Genghis Khan — green invader?

It has long been assumed that man’s effect on the atmosphere’s carbon levels began with the industrial revolution of the 18th century, but recent research suggests that human activity may have had a much earlier influence.

Until now, scientists have had difficulty in making accurate assessments of historic carbon levels, but a recently developed model demonstrates a link between population increase and deforestation. The relationship between population levels and agricultural land use is not simply proportional, as had been believed, but takes into account the varying efficiencies of different farming methods.

The earliest farmers did not have the know-how or techniques to make optimal use of the soil, therefore the amount of land that required clearing to feed one individual was relatively large. However, as farming methods became more efficient, the relative carbon increase slowed. Older, standard models simply stated that the bigger the population, the more forest needed to be cleared.

Results obtained using the new model suggest that the first major boom in carbon emissions occurred 2,000 years ago, corresponding to the expansion in civilisations in China and the Mediterranean area.

Other historical events have been identified through changes in carbon levels, but the only event of the preindustrial age with a profound effect was the Mongol invasion of Asia between 1200 and 1380, begun by Genghis Khan, in which 40 million people were killed. It decreased global carbon dioxide levels by almost 0.1 part per million, removing 700 million tons of carbon from the atmosphere, roughly equal to that produced annually by the global use of petrol. This extraordinary effect is attributed to the longevity of the invasion, since it can take a century for a tree to reach its full carbon storage capacity, and it takes years for forest regrowth to overcome emissions from material decaying in the soil.

It is unlikely, however, that Genghis Khan’s green credentials will improve his traditional image.