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| Photo: Nature |
The magazine Nature reports that 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.
The magazine says that 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.
Scientists, from the National Oceanography Center in Britain, measured sea temperature, currents and other conditions across the Atlantic from the Bahamas to Africa last year and found a 30 percent drop in the flow of warming waters since a similar set of measurements were taken in 1957.
The team, led by Harry L. Bryden, write in the current issue of Nature, that even though they had measurements from only 5 years out of the last 50, the pattern of change seen at various depths supported the idea that the shift was a significant trend and not random variability.
The researchers cite independent measurements of a long-term decline in the flow of water between some Arctic seas and the North Atlantic as evidence that a slowing of the overall Atlantic circulation was under way.
In a commentary in the magazine, Detlef Quadfasel of the University of Hamburg, says that it provided "worrying support for computer models that predict just such an effect in a world made warmer by greenhouse gas emissions."
A long-term trend?
US climate modeler Gavin A. Schmidt, who works at NASA's Goddard Institute for Space Studies, said the estimated decline in ocean circulation should have produced a perceptible dip in surface temperatures but that no such dip had been measured.
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 Detlef Quadfasel.
"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.
