Unlike modern day climate change, however, the Mongol invasion actually cooled the planet, effectively scrubbing around 700 million tons of carbon from the atmosphere.

So how exactly did Genghis Khan, one of history’s cruelest conquerors, earn such a glowing environmental report card? The reality may be a bit difficult for today’s environmentalists to stomach, but Khan did it the same way he built his empire — with a high body count.

Over the course of the century and a half run of the Mongol Empire, about 22 percent of the world’s total land area had been conquered and an estimated 40 million people were slaughtered by the horse-driven, bow-wielding hordes. Depopulation over such a large swathe of land meant that countless numbers of cultivated fields eventually returned to forests.