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.

It may not be popular, but if the population isn't decimated by war or "humane culling" then disease, famine and pestilence will do the job much more painfully.

It may not be popular, but if the population isn't decimated by war or "humane culling" then disease, famine and pestilence will do the job much more painfully.