While many think of climate change as the result of the power plants, factories and automobiles of the industrial era, our ancestors may have begun warming the planet 8,000 years ago.

Early farmers burned large tracts of land to plant crops, which released carbon dioxide into the atmosphere. Ocean-sediment and ice-core records reveal that the rise of agriculture coincides with the end of a natural cycle of greenhouse gas concentrations in the air.

Paleoclimatogolist and professor of environmental science William F. Ruddiman believes an ice age that should have arrived several thousand years ago has been forestalled by early farmers.

Critics argue that the population of early farmers was too small to affect the environment. “Many climate models assume that land use in the past was similar to land use today,” says Erle Ellis, Ruddiman’s co-author. “We are proposing that much smaller earlier populations used much more land per person.”

Several studies indicate that people today use 90 percent less land per person than those in early civilizations. “They used more land for farming because they had little incentive to maximize yield from less land, and because there was plenty of forest to burn,” says Ruddiman.