could trigger more severe winters and cooler summers across the region, scientists warned last week.
Researchers on a scientific expedition in the Atlantic Ocean measured the strength of the current between Africa and the east coast of America and found that the circulation has slowed by 30 per cent since a previous expedition 12 years ago.
The current, which drives the current known as the Gulf Stream, delivers the equivalent of one million power stations-worth of energy to northern Europe, propping up temperatures by 10 degrees Celsius in some regions.
The researchers found that the circulation has weakened by six million tonnes of water a second. Previous expeditions to check the current flow in 1957, 1981 and 1992 found only minor changes in its strength, although a slowing was picked up in a further expedition in 1998. The decline prompted the scientists to set up a network of moored instruments in the Atlantic to monitor changes in the current continuously.
The network should also answer the pressing question of whether the significant weakening of the current is a short-term variation, or part of a more devastating long-term slowing of the flow.
If the current remains as weak as it is, temperatures in Britain, for example, are likely to drop by an average of 1 degrees Celsius in the next decade, according to Harry Bryden at the UK's National Oceanography Centre in Southampton, England, who led the study. "Models show that if it shuts down completely, 20 years later, the temperature is 4 degrees Celsius to 6 degrees Celsius degrees cooler over north-wes-tern Europe," Dr. Bryden said.
Although climate records suggest that the current has ground to a halt in the distant past, the prospect of it shutting down entirely within the century are extremely low, according to climate modelers.
The current is essentially a huge oceanic conveyor belt that transports heat from equatorial regions towards the Arctic circle.
Warm surface water coming up from the tropics gives off heat as it moves north until eventually, it cools so much in northern waters that it sinks and circulates back to the south. There it warms again, rises and heads back north. The constant sinking in the north and rising in the south drives the conveyor.
Global warming weakens the circulation because increased meltwater from Greenland and the Arctic icesheets along with greater river run-off from Russia pour into the northern Atlantic and make it less saline which in turn makes it harder for the cooler water to sink, in effect slowing down the engine that drives the current.
The researchers measured the strength of the current at a latitude of 25 degrees North and found that the volume of cold, deep water returning south had dropped by 30 percent. At the same time, they measured a 30 percent increase in the amount of surface water peeling off early from the main northward current, suggesting far less was continuing up to Europe. The report appeared in last week's addition of the journal Nature.
Uncertainties in climate change models mean that the overall impact of a slowing down in the current are hard to pin down. "We know that if the current slows down, it will lead to a drop in temperatures in northern Europe of a few degrees, but the effect isn't even over the seasons. Most of the cooling would be in the winter, so the biggest impact would be much colder winters," said Tim Osborn, of the climatic research unit at the University of East Anglia, England.
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