Failing Ocean Current : We are nervous about the future say scientists.

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http://www.newscientist.com/article.ns?id=dn8398

Failing ocean current raises fears of mini ice age

 

18:00 30 November 2005

NewScientist.com news service

Fred Pearce

 

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.

The dramatic finding comes from a study of ocean circulation in the North

Atlantic, which found a 30% reduction in the warm currents that carry water

north from the Gulf Stream.

The slow-down, which has long been predicted as a possible consequence of

global warming, will give renewed urgency to intergovernmental talks in

Montreal, Canada, this week on a successor to the Kyoto Protocol.

Harry Bryden at the Southampton Oceanography Centre in the UK, whose group

carried out the analysis, says he is not yet sure if the change is temporary or

signals a long-term trend. " We don’t want to say the circulation will shut

down, " he told New Scientist. " But we are nervous about our findings. They have

come as quite a surprise. "

No one-off The North Atlantic is dominated by the Gulf Stream – currents that

bring warm water north from the tropics. At around 40° north – the latitude of

Portugal and New York – the current divides. Some water heads southwards in a

surface current known as the subtropical gyre, while the rest continues north,

leading to warming winds that raise European temperatures by 5°C to 10°C.

But when Bryden’s team measured north-south heat flow last year, using a set

of instruments strung across the Atlantic from the Canary Islands to the

Bahamas, they found that the division of the waters appeared to have changed

since previous surveys in 1957, 1981 and 1992. From the amount of water in the

subtropical gyre and the flow southwards at depth, they calculate that the

quantity of warm water flowing north had fallen by around 30%.

When Bryden added previously unanalysed data – collected in the same region by

the US government’s National Oceanic and Atmospheric Administration – he found a

similar pattern. This suggests that his 2004 measurements are not a one-off, and

that most of the slow-down happened between 1992 and 1998.

The changes are too big to be explained by chance, co-author Stuart Cunningham

told New Scientist from a research ship off the Canary Islands, where he is

collecting more data. " We think the findings are robust. "

Hot and cold But Richard Wood, chief oceanographer at the UK Met Office’s

Hadley Centre for climate research in Exeter, says the Southampton team's

findings leave a lot unexplained. The changes are so big they should have cut

oceanic heating of Europe by about one-fifth – enough to cool the British Isles

by 1°C and Scandinavia by 2°C. " We haven’t seen it yet, " he points out.

Though unseasonably cold weather last month briefly blanketed parts of the UK

in snow, average European temperatures have been rising, Wood says. Measurements

of surface temperatures in the North Atlantic indicate a strong warming trend

during the 1990s, which seems now to have halted.

Bryden speculates that the warming may have been part of a global temperature

increase brought about by man-made greenhouse warming, and that this is now

being counteracted by a decrease in the northward flow of warm water.

After warming Europe, this flow comes to a halt in the waters off Greenland,

sinks to the ocean floor and returns south. The water arriving from the south is

already more saline and so more dense than Arctic seas, and is made more so as

ice forms.

Predicted shutdown But Bryden’s study has revealed that while one area of

sinking water, on the Canadian side of Greenland, still seems to be functioning

as normal, a second area on the European side has partially shut down and is

sending only half as much deep water south as before. The two southward flows

can be distinguished because they travel at different depths.

Nobody is clear on what has gone wrong. Suggestions for blame include the

melting of sea ice or increased flow from Siberian rivers into the Arctic. Both

would load fresh water into the surface ocean, making it less dense and so

preventing it from sinking, which in turn would slow the flow of tropical water

from the south. And either could be triggered by man-made climate change. Some

climate models predict that global warming could lead to such a shutdown later

this century.

The last shutdown, which prompted a temperature drop of 5°C to 10°C in western

Europe, was probably at the end of the last ice age, 12,000 years ago. There may

also have been a slowing of Atlantic circulation during the Little Ice Age,

which lasted sporadically from 1300 to about 1850 and created temperatures low

enough to freeze the River Thames in London.

Journal reference: Nature (vol 655, p 438).

 

 

We have grasped the mystery of the atom and rejected the Sermon on the Mount.

Ours is a world of nuclear giants and ethical infants. We know more about war

than we know about peace, more about killing that we know about living.

-- General Omar Bradley

 

 

 

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