Mick Hartley

We've been here before, but they now claim to have some stronger evidence:

At least once in Earth’s history, global warming ended quickly, and scientists have long wondered why.

Now researchers are reporting that the abrupt cooling — which took place about 12,900 years ago, just as the planet was emerging from an ice age — may have been caused by one or more meteors that slammed into North America.

That could explain the extinction of mammoths, saber-tooth tigers and maybe even the first human inhabitants of the Americas, the scientists report in Friday’s issue of the journal Science.

The hypothesis has been regarded skeptically, but its advocates now report perhaps more convincing residue of impact: a thin layer of microscopic diamonds found in rocks across America and in Europe.

“We’re up over 30 sites, as far west as offshore California, as far east as Germany,” said Allen West, a retired geology consultant who is one of the scientists working on the research.

The meteors would have been smaller than the six-mile-wide meteor that struck the Yucatán peninsula 65 million years ago and led to the mass extinctions of the dinosaurs. The killing effects of the hypothesized bombardment 12,900 years ago would have been more subtle.

Climatologists believe that the direct cause of the 1,300-year cold spell, known as the Younger Dryas, was a sudden rush of fresh water from a giant lake in central Canada to the North Atlantic.

Usually a surface current of warm water flows northward in the Atlantic toward Greenland and Europe, then cools and sinks, returning south in the deep ocean. But the fresh water, which is less dense, blocked the sinking of the cold, salty water in the North Atlantic, disrupting the currents.

That sudden change in plumbing has long been known, but what caused it has never been satisfactorily explained.

The authors of the paper in Science say it was meteors.

At each site the scientists looked at, the diamond layer in the rocks correlates to the date of the hypothesized impact. Within the layer, the scientists report finding a multitude of diamond particles, all encased within carbon spheres. “We’ve yet to find a single diamond above it,” Dr. West said. “We’ve yet to find a single diamond below it.”

Perhaps more telling, the scientists reported last month at a meeting of the American Geophysical Union in San Francisco, the carbon atoms inside some of the diamonds are lined up in a hexagonal crystal pattern instead of the usual cubic structure. The hexagonal diamonds, formed by extraordinary heat and pressure, have been found only at impact craters and within meteorites and cannot be formed in forest fires or volcanic eruptions, Dr. West said.

The obvious explanation for the extinction of the American megafauna – sabre-toothed tigers, mastodons, giant wolves and the like – was the appearance of the first humans coming across the Bering Strait. As I wrote before:

It just seems to me too much of a coincidence that the Australian megafauna died out sometime around 50,000 years ago, just when humans first arrived. Then the American megafauna died out 12,000 years ago, just when humans first arrived. And the New Zealand megafauna (well, megabirds) died out around 1500 years ago, when humans first arrived. People and megafauna don't mix, except in Africa where they evolved together.

But the evidence is at least strong enough now to suggest that the picture is more complicated than that.