PARIS - New evidence has emerged that an ocean "pump" which drives the currents of the North Atlantic is in decline, apparently as a consequence of global warming, and with potentially big consequences for the climate in northwestern Europe.
The pump in question is a flow of cold, dense water that comes from the Arctic Ocean and Nordic Sea and heads southwards into the North Atlantic, racing through four deep channels between Greenland, Iceland, the Faroe Islands and Scotland.
This causes a circular convection in the North Atlantic that reaches down to tropical latitudes.
Rather like a conveyor belt, the current returns northward as a warm convection, the Gulf Stream, which bathes northwestern Europe and gives the region its mild climate in spite of its northerly latitude.
Now a study, published in Thursday's issue of the British scientific weekly Nature, supports the fears of climatologists that global warming may have damaged this vital ocean mechanism.
A team led by Bogi Hansen of the Faroese Fisheries Laboratory looked at the flow of water crossing one of the deep ridges, the Faroe Bank Channel, located between the Faroes and Shetland.
Using a tethered, upward-looking Doppler radar on the seabed to assess the strength of the current, and poring over hydrographic data, they estimate conservatively that the flow through the channel has declined by at least 20 percent since 1950.
"If this reduction in deep flow from the Nordic seas is not compensated by increased flow from other sources, it implies a weakened global thermohaline circulation," the authors say.
Hansen told AFP said that the pump works because it is driven by cold, dense, salty water.
But, he said, its action could be hampered by the shrinking of the Arctic icesheet or ice-bound coastline in Siberia.
This causes an influx of freshwater into the Arctic Ocean -- and as freshwater does not contain salt, the water becomes less dense and thus does not sink.
"When you add fresh water you reduce pumping efficiency," Hansen said, explaining the lower flow through the channel.
In the longer term, there could be a "cooling effect" on parts of northwestern Europe, as the region will have less return flow from the Gulf Stream, he said.
"One can expect that northern parts of Europe, like UK and Scandinavia, will be affected and one can expect effects further south," he said.
"You can also expect global consequences from this ... If you look at the deep water of the world's oceans, they are fed by only two sources. One of them is in the Antarctic and the other is up here, in this region. So if you reduce one of these sources, you reduce the whole circulation of one of the world's oceans."
Hansen said the study complemented two other pieces of research, showing that the flow from the Nordic seas had become warmer and less salty.
Taken together, they point to the effects of man-made global warming, he said.
"So far we have only seen a reduction and we don't know if this will be a closing or not, but this is in line with the predictions that have been made by climate people," he said.
There remained important knowledge gaps though, he said.
The Faroe Bank Channel accounted for about a third of the flow, and it was unclear whether other big channels, notably the Denmark Strait between Greenland and Iceland, which accounts for about half, had suffered any change.
Climatologists say that the unbridled burning of fossil fuels is causing the Earth's lower atmosphere to warm, a phenomenon with potentially catastrophic consequences for the climatic interplay of sea, land and wind.
One of the most pessimistic scenarios hypothesises that the melting of the Arctic icesheet will choke off the Gulf Stream entirely, plunging northwestern Europe back into the Ice Age.
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