Alarm over dramatic weakening of Gulf Stream
· Slowing of current by a third in 12 years could bring more extreme weather
· Temperatures in Britain likely to drop by one degree in next decade
The Guardian (U.K.) Dec. 1, 2005
The powerful ocean current that bathes Britain and northern Europe in warm waters from the tropics has weakened dramatically in recent years, a consequence of global warming that could trigger more severe winters and cooler summers across the region, scientists warn today.
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% since a previous expedition 12 years ago.
The current, which drives the Gulf Stream, delivers the equivalent of 1m power stations-worth of energy to northern Europe, propping up temperatures by 10C in some regions. The researchers found that the circulation has weakened by 6m 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 £4.8m 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 are likely to drop by an average of 1C in the next decade, according to Harry Bryden at the National Oceanography Centre in Southampton who led the study. "Models show that if it shuts down completely, 20 years later, the temperature is 4C to 6C degrees cooler over the UK and north-western 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 modellers.
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 N and found that the volume of cold, deep water returning south had dropped by 30%. At the same time, they measured a 30% increase in the amount of surface water peeling off early from the main northward current, suggesting far less was continuing up to Britain and the rest of Europe. The report appears in the journal Nature today.
Disruption of the conveyor-belt current was the basis of the film The Day After Tomorrow, which depicted a world thrown into chaos by a sudden and dramatic drop in temperatures. That scenario was dismissed by researchers as fantasy, because climate models suggest that the current is unlikely to slow so suddenly.
Marec Srokosz of the National Oceanographic Centre said: "The most realistic part of the film is where the climatologists are talking to the politicians and the politicians are saying 'we can't do anything about it'."
Chris West, director of the UK climate impacts programme at Oxford University's centre for the environment, said: "The only way computer models have managed to simulate an entire shutdown of the current is to magic into existence millions of tonnes of fresh water and dump it in the Atlantic. It's not clear where that water could ever come from, even taking into account increased Greenland melting."
Uncertainties in climate change models mean that the overall impact on Britain 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 Britain and 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 University of East Anglia climatic research unit.
The final impact of any cooling effect will depend on whether it outweighs the global warming that, paradoxically, is driving it. According to climate modellers, the drop in temperature caused by a slowing of the Atlantic current will, in the long term, be swamped by a more general warming of the atmosphere.
"If this was happening in the absence of generally increasing temperatures, I would be concerned," said Dr Smith. Any cooling driven by a weakening of the Atlantic current would probably only slow warming rather than cancel it out all together. Even if a slowdown in the current put the brakes on warming over Britain and parts of Europe, the impact would be felt more extremely elsewhere, he said.
The New York Times, Dec. 1, 2005
Scientists say they have measured a significant slowing in the Atlantic currents that carry warm water toward Northern Europe. If the trend persists, they say, the weather there could cool considerably in coming decades.
Some climate experts have said the potential cooling of Europe was paradoxically consistent with global warming caused by the accumulation of heat-trapping "greenhouse" emissions. But several experts said it was premature to conclude that the new measurements, to be described today in the journal Nature, meant that such a change was already under way.
The currents, branching off from the Gulf Stream, are part of an oceanic system that disperses tropical heat toward the poles and makes Northern Europe far warmer than its latitude would suggest.
Warming, in theory, could stall the salty, sun-heated, north-flowing currents by causing fresh water to build up in high-latitude seas as ice melts and more precipitation falls.
The scientists, from the National Oceanography Center in Britain, measured sea temperature, currents and other conditions across the Atlantic from the Bahamas to Africa last year and found a 30 percent drop in the flow of warming waters since a similar set of measurements were taken in 1957.
The team, led by Harry L. Bryden, wrote that even though they had measurements from only 5 years out of the past 50, the pattern of change seen at various depths supported the idea that the shift was a significant trend and not random variability.
They also cited independent measurements of a long-term decline in the flow of water between some Arctic seas and the North Atlantic as evidence that a slowing of the overall Atlantic circulation was under way.
In an accompanying commentary in Nature, Detlef Quadfasel of the University of Hamburg, who was not involved with the British study, said it provided "worrying support for computer models that predict just such an effect in a world made warmer by greenhouse-gas emissions."
Other scientists were more cautious. Gavin A. Schmidt, a climate modeler at the National Aeronautics and Space Administration Goddard Institute for Space Studies, said that the estimated decline in ocean circulation should have produced a perceptible decline in surface temperatures, but that no such dip had yet been measured.
Robert Dickson, of the British Center for Environment, Fisheries and Aquaculture Science, said that given the complexity and variability of the seas, much more data was needed to determine whether a slowdown was under way.
"However much statistical rigor is brought to bear, five transocean sections is still a small number on which to depend," he said.