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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 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.

Editorial Notes: Related stories: Ocean changes to cool Europe (BBC) Failing ocean current raises fears of mini ice age (New Scientist) Discussion at The Oil Drum Discussion at peakoil-dot-com The New Scientist article begins ominously:
The ocean current that gives western Europe its relatively balmy climate is stuttering, raising fears that it might fail entirely and plunge the continent into a mini ice age.
-BA UPDATE (Dec 5) Also see: Background on abrupt climate change by Union of Concerned Scientists. Excerpt:
Two high-profile events are putting the issue of "abrupt climate change" squarely in the public eye. The first is a February 2004 Fortune Magazine article that broke the news of a report prepared for the Pentagon on abrupt climate change and its implications for U.S. national security. The Pentagon report describes a scenario in which human-caused global warming leads to a near-term collapse of the ocean's thermohaline circulation, which brings warm surface waters from the tropics to the North Atlantic, warming parts of Western Europe. The authors propose dramatic impacts, including rapid cooling in Europe, greatly diminished rainfall in many important agricultural and urban centers and consequent disruptions in food supply and water supply with enormous geopolitical and security implications.
Fortune magazine has a related article: The Pentagon's Weather Nightmare ("The climate could change radically, and fast. That would be the mother of all national security issues.") Unfortunately, most of the article is behind a subscriber-only wall. WesTexas, who submitted those articles, writes:
Probably, the best guide for us regarding Europe is the "Little Ice Age" (LIA), which actually lasted all the way into the 19th Century. During the worst of the LIA, there were reports of glaciers moving out of the Alps at the rate of a "musket shot per day" obliterating everything in their paths. Recent research has shown that these rapid climate changes are like flipping a switch. So, recent warming patterns in Europe are irrelevant. In the Fortune article, the author noted that given a choice between starving to death in the cold or waging war, humans have traditionally chosen war, which is why the Pentagon has expressed so much interest in the subject. I was particularly taken by implications regarding the recent spike in U.K. natural gas prices to the equivalent of almost $180 per barrel. There were some very nasty comments being made by the Brits regarding their European cousins, basically because utilities on the continent were refusing to sell the U.K. additional gas at any price--they wanted to hold on to it for their own citizens. This is only the beginning to what could truly be a long winter of our discontent.
-BA

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