The Great Ocean Conveyor Belt
by Daniel Grossman
part 1 2 3 4 5 6
An Historic Parallel
Broecker's theory is now the leading explanation for these abrupt changes. It explains, for example, why Europe, which is warmed by the conveyor, cooled dramatically about 8,000 years ago. However, scientists are still uncertain how the shutdown reverberated worldwide. How did eastern China and India and Mesopotamia, where the Akkadians lived, become drier? How did a rain belt near Venezuela move? How did wind patterns off California change?
Nevertheless, Broecker says after years of research, scientists do know what caused the conveyor to shut off.
"One of the triggers that caused the conveyor to go off was the sudden release of a lot of fresh water," says Broecker.
Remember that salt water is denser than fresh water. If a lot of fresh water was mixed into the north Atlantic, the current would be too buoyant to sink, jamming the conveyor's piston. Researchers have discovered that at least three times in the past, that's exactly what happened. These incidents appear to be related to minute fluctuations in sunlight. The last such event occurred 8,200 years ago, a couple of thousand years after the last ice age ended.
Glaciologist Richard Alley has studied the incident.
Hudson Bay (center) was once the largest freshwater lake on the planet. As the ice covering Canada melted away 8,200 years ago, the bay quickly drained immense amounts of fresh water into the North Atlantic disrupting the Great Ocean Conveyor Belt. - (larger version)
Map courtesy of the Library of Congress
"The great ice sheet in Canada was really melting away. It was melting back toward Hudson Bay. And it had ponded around it an immense lake. And it is reasonably clear that that lake drained maybe in a summer. So you're taking maybe the biggest lake on Earth and putting it into the Atlantic in a summer."
Today, there's no such water poised to flood the North Atlantic, but Broecker says global warming has created a new mechanism that might flip the conveyor's off switch.
"As climate warms due to the steady creeping warmth, more water will evaporate in the warm parts of the planet and, hence, more of that water will get to the high latitudes. And the extra rain and snow in the regions around the north Atlantic will add enough fresh water to kill the conveyor."
Next - Poking the Angry Beast