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Researchers Measure 30% Weakening in Atlantic Circulation

30 November 2005

Thermohaline_large
The Atlantic Ocean circulation system. Click to enlarge.

Researchers in the UK have measured an apparent 30% weakening in the warm Atlantic Ocean currents that carry heat from the tropics to the high latitudes of Western Europe.

The team behind the new study are the first to spot these signs of decline in Atlantic currents. Harry Bryden of the National Oceanography Centre in Southampton, UK, and his team report their results in this week’s Nature.

Should this prove to be a sustained decline (there is a degree of uncertainty estimated in the paper), the findings would be extremely significant, and could mark an intensification of European winters in a relatively short period of time.

The Atlantic meridional overturning circulation—the current system that includes the warm Gulf Stream current—is a major contributor to the relatively mild weather experienced by Northern and Western Europe, even at its comparatively high northerly latitude. Both salinity and water density are key to the functioning of the transport.

The weakening of the system is likely caused by the additional fresh water flowing into the northern ocean from rivers, rain and melting ice, and this is thought to be linked to global warming. Despite no indication of climate cooling—on the contrary, average temperatures in Western Europe have increased—climate modellers are worried that the resulting weakening of ocean currents could ultimately lead to substantial cooling of the North Atlantic. Cooling resulting from warming, in other words.

A direct impact of the weakening circulation on air temperatures in western Europe has so far not been observed. Average temperatures have increased by around . Whether or not the true warming is partly eclipsed by an opposite oceanic cooling trend is not clear.

—Detlef Quadfasel, University of Hamburg

During a cruise in spring 2004 from the Bahamas to the Canary Islands, on board the British research vessel RRS Discovery, the research team measured water temperature and salinity along a latitude of 25º North, taking samples roughly every 50 kilometers.

Nature04385f12_1
Station positions for transatlantic hydrographic sections taken in 1957, 1981, 1992, 1998 and 2004. Note Cuba and Florida to the left, Africa to the right. The 1957 and 1992 sections each went zonally along 24.5° N from the African coast to the Bahama Islands. Because of diplomatic clearance issues, the 1981, 1998 and 2004 sections angled southwestward from the African coast at about 28° N to join the 24.5° N section at about 23° W. The 1998 and 2004 sections angled northwestward at about 73° W to finish the section along 26.5° N.

They then calculated from the density and pressure differences between each sample, the volume and velocity of the circulation at various depths, assuming that from coast to coast the balance of water flowing north and south must be zero.

Similar measurements along the same latitude were previously made in 1957, 1981, 1992 and 1998. Until now, the data never showed any significant decline in circulation.

Now, however, although the near-surface, and mostly wind-driven, Gulf Stream has remained almost constant since 1957, the deep-ocean return flow of cooler water has decreased dramatically. This cycle usually returns water to more southerly latitudes from as far north as Greenland and Scandinavia.

The warmer water now seems to be trapped in a loop in the subtropical Atlantic, instead of cycling all the way to the ocean's northern extremity.

Whereas the northward transport in the Gulf Stream across 25° N has remained nearly constant, the slowing is evident both in a 50 per cent larger southward-moving mid-ocean recirculation of thermocline waters, and also in a 50 per cent decrease in the southward transport of lower North Atlantic Deep Water between 3,000 and 5,000 m in depth. In 2004, more of the northward Gulf Stream flow was recirculating back southward in the thermocline within the subtropical gyre, and less was returning southward at depth.

—Bryden et. al.

This is quite sensational information in itself. But it is also an important message to politicians who negotiate the future of the Kyoto agreements: we do change our climate.

—Detlef Quadfasel

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November 30, 2005 in Climate Change | Permalink | Comments (12) | TrackBack (1)

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Comments

what does this mean? Trapped heat? These heat will transfer into water vapor, and extra heavy rain in some area while extra rigid weather in other.

Anyone havent watch ?

..."The Day After Tomorrow"

More like Kim Stanley Robinson's "40 Signs of Rain" series....

IF the Atlantic conveyor shuts down due to global warming causing too much fresh water to enter the ocean, it will result in a mini-Iceage for europe.

If we keep it up for too long, it will result in the return of a major Iceage event for the whole world. This could result in human extinction.

Mother nature has a way of dealing with greedy people.

This is the big 'Oh Shit!' scenario for Europe... let's hope these findings are wrong.

Or better yet, lets start doing something serious about combating global warming... like yesterday people!

Ok people, we must start doing something now. Let it be plant more trees, burn less fuel, or start digging into earth and find somewhere warm so we can start building a geothermal reactor there and then continue building our civilisation underground...

Its time to massively switch to green sources of energy
And reduce our CO2 production.
US Petrol prices shoud be taxed to help to make the shift to less fuel hungry vehicles.

Lucas you seem a bit confused. A mini-Iceage for Europe is conceivable if the currents stopped flowing. An iceage for the world makes no sense: the mean temperature of the earth is increasing! on average it is warmer...so i don't know where the ice age business comes from.

and since you are using a computer and presumably sitting in an heated/cooled room, you are apparently one of the greedy carbon-producing people you tacitly denounce...

This is just bad science. A group of scientist on a cruise ship, making one study and saying the sky is falling. Yes, the earth is warming. Yes, the earth is changing. But true science is still out on how much effect man has. Current weather in Europe seems to contradict these findings.
It is unfortunate that science is being used this way. It seems agenda is getting in the way of scientific study.

Actually, the researchers come from an institute with a decent reputation. And Nature is generally regarded as a journal with decent peer review.

So while more research has to be done, and the (immediate) effects of this reduction in heat circulation are not clear, it seems a little premature to yell 'bad science'. Feel free to come up with scientific arguments and data to validate that claim. The oceanographers have done so in Nature..

A planet like ours, if unpeopled, will evolve at a certain rate all by itself. However, intelligent life forms on such a planet WILL use what resources they need to survive, to invent things that make life easier or more enjoyable, to overpopulate, and to war against those who would have all the good things for themselves. Fossil fuel dependency, environmental degradation, and accelerated climate change are not surprising. Humans are giving planetary evolution a kick in the seat and there is nothing at this point that world governments (UN?) will actually agree to do which will forestall the inevitable. Look at the BIG picture folks! Kyoto Accords and hybrid cars can't stop what is happening now. It is WAY too late!

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