A professor emeritus at the University of Virginia has suggested that people began altering the climate thousands of years ago, as primitive farmers burned forests and built methane-bubbling rice paddies.

Has climate change been around as long as the pyramids?

It is an odd-sounding idea, because the problem is usually assumed to be a modern one, the product of a world created by the Industrial Revolution and powered by high-polluting fossil fuels.

But a professor emeritus at the University of Virginia has suggested that people began altering the climate thousands of years ago, as primitive farmers burned forests and built methane-bubbling rice paddies. The practices produced enough greenhouse gases, he says, to warm the world by a degree or more.

Other scientists, however, have said the idea is deeply flawed and might be used to dampen modern alarms over climate change.

Understanding the debate requires a tour through polar ice sheets, the inner workings of the carbon molecule, the farming habits of 5,000-year-old Europeans and trapped air bubbles more ancient than Rome.

"The greenhouse gases went up, and they should have gone down" many thousands of years ago, said U-Va.'s 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.

"Greenhouse gases do something they never did before," Ruddiman said. "And humans do something the Earth (had) never seen before."

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."

The assertion is the heart of Ruddiman's arguments — and his critics' complaints. Where he sees human impact on the curving plot of global temperatures, they see a misunderstanding of what nature was doing at the time.

"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. "I get really upset with him, because people who oppose global warming (legislation) can use this as some dodge."

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 civilizations 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.

"I think it's more or less a hypothesis without any evidence to support (it)," said Ken Caldeira, a researcher at the Carnegie Institution for Science in California.

(Optional add end)

In fact, critics of Ruddiman say, there is strong scientific evidence to prove him wrong. They say that recent studies of very deep ice samples show that ice ages did not always come and go on the same schedule. That could throw off Ruddiman's calculations for when the next round of cooling was supposed to start.

And there is evidence, they say, inside the carbon atoms themselves. Carbon atoms that come from plants can be tracked, by looking at the number of neutrons in their nucleus. If Ruddiman was right, and ancient farmers burned enough plants to change the climate, then the amount of carbon from plants in those bubbles would rise significantly.

But, they say, it did not.

"The general feeling (about Ruddiman's theories) in the community is 'Interesting — but probably not,' " said Gavin Schmidt, a climate researcher at NASA's Goddard Institute for Space Studies in New York.