The shutdown would cause global stocks of plankton, a vital early link in the food chain, to decline by a fifth while plankton stocks in the North Atlantic itself would shrink by more than half, it said.

"A massive decline of plankton stocks could have catastrophic effects on fisheries and human food supply in the affected regions," warned the research, authored by Andreas Schmittner of Oregon State University.

The circulation system is like a conveyor belt, taking warm water from the Caribbean in the tropical western Atlantic to the cold latitudes of the northeastern Atlantic.

There, the warm surface water cools and sinks, gradually getting hauled around back to the southwest, where it warms again and rises to the surface.

This movement is vital for northwestern Europe, for the warm water brings the region balmy, wet weather. Without it, Ireland, Britain, parts of France, Belgium, the Netherlands and Germany would be plunged into prolonged, bitter winters.

The circulation is also essential for plankton, providing an upwelling of deep-water nutrients on which these tiny creatures feed. In turn, the plankton feed fish and other marine animals, which in turn are harvested by humans.

Schmittner, writing in the British weekly science journal Nature, said his computer model of plankton loss was based on a disruption of the circulation system over 500 years, during which the conveyor belt lost more than 80 percent of its power.

Temporary slowdowns in the Atlantic's circulation system have occurred in the past, most notably after the end of the last Ice Age, about 10,000 years ago, said Schmittner.

Isotope traces from Greenland icecores suggest there were bursts of rapid warmings of 10 C (18 F), which melted huge amounts of Arctic ice.

This influx, because it comprised cold freshwater, sank to the bottom of the ocean floor, essentially acting like a giant sandbag thrown on the conveyor belt, braking its movement.

Today, Earth is considered to be in an "inter-glacial" period -- a balmy period between ice ages.

But scientists say there is a possibility of another big temperature rise induced by man-made global warming, caused by the spewing of fossil-fuel greenhouse gases into the atmosphere.

One scenario, considered outlandish only a few years ago but now increasingly taken seriously, is that a fast melt of part of the Greenland icesheet could slow or stop the warm-water circulation in the North Atlantic, with catastrophic, long-term results.

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