His empire lasted a century and a half and eventually covered nearly a quarter of the earth's surface. His murderous Mongol armies were responsible for the massacre of as many as 40 million people. Even today, his name remains a byword for brutality and terror. But boy, was Genghis green.

Genghis Khan, in fact, may have been not just the greatest warrior but the greatest eco-warrior of all time, according to a study by the Carnegie Institution's Department of Global Energy. It has concluded that the 13th-century Mongol leader's bloody advance, laying waste to vast swaths of territory and wiping out entire civilisations en route, may have scrubbed 700m tonnes of carbon from the atmosphere – roughly the quantity of carbon dioxide generated in a year through global petrol consumption – by allowing previously populated and cultivated land to return to carbon-absorbing forest.

An intriguing notion, certainly. But possibly not a guaranteed vote-winner for the Green party's next manifesto.