Published online 25 March 2011 | Nature | doi:10.1038/news.2011.184

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Models bolster case for early human effect on greenhouse-gas levels.

Researchers have found new evidence to support the theory that human agriculture was changing the climate 8,000 years ago. Punchstock

Scientists have come up with new evidence in support of the controversial idea that humanity's influence on climate began not during the industrial revolution, but thousands of years ago. Proposed by palaeoclimatologist William Ruddiman in 2003, the theory says that human influences offset the imminent plunge into another ice age and helped create the relatively stable climate that we are familiar with today. It has been repeatedly panned as implausible by palaeoclimate researchers, but eight years on, Ruddiman and others say that they have the data to support early anthropogenic climate change.

The argument centres on a curious trend in atmospheric carbon dioxide and methane levels since the last ice age ended about 11,000 years ago and the current Holocene epoch began. In previous interglacial periods, CO 2 levels spiked early and then gradually declined until the globe went into another ice age. The Holocene began by following this trend, but then CO 2 levels changed course and began to rise around 8,000 years ago. The same thing happened with methane levels around 5,000 years ago. These trends align with the expansion of human agriculture, and Ruddiman, of the University of Virginia in Charlottesville, argues that it is no coincidence — the clearing of land and expansion of irrigation released huge amounts of greenhouse gases into the atmosphere.

Critics say that human populations were probably too small to support such a hypothesis, and recent studies have raised serious questions about early anthropogenic carbon and methane emissions. But rather than backing down, Ruddiman and several other researchers will present their supporting evidence in a series of papers scheduled for publication in a special issue of The Holocene journal later this year. Researchers presented some of the work this week at the American Geophysical Union's Chapman Conference on Climates, Past Landscapes and Civilizations in Santa Fe, New Mexico.

"I'm of course hopelessly biased, but this year is going to be a good year for the early anthropogenic influence hypothesis," Ruddiman said as he presented his overview study1.

Slow and steady

One of the studies, led by Jed Kaplan at the Federal Polytechnic School of Lausanne, Switzerland, suggests that agriculture had a much larger impact than previously believed as it expanded in Europe and beyond2. Kaplan built a detailed model to analyse land-use change over time, building in historical and archaeological data where possible. In contrast to most previous estimates, the model assumes that humans cleared more land early on, with only gradual intensification as agriculture improved. "People are basically lazy," he says. "They only intensify their land use when they are forced to." The result is roughly double the carbon emissions compared with earlier estimates.

Ruddiman also took issue with a high-profile Nature study3 published in 2009 by a team at the University of Bern, Switzerland, led by climate modeller Thomas Stocker, who is co-chairman of Working Group I for the Intergovernmental Panel on Climate Change. The study takes advantage of the fact that plants preferentially take up the isotope carbon-12, subtly altering the ratio between carbon-12 and carbon-13 in the atmosphere. Stocker and his team analysed an Antarctic ice core and found no evidence of a change in the ratio, which would have been expected if carbon from cleared vegetation were released back into the atmosphere.

But that study underestimated the amount of carbon-12 taken up by peatlands, say Ruddiman and Kaplan. It assumed that just 40 gigatonnes of carbon were buried in peatlands during the late Holocene, whereas other estimates come in at 280 gigatonnes or more. That number would have to be offset by terrestrial emissions to maintain the atmospheric carbon isotope ratio.

In an e-mail to Nature, Stocker said that Ruddiman's latest paper merely "reiterates in extenso all of the points made earlier". Although Stocker acknowledges that peatland estimates need to be better quantified, he cited a recent analysis by his institute suggesting that carbon emissions from land-use change are neither sufficient nor properly timed to explain the rise in CO 2 levels in the Holocene4.

The rise of rice

Kaplan says that Stocker's land-use analysis contains some of the same problems and assumptions as others that have come before. Another study in The Holocene5 by Dorian Fuller, an archaeologist at University College London, explores methane emissions from livestock and the spread of rice agriculture in southeast Asia. Fuller says that the expansion of rice could account for up to 80% of the additional atmospheric methane as of 1,000 years ago, and suggests that the expansion of livestock could help to plug the gap in previous millennia.

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Palaeoclimatologist Eric Wolff of the British Antarctic Survey in Cambridge, UK, acknowledges that whereas no one can refute the idea that humans played a significant part in influencing the Holocene climate, no one can prove that they did. Wolff points out that a modelling study6 that appeared in Nature in February this year, led by Joy Singarayer at the University of Bristol, UK, shows that orbital variations and tropical sources can explain the Holocene methane trends. "This does not prove there was not an anthropogenic influence, but it removes the need for one," Wolff explains.

Both Kaplan and Fuller say that their focus is not so much on Ruddiman's specific hypothesis as on the idea that humans might have influenced climate well before the industrial revolution. "The human influence is there," says Fuller. "We can see that." Researchers have plenty of work to do in terms of quantifying early human emissions, adds Kaplan, "but it is getting hard to support the idea that anthropogenic influence was negligible before the industrial era".