During the past two decades—as concern about climate change, and especially abrupt climate change, has mounted—Exhibit A has been a compelling scenario that explains a sharp cold snap that occurred in the Northern Hemisphere during what's called the Younger Dryas period. The snap started about 12 800 years ago and lasted about 1200 years. In 1987 Wallace S. Broecker postulated that fresh waters from the southern rim of the North American ice sheet spontaneously spilled into the North Atlantic through what’s now the St. Lawrence River. Such a deluge would have shut down the salt-and-temperature driven currents that draw warm waters into the ocean there and keep Europe temperate. Broecker, an eminent geochemist and climatologist at Columbia University’s Lamont-Doherty Earth Observatory, north of New York City, postulated that the shutdown of the Gulf Stream led to the observed sharp cooling—an 8 C drop on average. He dubbed the scenario ”the biggest chill.”
Since 1987, whenever scientists have produced major new findings about what melting Arctic ice will mean, questions inevitably arise as to whether global warming could produce another big chill, plunging Western Europe into another miniature ice age. (Although the answers are almost always reassuring, the accelerated melting of Arctic ice this year and concerns about the long-term fate of Greenland’s ice sheet have kept the issue alive.) When a blue-ribbon panel of scientists produced a report about abrupt climate change for the U.S. National Academies’ National Research Council in 2002, naturally the very first reference was to Broecker’s work.
Now research reported in the 9 October issue of The Proceedings of the National Academy of Sciences suggests that the abrupt onset of the Younger Dryas cooling and the freshwater infusion into the North Atlantic did not arise entirely from the inner workings of earth, ocean, and climate systems—but instead got a big nudge from something extraterrestrial. The article, signed by 26 scientists from well-known institutions in the United States and Europe, identified a carbon-rich black layer of sediment at about 50 North American sites, dating from roughly 12 900 years ago—near the start of the chill—and strongly argues that the layer was left by something from space. The scientists propose that one or more large, low-density objects—such as a comet or asteroid—exploded over what is now Canada, destabilizing the glaciers there, spilling fresh water into the Atlantic, and triggering the Younger Dryas event.
The new scenario is reminiscent of the theory that a huge impact caused the extinction of the dinosaurs—which itself has become enormously controversial in science circles. The authors of the new theory acknowledge that their idea may never be provable, because virtually all the direct physical evidence of the explosion—such as a crater, if it occurred over the kilometers-thick ice sheets of the time—might have disappeared when the sheets melted. But the sophisticated radiochemical techniques they used, together with the variety of circumstantial physical evidence they discovered, give credence to their claims.
If the new theory gains ground, how much of an amendment will it be to Broecker’s scenario, and will it have much influence on public opinion about the possibility of catastrophic climate change? Richard Alley, professor of geosciences at Pennsylvania State University, in University Park, and chairman of the National Academies’ abrupt climate change report, argues that some proximate cause had to set Broecker’s postulated mechanisms in motion, and that it doesn’t really matter if it turns out to have been an extraterrestrial object.
Broecker himself says that if telltale signatures of an impact—such as buckyballs, nanodiamonds, or iridium—are found, then the authors ”have something.” But for now, reserving judgment, he declares himself suspicious of ”uncritical catastrophists.”
However scientists come down on that issue, might the new scenario have some influence on northern Europeans who have been worrying that runaway greenhouse gas emissions could melt the Arctic ice and plunge them into an ice age? It might.
This article was corrected on 9 January 2009.