One of history's most famous rulers, Genghis Khan founded the Mongol Empire and, by the time of his death, had conquered everything from the Sea of Japan to the Caspian Sea. And while the Mongol Invasions are usually associated with barbarism and chaos, the Great Khan may have been behind the first man-made drop in global temperatures.
The nomadic Mongols were unimpressed by agricultural societies and didn't mind killing enemies by the thousand, qualities that came with an unexpected result. As they swept across Asia, they decimated populations and destroyed huge swathes of farmland, which reverted to natural forest (not that we think mass slaughter of mankind is a solution).
In a study by published in The Holocene, Julia Pongratz of the Carnegie Institution's Department of Global Ecology posited that the scale of the area returned to forest would have cooled the planet.