Ancient farmers were responsible for increasing the Earth's temperature by about 0.9 degree Celsius over a period of 8,000 years - almost as much as global warming has caused in the past 150 years.
The finding, by a new study, suggests that early agriculture was as powerful as the whole industrial revolution, says Feng He, lead author and a climate scientist at the University of Wisconsin, Madison.
The study, however, says that the "net warming" caused by early humans was only 0.73 degrees Celsius, thanks to a slight cooling due to more sunlight reflecting from cleared land.
The study says that while early cultures were "global warming turtles" that slowly raised temperatures by adding greenhouse gases such as carbon dioxide and methane to the planet's atmosphere over thousands of years, post-industrial revolution societies have "climate change rabbits".
They were responsible for temperatures rising about 0.85 degrees C between 1880 and 2012, according to the Intergovernmental Panel on Climate Change (IPCC).
Usually, 1850 is picked as the kickoff for global warming, but the study suggests human activities such as deforestation and agriculture could have shifted the climate earlier.
And ice cores suggest this is indeed the case: carbon dioxide and methane levels over the past 8,000 years do not follow their usual post-ice age trends. The gases go up as human population booms, instead of their usual decline.
Feng He and his co-authors estimated past global temperatures with climate models that calculated the effects of land-cover changes such as deforestation and irrigation.
They compared climate models of a human-free Earth to a planet crawling with hunter-gatherers and farmers. The researchers used estimates of past land-use from a past study that built a detailed model of land-use over time based on historical and archaeological data.
The study says that after the last ice age ended, carbon dioxide and methane levels should have dropped to about 245 parts per million (ppm) and 445 ppm without human influence on the planet.
Instead, carbon dioxide rose about 40 ppm, to 285 ppm, and methane jumped to 790 ppm, a 345 ppm rise, as early humans chopped down trees and irrigated rice fields.
"In terms of long-term climate change, the last several thousand years are unique because of this human factor in it," Feng He said. "It's almost like we're on a speeding train without a brake, but we are continually putting in the coal into the engine."
The findings of the study have been published in the latest edition of the journal Geophysical Research Letters.