Man-made global warming 8,000 years ago may have helped stave off an ice age. Sumitra Rajagopalan reports

Forget Kyoto. By the time Christ appeared on Earth, the planet was already belching enough gas to cause global warming. And we have our ancestors to blame. Or thank.

Flying in the face of conventional wisdom, William Ruddiman, professor of environmental sciences at the University of Virginia, in the US, is behind a theory suggesting that humans had a hand in warming the planet nearly 8,000 years ago and, in doing so, might have prevented another ice age.

In his book, Plows, Plagues and Petroleum: How Humans took Control of the Climate , Ruddiman delves further into the theory that first made waves in the winter of 2003. It was then that Ruddiman, now retired from academia and living in western Virginia, presented his results at the American Geophysical Union in San Francisco. His findings threw a monkey wrench into a time-honoured theory - that global warming began only 150 years ago, shortly after the Industrial Revolution.

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The idea first occurred to him in the late 1990s. Even as he juggled the multiple demands of university life, Ruddiman would often snatch a few moments to reflect on the bigger picture. Earth pirouettes like a ballerina around its axis. Every now and then - once in 22,000 years to be exact - the axis tilts, causing the planet to wobble. This clumsy movement is enough to cause warmer summers in the Northern hemispheres and drive methane levels up in the atmosphere through the breakdown of plant matter in the wetlands. Then, as the Northern hemisphere moves away from the sun, methane emissions plummet, reaching a nadir 11,000 years later. Thus methane waxed and waned throughout recorded history, up until 5,000 years ago.

Then Ruddiman saw a wrong turn.

As he pored over data collected from ancient air trapped inside Antarctic ice cores, Ruddiman found that the methane levels reversed direction 5,000 years ago, soaring back to 700 parts per billion when they should have ebbed to 450ppb, akin to previous cycles.

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Intrigued, he turned his attention to carbon dioxide. Here, a similar picture emerged. During the interglacial period, the CO2 level peaked about 10,500 years ago as expected and continued its slow decline through modern times. Then, it reversed course 8,000 years ago. By the start of the industrial era, CO2 concentrations had soared to 285ppm, about 40ppm higher than expected.

Ruddiman suspected that these discrepancies were not driven by natural causes. "Which led me to wonder," he recalls, "could humans have been responsible for this anomaly?"

He found his answers in the ancient civilisations of China and Mesopotamia.

Ruddiman found evidence from studies in archaeology and human historical records to show that Europeans began clearing forests to make way for new crops such as wheat, barley and peas about 8,000 years ago.

"This significant deforestation would have pushed levels of CO2 upwards," he says.

Likewise, he also noted that about 5,000 years ago Chinese farmers began flooding lowlands near rivers to grow rice, which would have contributed to a rise in atmospheric methane. "That was my 'aha!' moment," he chuckles.

Thus, ancient agriculture, not modern industry, was responsible for the onset of global warming.

Ploughs, not petroleum.

"Fascinating," exclaims Alex Ellery, lecturer in space and planetary sciences at Surrey University. As a long-time observer of climate change, Ellery finds Ruddiman's theory entirely plausible.

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"I suspect he would have to face brickbats from some quarters," he adds.

Indeed, as with all radical theories, this one has its sceptics. In particular, Ruddiman found two critiques that warranted a closer look.

The first came from the European Project for Ice Coring in Antarctica (Epica), a consortium of European climate scientists. At odds with Ruddiman's claims, they believed that there was nothing anomalous about current greenhouse gas levels. In fact, they stated, a similar pattern was prevalent many glacial cycles ago.

In a paper published in Nature last year, the Epica community compared the current interglacial period to an analogous period 400,000 years ago. By comparing the amount of solar radiation reaching Earth for both these periods, they came to a very different conclusion: today's climate is on track, no different from the trends 400,000 years ago. By their estimates, we have 16,000 years to go before we hit the next ice age.

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"In essence, what they were saying was that this is all deja vu," explains Ruddiman, "that what we see today is business as usual, with no human hand tinkering with the pre-industrial era climate."

As it turns out, Ruddiman found the Epica comparison to be based on an inaccurate alignment of data points. "One always has to be careful when drawing comparisons," Ruddiman warns. "One should choose the best possible analogue to the current interglaciation before drawing any conclusions."

No sooner had Ruddiman refuted Epica's first argument than a second scientist weighed in. Fortunat Joos is a professor of environmental physics at the University of Bern, Switzerland. Joos has developed models of carbon cycling that can predict the amount of CO2 in the atmosphere at a given period of time. Joos jumped on Ruddiman's assertions and stated that forest clearing could not account for the anomalous 40ppm rise in CO2. With the area of forestland prevalent in the day, any rise in CO2 would level off at 4ppm. So Ruddiman's assertions were off the mark - by a factor of ten.

Joos had a point. "Indeed," confesses Ruddiman, "he was right. Deforestation could only account for 35 to 45 per cent of the anomalous CO2 rise." So where did the rest of the CO2 come from? After much rumination, Ruddiman hit on a possible answer. "It dawned on me that, alongside human factors pushing up CO2 levels, there might be another mechanism keeping the CO2 levels from going down."

Further research revealed a cooling trend prevalent in previous interglacial cycles. Looming sea ice in Antarctica is believed to drive down atmospheric CO2 values by reducing carbon exchanges between Southern Ocean surface water and the atmosphere. This cooling mechanism is absent today, thus possibly adding up to an already increased level of gas.

Still, Ruddiman admits that this doesn't explain the whole story and there still remain "lingering uncertainties". But even as he ponders a few loose ends in his theory, he emphasises that its central tenet still stands: that humans did have a hand in choreographing the climate long before the industrial revolution.

Many of Ruddiman's colleagues have welcomed his assertions. A recent article in an international climate newsletter called for further investigation into the effects of humans on deforestation in the pre-industrial era. Many credit Ruddiman for having brought about this paradigm shift.

Ellery agrees. "Climate change is a complex science with innumerable factors at play," he observes. He marvels at Ruddiman's ability to piece together facts and "weave an intricate theoretical tapestry around it".

Ruddiman himself likens his work to a giant jigsaw puzzle with only a few pieces left to complete the picture. "I am having the time of my life." he says.

Ruddiman's theory brings up an intriguing prospect. Greenhouse gases distanced us from an impending ice age. Could this mean that global warming was a good thing?

"Ah, you've hit the nail on the head," he says. Yet, today, the face of global warming is decidedly ugly: floods, melting glaciers, droughts and disease. How then to reconcile the devastating effects of global warming with this new image of greenhouse gases as a warm, cuddly blanket, shielding Earth from the next ice age?

"My theory is about the past," he emphasises, "and we simply cannot make inferences about the beneficial effects of global warming today based on yesterday's records."

Ellery has similar concerns. "I can see the powerful anti-Kyoto lobby jumping on this," he warns. "These chaps are looking for reasons to not reduce emissions and they will likely jump on a theory that seems to tell them that global warming is really OK."

Ruddiman certainly hopes it doesn't come to that. "If nothing else," he says, "this study should be a lesson in humility. As humans, we were able to alter the climate simply by growing food," he says. "Now with all the technology at our disposal, just imagine what we can do with the climate in the near future."

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Sumitra Rajagopalan is adjunct professor of mechanical engineering at McGill University, Montreal, Canada, and science columnist for the Canadian Broadcasting Corporation. Plows, Plagues and Petroleum: How Humans took Control of Climate , by William F. Ruddiman, is published this week by Princeton University Press, ?15.95.