As massive grazing animals were killed off by early human hunters, methane levels dropped, contributing to a chilling planet.
The influx of human hunters to the Americas may have helped trigger a plunge in global temperatures some 12,800 years ago.
Extrapolating from data on cows, scientists estimated the methane output of pre-historic megafauna was nearly 10 trillion grams per year.
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.