Also: Press release from Woods Hole Oceanographic Institute:

Ancient clues to ocean currents

The Ocean Conveyor transports heat worldwide. White sections represent
warm surface currents. Purple sections represent cold deep currents.

The close link between temperatures in the North Atlantic and the strength of ocean circulation is underlined by a new analysis of sea-floor sediments. These contain traces of naturally occurring radioactive atoms in ratios that indicate the speed of ocean waters over the past 60,000 years.

Rapid climate change may be related to how vigorously ocean currents move heat between low and high latitudes. If human activity alters circulation speeds, as many scientists suspect may happen, it could have a dramatic impact on temperatures.

This is of particular concern in northern Europe, which benefits from the strong flow of warm waters that move up from the tropics and keep winters mild. These waters eventually sink, or overturn, at high latitudes and return to the tropics as a deep, cold flow.

If this great circulation, or conveyor, slows – and there is some evidence this is happening – then European winters should become harsher.

“Warm periods in the past are generally associated with strong ocean circulation, or overturning; and cold periods are generally associated with a weak overturning circulation.”

See also: Failing ocean current raises fears of mini ice age.