The Mongol warlord built the world's largest land-based empire. But he couldn't have done it without a change in climate. The difference was Genghis Khan, the warlord who united the tribes and launched them on their wave of unstoppable conquest. But the Mongol Empire wasn’t solely the product of Genghis’s will. As a fascinating new study in the Proceedings of the National Academy of Science (PNAS) demonstrates, the rise of the Mongols may have owed just as much to beneficial changes in the climate that made the grasslands of the Mongol steppes green and verdant, fueling the horses that were...