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Event

 
Atlantic currents show signs of weakening
Published online: 30 November 2005
http://www.nature.com/news/2005/051128/full/051128-9.html

Atlantic currents show signs of weakening
Research cruise reveals evidence of half-century of wane.
Quirin Schiermeier

The North Atlantic's natural heating system, which brings clement weather to 
western Europe, is showing signs of decline. Scientists report that warm 
Atlantic Ocean currents, which carry heat from the tropics to high 
latitudes, have substantially weakened over the past 50 years.

Oceanographers surveying the 'Atlantic meridional overturning circulation', 
the current system that includes the warm Gulf Stream current, report that 
it seems to be 30% weaker than half a century ago.

Failures of the Atlantic Ocean's circulation system are thought to have been 
responsible for abrupt and extreme climate changes during the ice age that 
lasted from 110,000 to 23,000 years ago. More recently, a fictional shutdown 
of the Gulf Stream inspired the 2004 Hollywood blockbuster The Day after 
Tomorrow.

The climate shifts depicted in the movie, in which New York is engulfed by 
an instant ice age, are mere fancy. But scientists are worried about the 
real changes measured in the North Atlantic. Both salinity and water 
density, which influence the transport of warm waters, have previously been 
found to be decreasing.

Stuck in a loop

The likely cause is more fresh water flowing into the ocean from rivers, 
rain and melting ice, and this is thought to be linked to global warming. 
But climate modellers are worried that the resulting weakening of ocean 
currents could ultimately lead to substantial cooling of the North Atlantic.

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 Nature1.

During a cruise in spring 2004 from the Bahamas to the Canary Islands, on 
board the British research vessel RRS Discovery, the team measured water 
temperature and salinity along a latitude of 25 North, taking samples 
roughly every 50 kilometres. 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. But until now, the data never showed any significant 
decline in circulation. "In 1998 we saw only very small changes," says 
Bryden. "I was about to give up on the problem."

However, this time things were very different. The near-surface, and mostly 
wind-driven, Gulf Stream has remained almost constant since 1957. But 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.

But much of this 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. 
Bryden and his colleagues estimate that, overall, the circulation has slowed 
by about 30% since 1957.

"This is quite sensational information in itself," says Detlef Quadfasel, an 
oceanographer at the University of Hamburg in Germany. "But it is also an 
important message to politicians who negotiate the future of the Kyoto 
agreements: we do change our climate."

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 0.6 C since 1900. Whether or not the true warming is partly eclipsed 
by an opposite oceanic cooling trend is not clear, says Quadfasel.

A long-term trend?

Other oceanographers warn that this is not proof of a long-term trend. 
Possible disturbances such as ocean eddies, and natural fluctuations in the 
strength of the circulation system, must be considered, they say.

"Something is clearly going on," says Jochem Marotzke, an oceanographer at 
the Max Planck Institute for Meteorology in Hamburg. "But we still have only 
a series of snapshots. The crux is to determine how representative they 
really are." He adds that the chances of imminent collapse of the 
circulation system is small.

Sensor-equipped moorings installed at 25 locations across the subtropical 
Atlantic have now begun to monitor continuously the circulation at all 
depths. The next four years or so should tell us whether the Atlantic 
heating system is still working well, says Marotzke.

References
BrydenH., LongwortH. & CunninghamS. Nature, 438. 665 - 657 (2005).

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