Why did mammoths, mastodons, and other mega-beasts vanish from North America?
Was it because:
1) humans killed them;
2) they couldn’t hack the climate after the Ice Age ended; or
3) an exploding comet ignited continent-wide wildfires, sent hundred-mile-an-hour winds and tornadoes howling across the land, and shattered the North American ice sheet, while also maybe gouging out the Great Lakes?
Let’s talk about option number three.
The idea that a comet struck Earth 12,900 years ago, at the beginning of a strange interlude of climate cooling called the Younger Dryas was first proposed in 2007. In the bitter scientific debate that has flared sporadically ever since, the latest evidence includes:
Tiny, glassy “spherules” of rock found in a Pennsylvania flowerbed by a woman who had seen a NOVA program about the comet hypothesis. In a paper that got wide coverage last week, Dartmouth researchers argue that those spherules were hurled to Pennsylvania by an impact in Quebec 12,900 years ago.
Traces of platinum deposited on the Greenland ice cap at about the same time. Harvard researchers argue that the platinum probably came from an extraterrestrial object—not a comet, however, but a rare type of iron-rich meteorite.
Spherules in Syria. In their latest paper, some of the original proponents of the impact hypothesis now say it deposited 10 million metric tons of spherules over an area of 20 million square miles, stretching from Syria through Europe to the west coast of North America.
Some opponents of the hypothesis—and there are many—want so badly for it to go away that they have attempted to declare it dead. “My only comment is that the pro-impact literature is, at this point, fringe science being promoted by a single journal,” one of them, Nicholas Pinter of Southern Illinois University, said last week. The journal in question is Proceedings of the National Academy of Science.
Other researchers are trying to keep an open mind.
“Most people were trying to disprove this,” said Wallace Broecker, a geochemist and climate scientist at Columbia University’s Lamont-Doherty Earth Observatory. “Now they’re going to have to realize there’s some truth to it”—though maybe only a spherule or two.
Why Care About the Younger Dryas?
Even if you’re inclined to let sleeping mammoths lie, this debate matters: It bears on the question of just how fragile Earth’s climate is. Does it need an extraterrestrial whack to go haywire, or can it do that on its own?
The alleged mammoth-killing impact is also alleged to have triggered the Younger Dryas. At the time, 12,900 years ago, the continental ice sheets were in full retreat from the last Ice Age, and the planet was nearly as warm as it is now.
Suddenly, in a matter of decades, glacial temperatures returned, and the ice advanced again. The cold lasted 1,500 years, then ended even more suddenly than it had begun.
In the 1980s Broecker helped bring the Younger Dryas to wide attention. He explained the sudden cold snap with a mechanism that’s internal to the climate system.
At the start of the Younger Dryas, he said, a conveyor belt of ocean currents that normally transports heat into the North Atlantic—the Gulf Stream is part of it—had gotten jammed by fresh meltwater flowing off the receding ice sheet. With no heat flowing north in the ocean, the North Atlantic region relapsed into bitter cold.
The Younger Dryas became the paradigm for the idea that Earth’s climate was an intrinsically flighty creature, capable of shifting abruptly to a radically different state. That idea has made the prospect of future climate change even more worrisome.
Conceivably a comet might have triggered the Younger Dryas by helping to break up the ice sheet and send meltwater into the North Atlantic. It would have had to have been a big comet, with about a million times the energy of the bolide that excavated Meteor Crater in Arizona, which is three-quarters of a mile wide.