The Atlantic Ocean circulation system. Click here to see enlarged view.© Nature

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 25º 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 ºC 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. 

University of Hamburg

• References

  1. Bryden H., Longwort H. & Cunningham S. Nature, 438. 665 - 657 (2005).