As North Atlantic current slows, concern rises

Each second, millions of cubic meters of cold, dense Arctic seawater slip over the top of an undersea ridge stretching between Greenland and Scotland, then slide thousands of meters to the floor of the Atlantic to begin a journey of global proportions.

Now, a team headed by oceanographer Bogi Hansen, with the Faroese Fisheries Laboratory in Torshavn, Faroe Islands, reports that during the past half century, the flow of cold water south through a key gap in the ridge has slowed measurably.

If that reduction isn't offset by higher flows elsewhere along the ridge, they say, their measurements could signify that human-induced climate change is beginning to apply the brakes to the main engine-driving North Atlantic Ocean circulation - which in turn affects conditions ranging from regional climate patterns to economically important fisheries worldwide.

The results, published in today's edition of the journal Nature, indicate that the flow into the Atlantic through the Faroe Bank channel has fallen by more than 20 percent since 1950. The team notes that its results are consistent with climate-model forecasts of the effects of rising greenhouse-gas concentrations from human activities.

"This is the latest in a chain of evidence" stretching back to the 1980s "that the pump in the north is slowing down," says William Turrell, an oceanographer at the Fisheries Research Services Marine Laboratory in Aberdeen, Scotland, and a member of the team.

In broad terms, differences in sea water's heat and salt drive the formation of North Atlantic deep water, the subject of the study. As relatively warm sea water moves north over the Greenland-Scotland undersea ridge and cools, it grows more dense until it begins to sink.

Its density also is affected by the water's salt content - the saltier the water, the greater its density at a particular temperature.

In the Nordic Sea, north of the ridge, sea water typically begins to sink when it reaches a toe-numbing 37 degrees F.

Along much of the ridge, punctuated by Iceland and the Faroe Islands, this cold, dense water gathers at depths below 500 meters - the typical depth for the top of the ridge, the researchers note.

Between the Faroe Islands and Scotland, however, a gap in the ridge dips to about 840 meters, forming a natural "spillway" for the deep water. This is where the team directly measured the outflow for more than 4-1/2 years, beginning in November 1995.

During that period, they noted a continuous weakening in the flow, ranging from 2 to 4 percent a year. To reach deeper into the past, they used less direct measurements and mathematical models to estimate the flow rate prior to 1995.

How closely these observations can be tied to human-induced climate change remains an open question, some researchers say.

They note that changes in North Atlantic thermohaline circulation, as the overall circulation regime is called, can vary widely over time scales ranging from decades to millenniums.

Cecelie Mauritzen, an oceanographer at the Woods Hole Oceanographic Institution in Woods Hole, Mass., notes that the Hansen team's results are "suggestive, but are a long, long way from conclusive."

Dr. Mauritzen, who also has worked in the region Hansen and his colleagues are studying, applauds the team for compiling the 1995-2000 data, the longest time-series of direct measurements through the channel.

But, she adds, the region remains data-poor, in both time and space, forcing researchers to rely on mathematical simulations to reconstruct historical flow patterns.

More-conclusive results are likely to come, she continues, if researchers can deploy a series of remote-sensing buoys throughout the Atlantic to gauge not only what is happening at the surface, but at depth, over long periods of time.

(c) Copyright 2001. The Christian Science Monitor