As Mammoths Died Out, Earth Chilled

The rapid decline of mammoths and other megafauna after humans spread across the New World may explain a bone-chilling plunge in global temperatures some 12,800 years ago, researchers reported Sunday.

The 100-odd species of grass-eating giants that once crowded the North American landscape released huge quantities of methane -- from both ends of their digestive tracks.

As a heat-trapping greenhouse gas, methane is 30 times more potent than carbon dioxide (CO2).

It was not enough to trigger runaway global warming. But when all that gaseous output suddenly tapered off, it caused or at least contributed to a prolonged freeze known as the Younger Dryas cold event, they argue.

If so, the "Anthropocene epoch" -- the era of major human impacts on Earth's climate system -- began not with the industrial revolution in the 1800s, but the large-scale influx of two-legged predators to the Americas more than 13,000 years earlier.