 Image copyright National Palace Museum Image caption Genghis Khan was the founder of the Mongol empire, which historians say was the largest contiguous empire in history 
 
 The rise of Genghis Khan and the huge Mongol Empire in the early 13th Century may have been helped by good weather, scientists suggest.
– Scientists who study tree rings for a living have discovered that central Mongolia had an usually warm and wet spell from 1211 to 1225. This would probably remain of note only in tree-ring-studying circles if not for one other thing: Those dates happen to coincide with the rise of none other than Genghis Khan, reports the BBC. And the researchers say it's no coincidence. They argue in the Proceedings of the National Academy of Sciences that the change in weather from severe drought conditions prior to 1211 to fertile ones afterward directly helped Khan build his empire. Think rain—specifically the lush grasslands it created to fuel to Khan's horse-driven army, along with the livestock his soldiers needed to survive. "It wasn't the only thing, but it must have created the ideal conditions for a charismatic leader to emerge out of the chaos, develop an army, and concentrate power," West Virginia University researcher Amy Hessl tells National Geographic. "Where it's arid, unusual moisture creates unusual plant productivity, and that translates into horsepower—literally. Genghis was able to ride that wave." By the time he died in 1227, Khan's empire spanned modern-day Korea, China, Russia, eastern Europe, India, and southeast Asia. Researchers stumbled onto his meteorological good luck while studying ancient Siberian pine trees. (Click to read about the 800-year search for Genghis Khan's tomb.)