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Climate Scientist Charts an Early Start for the Epoch of Man

Ancient rice paddies released significant amounts of methane into the atmosphere, but scientists disagree on whether they helped trigger a change in climate.

When did the Epoch of Man begin?

In recent years, it’s become common to hear that the earth has entered the Anthropocene, a new geological time dominated by humanity. The term, very much a meme, unifies a host of environmental concerns—climate change, biodiversity loss, pollution. It’s so influential that the body governing geological time is now studying, as I detailed last year, whether to consider the Anthropocene as a formal epoch—like the Pleistocene or Miocene—to the chagrin of some stratigraphers, the fastidious adherents to the discipline that judges such things.

If we are to enter a new epoch, though, geologists will have to decide when the Anthropocene began. That debate has mostly revolved around a few camps: those who believe it started with the Industrial Revolution; others who see the economic acceleration of the 1950s as the true breakaway point; and stratigraphers who hate the whole damn thing. But off to the side, there is also another, more provocative camp. The Anthropocene isn’t anything new, they argue. It kicked off thousands of years ago.

For the past decade, that’s been the contention of William F. Ruddiman, a climate scientist at the University of Virginia. As he proposed in a famed 2003 paper, human beings—armed with a powerful new tool called agriculture—burned, plowed, and seeded enough land to cause real, if modest, climate change millennia ago. And Ruddiman went further still: Not only did farming cause warming; it also, in effect, saved human existence. It prevented, he said, a slide into the next ice age.

Ruddiman’s idea caused a flurry of scientific, and sometimes vitriolic, rebuttals. It challenged assumptions about what many scientists thought was a sufficiently explained natural phenomenon, a peculiar, steady rise in greenhouse gases that began some 7,000 years ago. And it also tapped into deeper values. Many scientists view modernity as an anomaly in its human domination; Ruddiman argued, in effect, that this influence stretched as far back as the origins of culture.

With his hypothesis reaching its 10th year, it seemed a good time to check in with Ruddiman and see how it is faring now. He has published a new review paper, out this month from Annual Reviews, encapsulating the debate as he sees it. And later this year he’ll have a textbook out, tentatively titled Earth Transformed.

It’s time for researchers to take another look at the evidence, he said over the course of two phone interviews, this month and last summer. Ruddiman’s hypothesis has proved resistant to falsification, to the point that he challenged scientists to defeat it in a 2011 paper.

“I’m being a little bit intentionally provocative. … Any book on the theory of science would say that’s how science works,” he said. “I’m just hastening the process a little bit.”

No one has debunked his theory yet—but neither has it been highly corroborated.

Scientists see the debate between Ruddiman and his intellectual foes as analogous to a heavyweight fight eight rounds in; no knockouts landed, and everyone is weary. The dispute centers on reconstructions of how our ancestors used the land, which rely on proxies like fossilized pollen and ice cores to build models. It’s a stubborn, immovable kind of science, one not uncommon in the geosciences. For example, the debate over early CO2 comes down to models of how much carbon ended up sunk in northern peat bogs.

“You can’t really measure your way out of this,” said Erle Ellis, a geographer at the University of Maryland at Baltimore. Ellis remains on the fence about Ruddiman’s hypothesis. “It’s going to remain a raging argument for quite some time,” he said.

Ruddiman’s idea grew out of an incongruity that has puzzled scientists for decades. Over the past few million years, the earth, for the most part, has followed a pattern. As it shuffles out of an ice age, the amount of greenhouse gases in the air slowly falls as trees and grasses begin to grow on formerly ice-covered land. But along our way to modernity, something happened. As the glaciers’ bulk retreated, dropping moraines like dandruff around our neolithic ancestors, the gases didn’t fall. Instead, they rose.

What could explain that? Ruddiman looked around, and mostly he saw a lot of us.

The “early anthropogenic” hypothesis is supported by a few interconnected struts. Human populations rose much faster than past projections, Ruddiman said, and this led to widespread deforestation for farming. That released carbon into the air, prompting additional CO2 releases from the environment. The invention and spread of rice paddies in Asia, notorious for their methane emissions, was particularly effective. With all of that activity, human beings increased the amount of carbon dioxide in the air from 240 to 280 parts per million before industrialization. (By now, humanity has fired up those concentrations to 395 ppm.) That, in turn, forestalled a slide into the next ice age.

Certainly some recent archaeological work has supported Ruddiman’s contention, especially in Asia. Many studies have found early evidence for rice farming. (And some have not.) There’s some evidence that prehistoric populations grew faster than previously expected. And a Swiss scientist, Jed O. Kaplan, put together a new method of estimating historical land use that, banking on humanity’s essential laziness, saw nascent farmers use much more land per person than their descendants.

