How long has climate change been around?

“The greenhouse gases went up,and they should have gone down” many thousands of years ago,said William Ruddiman. “Why did that happen?” His answer is based on circumstantial evidence. Ruddiman said two events in world history—an apparent shift in the composition of the atmosphere and the first explosion of human agriculture—took place at nearly the same time.

Ruddiman first presented his idea of ancient climate change in 2003. But he returned to the subject last month,in a paper intended to rebut one major criticism—that there were not enough people on the planet thousands of years ago for their emissions to make a difference. Ruddiman’s response: yes,there were. And in those days,one farmer was as destructive as multiple farmers are today.

He and Erle Ellis,a professor at the University of Maryland Baltimore County,wrote in the journal Quaternary Science Reviews that early farmers did not have modern fertilizer or factory-made tools,but they did have a lot of land. They would clear an area by cutting or burning it,farm the ground until it was nearly barren and move on. “Those tens of millions (of people) had the impact of hundreds of millions,because per person,they had 10 times the impact,” Ruddiman said. “And that’s enough to start the curve turning around.”

“I think it’s a bunch of bosh,” said Wallace Broecker,a professor at Columbia University. Broecker said he worried that the idea of pre-modern people as carbon emitters would turn into an argument that the modern world need not worry so much about its own pollution.

The science of the debate begins with the idea that Earth has natural freeze-and-thaw cycles,driven heavily by changes in its orbit. The planet is in a warm “interglacial” period,which began 10,000 years ago with the end of the last Ice Age. Beginning more than 8,000 years ago,Ruddiman said,things should have started slowly cooling off again.

But for some reason,he said,the cooling was less than expected. Ruddiman thinks the explanation is revealed in long “cores” taken from polar ice,in which tiny bubbles of air have been trapped for thousands of years. He has examined the bubbles and found that about 5,000 years ago,they began showing unexpected increases in carbon dioxide and methane.

His theory is that the gases were pollutants,produced by civilisations on several continents that were picking up the settled life of farmers. The carbon dioxide,Ruddiman said,could have come from smoke,from forests burned to create farmland on several continents. It could have seeped out of felled trees as they rotted. The methane,a byproduct of decay in swampy water,could have come from areas of Asia newly flooded to grow rice. It also might have been expelled by livestock.

In the atmosphere,Ruddiman says,the gases trap solar heat that might otherwise have bounced back out to space. They were greenhouse gases,the same as now. His theory is that the trapped heat,amplified by natural feedback cycles,may have kept Earth’s temperature steady,when it otherwise might have slipped back toward an ice age. That effect lasted until modern times,he said: temperatures might be more than one degree Fahrenheit higher than they would have been.

The early farmers “did not change the actual climate,” said Ellis,Ruddiman’s collaborator on the recent paper. “They kept the climate from changing.” But Ruddiman’s critics say he is wrong to see human impact here: nature was in control all along.