Ruddiman said that starting thousands of years ago, people would burn down a forest, poke a hole in the soil between the stumps, drop seeds in the holes and grow a crop on that land until the nutrients were tapped out of the soil. Then they would move on.

“And they’d burn down another patch of forest and another and another. They might do that five times in a 20-year period,” he said.

That slashing and burning on such a large scale spewed enormous amounts of greenhouse gasses into the atmosphere and warmed the planet, the study says.