London: Genghis Khan, who established the world's largest contiguous empire between the 13th and 14th centuries, has been branded the 'greenest invader' in history as his murderous invasion actually helped scrub about 700million tonnes of carbon from the atmosphere. According to a new research, the ruthless Mongol warrior killed so many people during his bloody invasions that huge swathes of cultivated land depopulated and returned to forest.Nearly 40 million people were believed to be killed in Mongol invasions which led to an empire that spanned 22 per cent of the Earth's surface.This resulted in huge cultivated land becoming forests again, absorbing an estimated 700 million tonnes of carbon from the atmosphere, found the research by the Carnegie Institution's Department of Global Ecology.Though his methods may be difficult for environmentalists to stomach, ecologists believe it may be the first ever case of successful manmade global cooling."It's a common misconception that the human impact on climate began with the large-scale burning of coal and oil in the industrial era," said lead researcher Julia Pongratz."Actually, humans started to influence the environment thousands of years ago by changing the vegetation cover of the Earth's landscapes when we cleared forests for agriculture," Pongratz was quoted as saying by the Daily Mail.The 700 million tonnes of carbon is equivalent to the quantity produced in a year from the global use of petrol.The Carnegie study measured the carbon impact of a number of historical events that involved a large number of deaths.Time periods also looked at included the Black Death in Europe, the fall of China's Ming Dynasty and the conquest of the Americas.All of these events share a widespread return of forests after a period of massive depopulation. But the bloody Mongol invasion immediately stood out for its longevity.And this is how Khan, who repeatedly wiped out entire settlements, was able to scrub more carbon from the atmosphere than any other despot, the researchers said.Dr Pongratz explained: "We found that during the short events such as the Black Death and the Ming Dynasty collapse, the forest re-growth wasn't enough to overcome the emissions from decaying material in the soil."But during the longer-lasting ones like the Mongol invasion there was enough time for the forests to re-grow and absorb significant amounts of carbon."Though the Khan will remain known as Genghis the Destroyer and not Genghis the Green, Dr Pongratz hopes that her research will lead to future historians examining environmental impact as well as the more traditional aspects of study.