From the Cover: Pluvials,droughts, the Mongol Empire,and modern Mongolia

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.Abrupt climate changes have immediate and long-lasting consequences for ecosystems and societies. Although studies have linked the demise of complex societies with deteriorating climate conditions (1–4), few, if any, have investigated the connection between climate, surplus resources, energy, and the rise of empires. The rapid expansion of the Mongols under Chinggis Khan (also known as Genghis Khan) from 1206 to 1227 CE resulted in the largest contiguous land empire in world history (Fig. 1, Inset). The Mongol conquests affected the history of civilizations from China to Russia, Persia to India, and even left a genetic fingerprint on the people of Eurasia (5). Although historians have proposed climate as a possible factor in Mongol history (6), few paleoenvironmental data of the necessary temporal resolution are available to evaluate the role of climate, grassland productivity, and energy in the rise of the 13th-century Mongol Empire.Open in a separate windowFig. 1.Tree-ring drought reconstruction site (green cross) and inferred temperature site (8) (white cross) are 50 km apart. Map of the Mongol Empire near its zenith (aqua) in 1260 CE (Inset). The ancient capital city of Karakorum (black triangle) and current capital of Mongolia, Ulaanbaatar (black star).Lake sediment data from central Mongolia suggest that the climate of the Mongol Empire may have been unusually wet (7), but the temporal resolution of these records is too coarse to capture conditions during the 2 decades of rapid growth of the Mongol Empire. Annual tree-ring records of past temperature from central Mongolia extending back to 558 CE document warm conditions during the 11th century, consistent with other Northern Hemisphere records, but also indicate a subsequent warm period during the 12th and 13th centuries (8, 9). Millennium-long reconstructions of past precipitation in western China, mostly located on the Tibetan Plateau and north central China (10–15), document drought during the early 1200s. However, periods of drought in central Mongolia are generally out of phase with drought on the Tibetan Plateau (2), and there is little reason to believe that moisture conditions on the Tibetan Plateau and north central China would be consistent with that of central Mongolia. Here we present the first, to our knowledge, annually resolved record of moisture balance covering the last millennium for the Asian steppe. This new record allows us to evaluate the hypothesis that drought drove the 13th-century Mongol expansion into Eurasia (6, 16).