By killing Ice Age mammals, humans cut greenhouse gas emissions, study says.
When mammoths and other Ice Age "megafauna" disappeared from the Americas about 12,800 years ago, the animals took with them their planet-warming burps—spurring the mysterious cooling period known as the Younger Dryas, a new study says.
And because humans are thought to have killed the creatures off, the deaths hint that we've been changing the climate since long before the first Model T chugged out of Mr. Ford's factory.
According to ice core studies, the Younger Dryas event began about a thousand years after mass human migrations into the Americas 13,400 years ago, near the end of the last ice age.
The world had been starting to warm, but the Younger Dryas brought on a freeze that lasted roughly 1,300 years, with estimated temperature drops of 7.2 to 14.4°F (4 to 8°C) in eastern North America and northern Europe.
Also within a thousand years of the human migrations, more than 114 species of large plant-eaters—including woolly mammoths, giant camels, and ground sloths—had gone extinct. (See pictures of a stunningly preserved baby mammoth.)
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