Big Chills

As every educated European knows--that is to say, virtually every European--the continent's benign climate depends on an anomaly: Atmospheric warming by the Gulf Stream, which Benjamin Franklin first noticed. Were it not for the Atlantic's warming surface currents--and, perhaps too, deflection of high-atmosphere winds by the Rocky Mountains--Paris and London might resemble Winnipeg, and Scandinavia would be virtually uninhabitable. Because of this precariousness, notions of abrupt or catastrophic climate change have more currency in Europe than in the United States--especially the "big chill" scenario developed, ironically, by the American geochemist Wallace Broecker.

Broecker's 50 years of work at Columbia University's Lamont Doherty Earth Observatory got a recent celebration at the laboratory. Anybody wanting a quick and easy introduction to his main accomplishments can do no better than to watch and listen to the songs written for the occasion by folksinger Tom Chapin, who happens to be Broecker's brother-in-law, and Penn State geologist Richard Alley, author of a nice general-reader book about ice coring.

Broecker's big chill started about 10,500 years ago and lasted about 1,200 years, in a period now known as the Young Dryas, named after a Scandinavian flower whose wanderings testified to sudden climate change. In that event, an ice dam blocking a huge inland lake, Agassiz, burst, sending a flood of freshwater into the North Atlantic; the effect was to shut down the North Atlantic conveyor, plunging western and northern Europe into a mini-ice age.

In 1997, Alley published a paper identifying a similar event, one that occurred about 8,200 years ago, in which freshwater abruptly flooded the Hudson Bay. Now, in a recent Science paper, Shi-Yong Yu and colleagues report on an event about 9,300 hundred years ago, with a similar pattern yet again. That episode had "a Northern Hemispheric expression with a spatial pattern nearly identical to that of [Alley's] '8.2 kyr event,' a widespread cooling associated with the sudden drainage of the glacial Lake Agassiz-Ojibway complex through the Hudson Straight."

Commenting, Alley says there do indeed seem to be several significant "wiggles" in Earth's recent temperature record. The basic mechanism starts with the relative saltiness of the Atlantic, a result of trade winds carrying vapor from the Atlantic across Central America to the Pacific. When the salty Atlantic waters reach the region around Greenland and Norway they sink, to start their return journey south. But when there's a sudden freshwater infusion, the waters fail to sink, temporarily shutting down the conveyor mechanism. In the extreme case, says Alley, the surface freshwater freezes off Norway, giving the regional climate a really nasty kick. In any case, "the emerging picture is that the North Atlantic [climate] does care about freshwater."

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