Eight hundred years ago, relatively small armies of mounted warriors suddenly exploded outward from the cold, arid high-elevation grasslands of Mongolia, and conquered the largest contiguous empire in history. Led by Genghis Khan and his sons and grandsons, the Mongols briefly ruled most of modern-day Russia, China, Korea, southeast Asia, Persia, India, the Middle East and eastern Europe. They reshaped world geography, culture and history in ways that still resound today. How did they do it? Among the forces at work: the Mongols’ fast horses and brilliant cavalry tactics; their openness to new technologies; and the political genius of Genghis himself. Now, a research group is looking into a possible other factor: climate change. The idea may have implications not only for our understanding of history, but for modern Mongolia and the wider world.