State of the Planet

News from the Columbia Climate School

Early Human Influence on the Climate

I’ve been meaning to blog about our visit from Bill Ruddiman, professor emeritus at the University Virginia and former Doherty Senior Research Scientist, who stopped in at his old stomping grounds last month. Ruddiman just wasn’t here to renew old ties, though. Rather, he used his visit as an opportunity to continue a rather heated conversation he started six years ago with the publication of his early anthropocene hypothesis.

Ruddiman’s basic premise is that anthropogenic climate change did not begin with the Industrial Revolution, but can be traced back 8,000 years to the advent of agriculture. If you think his idea sounds a little wacky, you’re not alone. No less than the estimable Wally Broecker has called Ruddiman’s hypothesis “complete and utter nonsense” – and he’s not the only one. But if the critics haven’t exactly died off since Ruddiman first debuted his theory six years ago, Ruddiman has certainly managed to answer a fair number of them with due diligence and further research.

Ruddiman’s work highlights the breakdown of certain cycles in the Earth’s atmosphere. He uses ice core records to show that levels of atmospheric methane – which have waxed and waned every 23,000 years for the last 40,000 – bucked a downward trend roughly 5,000 years ago and have increased steadily more-or-less ever since. Similarly, CO2 – which fluctuates in the atmosphere on 100,000-year and 41,000-year cycles – showed an unexpected upswing 7,000 years ago and has continued to rise.

According to Ruddiman, there is no natural explanation for this. If the climate system had followed cycles similar to those it had followed for many thousands of years, the Earth would have experienced an ice age. Instead, atmospheric methane and CO2 began to rise at a time when we would have otherwise expected it to decline. According to Ruddiman, it’s not simply coincidence that this occurred at the same time as the rise of slash-and-burn agriculture in Eurasia and increases in methane to the advent of wet-rice farming in China.

Because he’s been challenged quite a few times on this theory, Ruddiman is ready for the skeptics. He does not argue, for instance, that human activity alone accounts for the observed increases in greenhouse gases; rather, he contends that manmade gases warmed the planet enough to create a feedback cycle which ultimately amplified the influence of early farmers. Also, though Ruddiman has gathered quite a bit of information on ancient agriculture, he suggests that there is simply too much we don’t know about early peoples to allow him to answer questions about early human per capita greenhouse gas emissions.

One thing Ruddiman feels he can explain are the several dips in CO2 that occurred over the last 2,000 years. As far as he’s concerned, these dips were caused by “pandemic mortality” — widespread death associated with epidemics and civil strife, which led to the abandonment of land, and then reforestation and, ultimately, carbon sequestration. Interestingly, Ruddiman finds easy correlates between periods of declining CO2 and the conquest of Genghis Khan and outbreaks of the bubonic plague.

Of course, here is where I should probably should reiterate that Ruddiman’s theory is wildly controversial. But so interesting! Also, compelling and a bit portentous. If ancient peoples started changing the climate 7,000 years ago and managed to avert an ice age, what sort of influence should we expect from our industrial, and much more crowded, society?

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13 years ago