Is Earth's climate vulnerable to small disturbances? Two geologists writing in Scientific American described the past vulnerability of an ocean current sometimes called the North Atlantic conveyor.

How The Conveyor Works

The conveyor works like this. At about 2,000 feet beneath the surface of the Atlantic Ocean extra-salty water flows northward from the tropics. Winds over the North Atlantic sweep surface water aside, allowing this warm salty water to rise to the surface and come into contact with the cold air.

Heat passes from the water to the air. The extra-salty water, now extra-dense because it is only a few degrees above freezing, sinks to near the bottom of the Atlantic and flows southward around Africa. Meanwhile, the warmed air passes over Europe and makes European winters warmer than they would be otherwise.

Flowing Water

During a one-thousand-year period about 11,000 years ago, this conveyor was shut down, because the North Atlantic was flooded with melt water from Canadian glaciers. Previously, that melt water had flowed down the Mississippi River. But at some point the retreat of the glaciers opened a channel to the east.

Melt water then flowed down the St. Lawrence River into the North Atlantic, blocking the flow of warm water from the South Atlantic. The conveyor was stopped, and Europe was hit with a thousand-year cold spell.

Canadian Ice Sheet

The conveyor started again when another change in the shape of the Canadian ice sheet diverted melt water back to the Mississippi.

The events of 11,000 years ago suggest that small disturbances may sometimes cause sudden, dramatic changes in Earth's climatean idea we may be concerned about as we try to predict the effects of human activity on our planet.