Early farmers warmed Earth's climate
William Ruddiman, a climate scientist at the University of Virginia in Charlottesville, started to suspect that ancient human activities have affected the climate when he noticed a telltale discrepancy in levels of greenhouse gases revealed by ice cores.
During the previous three periods between ice ages, levels of carbon dioxide and methane in the air fell in lockstep with decreases in summer sunshine caused by cyclical changes in Earth's orbit. But after the most recent ice age, which peaked about 12,000 years ago, the two gases broke the pattern (see graphic).
He estimates that over time this activity laced the atmosphere with about 40 parts per million of carbon dioxide and 250 parts per billion of methane, enough to produce nearly 0.8 °C of warming before 1700, around the dawn of industrialisation. If he is right, that just about equals the warming humans are thought to have caused since then.
Intriguingly, Ruddiman thinks the anomalous cooling of the "little ice age" that gripped the world for several centuries from around 1300 was caused by a specific setback to agriculture plague. He notes that pandemics of bubonic plague depopulated Eurasia during those same centuries.
Fields and villages were abandoned and reclaimed by fast-growing forests that sucked carbon dioxide out of the atmosphere, resulting in the cooler temperatures felt worldwide. This completely reverses the widely held idea that it was the little ice age that caused the famine, depopulation and disease.
Another surprising implication of Ruddiman's theory is that the warming before 1700 0.8 °C globally, but nearly 2 °C in far northern latitudes may have saved Canada from renewed glaciation.
If levels of greenhouse gases had continued to fall after the most recent ice age, as they did after the three preceding ice ages, glaciers would once again have spread across north-eastern Canada about 4000 years ago.
Geochemist Jeff Severinghaus at the Scripps Institution of Oceanography in San Diego, California, is wary. "I think it's very interesting," he says, "but very speculative. I doubt that ancient humans could have done that."
Others are more positive. "It's provocative," says Pennsylvania State University glaciologist Richard Alley, but "absolutely worth following up".
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