The Great Ocean Conveyor Belt
by Daniel Grossman

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A Flip of the Switch

A 300-year drought today would be catastrophic, says Peter deMenocal. And he says, although researchers aren't certain exactly how these past droughts were caused, there is some reason to believe humans might trigger one. Think back to the massive abrupt incidents noticed in Greenland. Before they were discovered, scientists believed that Earth's climate always changed gradually, as if controlled by the dial of a global thermostat. Professor Richard Alley says the Greenland events made scientists realize that climate sometimes behaves like it's controlled by a switch.

"And a switch - you push on it a little and nothing happens. But if you push on it a little more and boom something's changed. And what the ice cores in Greenland forced us to believe is that the climate has switches as well as dials."

Columbia University climatologist Wally Broecker developed the theory of abrupt climate change. - photo by Daniel Grossman

If you want know how these switches work, you need to talk to Columbia University professor Wally Broecker. Broecker has come up with a theory that has gained widespread scientific acceptance.

In a spacious office at Columbia's Lamont Doherety Earth Observatory north of New York City, Broecker leans over a white board. His thinning gray hair appears tussled by high winds. He's making a series of lines and arrows on a map he's drawn of the Earth.

"It starts out here in the South Atlantic. Goes through the Caribbean. Swings around Florida where it becomes the Gulf Stream that we know and love. And some of the water goes up into the Norwegian and Greenland sea keeping that region of the ocean unusually warm because heat is being pumped up there all the time."

(larger version) - courtesy of The Intergovernmental Panel on Climate Change

Broecker is sketching the path of an ocean current that flows with the strength of 150 Amazon Rivers he's dubbed it the Great Ocean Conveyor Belt.

Tropical heat from the great conveyor warms Western Europe, which is, in part, why the English tend gardens at the same latitude Canadian Inuit build igloos. Two things happen as the conveyor approaches the north Atlantic, both of which cause its waters to become denser or less buoyant. First, as the conveyor gives off heat, its waters become cooler. And cooler water is denser. Second, the conveyor's water becomes saltier, because fresh water evaporates on the odyssey from the equator. Saltier water is also denser. Between Iceland and Norway, the now very dense conveyor sinks below surrounding lighter waters. Like the piston of a pump, this downward flow pushes the current on its circuitous path around the globe. The conveyor's water flows south along the ocean floor and into the Pacific where it warms up and returns to the surface. Then it begins the long trip back.

Wally Broecker has proposed that changes in the great conveyor altered the North Atlantic's climate and caused Greenland's abrupt events.

"This conveyor was essentially turning on and off, so all that heat no longer came up there and we think during glacial time that allowed the surface ocean to freeze over."

Next - An Historic Parallel

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