It hasn’t all been rosy for Ruddiman’s hypothesis, however.

The most vociferous in their opposition to Ruddiman have been geochemists and modelers studying ice cores, perhaps the climate record’s most influential dipstick. Wallace Broecker, Ruddiman’s former colleague at Columbia University, led the early battle against the hypothesis, growing angry at Ruddiman for not accepting his debunking. (“He’s furious at me and just thinks I’m doing bad science and won’t listen to common sense,” Ruddiman said.) And two Swiss scientists, Fortunat Joos and Thomas Stocker, have continued to trade publications with Ruddiman and his peers.

You simply don’t need people to account for the CO2 rise, a recent modeling study by Joos contended. A complex climatic mix led to the gas rise, those scientists say. There was an influx of freshwater into the ocean that may have altered circulations and emissions. Coral-reef growth played a role. There were changes in the ocean’s CO2 solubility in response to warming ocean temperatures. And much more still. It’s a thorough case, but one that’s full of uncertainty and difficult to test.

Ruddiman’s response to the climatic case remains what it has always been, he said. If this is the natural response, he asks, then why didn’t it happen after the other ice ages?

“That criticism still stands,” Ruddiman said.

The most contentious bit of evidence against Ruddiman right now seems to involve carbon isotopes. Plants prefer to absorb a lighter version of carbon, and if people had caused large-scale deforestation, the carbon ratios found in ice cores would have shifted markedly. It didn’t, Stocker’s 2009 study showed. But wait! Ruddiman countered. Even such definite-sounding evidence requires modeling and conjecture, particularly over how carbon would have sunk into peat bogs. Increase that budget—a reasonable case can be made to do so—and the hypothesis remains.

And that’s pretty much how Benjamin Stocker, a Ph.D. student of Joos’s—and no relation to Thomas Stocker—found himself enmeshed in historical peat reconstructions. After his past work, published in 2011, found that even with fast, early population growth, human beings couldn’t account for the CO2 rise, Stocker turned to answering Ruddiman’s peat criticisms. And his forthcoming model finds that the peat didn’t hide deforestation during the early carbon spike. But it’s hardly evidence enough to end the debate.

Another reason Ruddiman’s hypothesis has proved resistant is its flexibility, said Jennifer Marlon, a geographer at the University of Wisconsin at Madison. Partially prompted by Ruddiman, Marlon spent a decade assembling a global index of charcoal records, which can be used to probe the spread of fire over millennia. Fire would have been a major tool of deforestation, but Ruddiman has never predicted exactly how much fire would be needed to support his case.

Anyway, the charcoal doesn’t seem to support him, Marlon added, a fact she lays out in a forthcoming study. Fire incidence does slowly spread after the ice age, but there’s no surge, as would most likely be necessary to account for the gas rise. The records don’t match up.

“There isn’t an obvious visual similarity,” Marlon said.

More broadly, even with earlier farming and faster population growth, many scientists still can’t imagine there were enough people around early enough to cause the greenhouse-gas rise. The world was an enormous place to those remote populations. It’s a perspective that’s easy to lose in our modern time of jet travel and ubiquitous global information.

“We tend to underestimate just how vast the landscape is,” Marlon said.

Ruddiman’s hypothesis will very likely linger in uncertainty, and it’s certainly possible that he and the geochemists are both partially correct. But in many ways, the idea already has a legacy. It has provoked an incredible diversity of work, challenging preconceptions and improving the science of climate past. Indeed, many scientists say its most important legacy will be how widely accepted it is in climate science that, even if human beings didn’t cause the early gas rises, they have almost certainly been changing atmospheric gas levels for the past two millennia, at least.

Marlon is open to the idea that there’s a large human signal in the recent charcoal record; indeed, there seems to be a global peak in burning 2,000 years ago that the climate cannot explain. The peat record also leaves room for a human influence over the past couple-thousand years, Stocker added. New work has pointed to the importance of the past 1,000 years to current emissions, and a study last year in Nature confirmed that methane emissions have seemed to jag along with human population trends since Roman times.

“The evidence does seem to show that humans were having a big ecological impact early in the Holocene and before, with some record in the atmosphere,” said Richard Alley, a climate scientist at Pennsylvania State University. “I expect that Bill is right that early humans affected CO2” and methane. “Whether we affected them enough to make a difference to the climate evolution is a much tougher question.”

[Photograph from iStock]

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