In Europe, slash-and-burn techniques for clearing forested land allowed the farming of crops that had spread from the Fertile Crescent. This practice loosed the forests’ stored carbon into the atmosphere in the form of carbon dioxide. In eastern Asia a couple of millennia later there was a tenfold increase in the growth of rice as the region’s principal foodstuff. That meant the destruction of vast grasslands, which released equally vast amounts of methane—a gas far more efficient at trapping heat than carbon dioxide is.The ice-core record shows that the level of carbon dioxide in the atmosphere made an anomalous upturn about 7,000 years ago, and that methane levels, which were also falling, began to increase about 5,000 years ago (see chart).
He and his colleagues have turned to archaeological and anthropological data to show that early farmers used ten times as much land per person as modern farmers. Burning off large areas of forest or grassland, they would farm the enriched soil until its yield began to drop, and then move off to do the same elsewhere—as practitioners of slash-and-burn agriculture do to this day.
Such profligacy would make the contributions of early farmers large enough to have an effect on worldwide levels of greenhouse gases. So although the size of the effect has increased markedly since the industrial revolution, it looks as if humanity has been interfering with the climate since the dawn of civilisation.
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