Paris: It has been called the Big Freeze, the Last Blast of the Ice Age or -- to use the scientific name -- the Younger Dryas, and for climate experts it is one of the big mysteries of their field.
Around 12,900 years ago, Earth was on a steadily warming trend.
For nearly 100,000 years, the planet had been gripped in glaciation. Ice sheets placed a swathe of the northern hemisphere under a dead hand, extending their thrall as far as south as New England and Wales.
But just as the glaciers were beginning to retreat, and an easier life at last beckoned for Earth's small population of humans, everything went into reverse.
Temperatures fell dramatically by up to eight degrees Celsius (14.4 degrees Fahrenheit), heralding a cruel winter that would last 1,300 years.
But what caused it?
Crunching powerful equations and weighing fresh evidence, an astrobiologist in Britain is pointing the finger at an unusual culprit.
Earth collided with debris from a vast comet, measuring 50-100 kilometres (31-62 miles) across, that had wandered into the inner Solar System some 30,000 years ago before breaking up, says Bill Napier, a professor at Cardiff University's Astrobiology Centre.
The impact unleashed a firestorm that blanketed the atmosphere with ash and dust, reducing heat and light from the Sun, Napier suggests in Monthly Notices, a journal of the Royal Astronomical Society.
What is worrying, adds Napier, is that our planet still crosses the path of the remaining orbiting cometary rubble, a well-observed, although still enigmatic, phenomenon called the Taurid Complex.
Many of these fragments are tiny and their burnup in the atmosphere causes pretty shooting stars.
Other pieces, though, may not be so enchanting.
They are not big enough to inflict global extinctions like the event 62 million years ago that wiped out the dinosaurs, but they could still devastate entire regions.
"It (the Taurid Complex) includes at least 19 of the brightest near-Earth objects," says Napier.
"Sub-kilometre bodies [objects measuring 1,000 yards across or less] in meteor streams may present the greatest regional impact hazard on timescales of human concern."
Previous research buttresses Napier's theory that only an impact could explain the plunge in temperature, as opposed to a natural quirk in Earth's orbit or drop in the Sun's heat output.
At 15 sites in North America, archaeologists have uncovered a thin layer of soot deposited at the time of the Younger Dryas.
It includes microscopic hexagonal "nano-diamonds," imbedded within melted plant resin, which elsewhere have only ever been found in meteorite craters.
Then there is evidence of a die-out. If the fossil record is a guide, at least 33 mammals abruptly disappeared, including types of mammoth, horse and camel as well as birds and smaller mammals.
Under some scenarios, an asteroid or comet measuring some four kilometres (2.5 miles) across smashed into the two-km- (1.2-mile) -thick Laurentide ice sheet covering present-day Canada and part of the northern US.
Other astronomers, though, have railed at this theory.
At the time, they note, the Solar System had been largely cleansed of the monster rocks of its infancy, and the chance of a collision with an object of this magnitude was a thousand to one against.
And the heat unleashed by the rising fireball would be constrained by the curvature of the horizon, and this would not explain why wildfires erupted across the continent.
But Napier says the riddle can be explained.
According to his model, Earth ran into a swarm of fragments, not a single vast chunk.
For one hellish hour, North America was bombarded by thousands of impacts, each releasing the energy of a megaton-class nuclear bomb, and this explains why the fires were so extensive.
"A large comet has been disintegrating in the near-Earth environment for the past 20,000-30,000 years, and running into thousands of fragments from this comet is a much more likely event than a single large collision," says Napier.
First Published: Saturday, April 03, 2010, 09:50