( water. That would not only disrupt the...)Currents could disrupt ocean food chain

"When the Atlantic Conveyer current works, the dead plankton sink to the bottom and are replaced at the surface with nutrient-rich water that encourages further production," Schmittner said. "When the current is disrupted, and the mixing slows, that production also is disrupted."

The shutdown of the Atlantic Conveyer current isn't just idle speculation. A growing body of evidence suggests that it switched on and off 20 to 25 times during the last ice age.

"During the last ice age, from about 100,000 years before present to 20,000 years B.P., thick ice sheets over Canada sporadically dropped armadas of icebergs into the North Atlantic where they melted, sufficiently freshening the water to disrupt the conveyer," Schmittner said.

"There is some evidence backing that up," he added. "Deep ocean sediment core samples show pebbles from land delivered by the floating icebergs."

Schmittner said scientists also have examined ice cores from Greenland and measured isotopes that show rapid temperature changes, which coincide with changes in ocean nutrient concentrations measured in deep-sea sediment cores.

"One full oscillation of these switches took 1,500 years," Schmittner said, "but the individual transitions happened surprisingly fast. The climate went from a cold state to a warm state in as little as 20 to 50 years. Surface temperatures in Greenland increased 20 to 30 degrees Fahrenheit and water temperatures increased 10 to 20 degrees."

Schmittner said the impact of the current on the Pacific Ocean generally isn't as great, even though the system is a global one. Still, he added, plankton production would also decrease in the Pacific if the current was reduced.