A 1,112-y tree-ring record of moisture shows that in opposition to conventional wisdom, the climate during the rise of the 13 th -century Mongol Empire was a period of persistent moisture, unprecedented in the last 1,000 y. This 15-y episode of persistent moisture likely led to a period of high grassland productivity, contributing fuel to the Mongol Empire. We also present evidence that anthropogenic warming exacerbated the 21st-century drought in central Mongolia. These results indicate that ecosystems and societies in semiarid regions can be significantly affected by unusual climatic events at the decadal time scale. 
 
 Abstract 
 
 Although many studies have associated the demise of complex societies with deteriorating climate, few have investigated the connection between an ameliorating environment, surplus resources, energy, and the rise of empires. The 13th-century Mongol Empire was the largest contiguous land empire in world history. Although drought has been proposed as one factor that spurred these conquests, no high-resolution moisture data are available during the rapid development of the Mongol Empire. Here we present a 1,112-y tree-ring reconstruction of warm-season water balance derived from Siberian pine (Pinus sibirica) trees in central Mongolia. Our reconstruction accounts for 56% of the variability in the regional water balance and is significantly correlated with steppe productivity across central Mongolia. In combination with a gridded temperature reconstruction, our results indicate that the regional climate during the conquests of Chinggis Khan’s (Genghis Khan’s) 13th-century Mongol Empire was warm and persistently wet. This period, characterized by 15 consecutive years of above-average moisture in central Mongolia and coinciding with the rise of Chinggis Khan, is unprecedented over the last 1,112 y. We propose that these climate conditions promoted high grassland productivity and favored the formation of Mongol political and military power. Tree-ring and meteorological data also suggest that the early 21st-century drought in central Mongolia was the hottest drought in the last 1,112 y, consistent with projections of warming over Inner Asia. Future warming may overwhelm increases in precipitation leading to similar heat droughts, with potentially severe consequences for modern Mongolia. ||||| 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. 
 
 American researchers studying the rings of ancient trees in central Mongolia have discovered that his rise coincided with the mildest, wettest weather in more than 1,000 years. 
 
 Grass grew at a rapid rate, providing fodder for his war horses. 
 
 Genghis Khan united the Mongol tribes to invade and rule a vast area. 
 
 It covered modern-day Korea, China, Russia, eastern Europe, India and south-east Asia. 
 
 Charismatic leader 
 
 The research shows that the years before Genghis Khan's rule were characterised by severe drought from 1180 to 1190, the study in the Proceedings of the National Academy of Sciences said. 
 
 Image caption Present-day Mongolians regard Genghis Khan as the founding father of their country 
 
 But as the empire expanded from from 1211 to 1225, Mongolia saw an unusual spell of regular rainfall and mild temperatures. 
 
 "The transition from extreme drought to extreme moisture right then strongly suggests that climate played a role in human events," study co-author and West Virginia University tree-ring scientist Amy Hessl told the AFP news agency. 
 
 "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. 
 
 "Where it's arid, unusual moisture creates unusual plant productivity, and that translates into horsepower. Genghis was literally able to ride that wave." 
 
 Allied to the good weather, Genghis Khan was able to unite disparate tribes into an efficient military unit that rapidly conquered its neighbours. 
 
 For the oldest samples, Ms Hessl and lead author Neil Pederson, a tree-ring scientist at Columbia University's Lamont-Doherty Earth Observatory, concentrated on an unusual group of stunted Siberian pines found while researching wildfires in Mongolia. 
 
 The trees were growing from cracks in an old solid-rock lava flow in the Khangai Mountains, according to a statement from Columbia. 
 
 Trees living in such conditions grow more slowly and are particularly sensitive to weather changes - and as a result provide an abundance of data to study, scientists say. 
 
 Some of the trees had lived for more than 1,100 years. the experts say, and one piece of wood they found had rings going back to about 650 BC.
– 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.)