Pluvials, droughts, the Mongol Empire, and modern Mongolia Neil Pedersona,1,2, Amy E. Hesslb,1,2, Nachin Baatarbilegc, Kevin J. Anchukaitisd, and Nicola Di Cosmoe aTree Ring Laboratory, Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964; bDepartment of Geology and Geography, West Virginia University, Morgantown, WV 26506; cDepartment of Forestry, National University of Mongolia, Ulaanbaatar, Mongolia 14201; dDepartment of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543; and eSchool of Historical Studies, Institute for Advanced Study, Princeton, NJ 08540 Edited by Karl W. Butzer, The University of Texas at Austin, Austin, Texas, and approved February 11, 2014 (received for review October 2, 2013) Although many studies have associated the demise of complex warm period during the 12th and 13th centuries (8, 9). Millennium- societies with deteriorating climate, few have investigated the long reconstructions of past precipitation in western China, connection between an ameliorating environment, surplus re- mostly located on the Tibetan Plateau and north central China sources, energy, and the rise of empires. The 13th-century Mongol (10–15), document drought during the early 1200s. However, peri- Empire was the largest contiguous land empire in world history. ods of drought in central Mongolia are generally out of phase Although drought has been proposed as one factor that spurred with drought on the Tibetan Plateau (2), and there is little reason these conquests, no high-resolution moisture data are available to believe that moisture conditions on the Tibetan Plateau and during the rapid development of the Mongol Empire. Here we north central China would be consistent with that of central present a 1,112-y tree-ring reconstruction of warm-season water Mongolia. Here we present the first, to our knowledge, annually balance derived from Siberian pine (Pinus sibirica) trees in central resolved record of moisture balance covering the last millennium Mongolia. Our reconstruction accounts for 56% of the variability in the regional water balance and is significantly correlated with for the Asian steppe. This new record allows us to evaluate the steppe productivity across central Mongolia. In combination with hypothesis that drought drove the 13th-century Mongol ex- a gridded temperature reconstruction, our results indicate that the pansion into Eurasia (6, 16). regional climate during the conquests of Chinggis Khan’s (Genghis Results Khan’s) 13th-century Mongol Empire was warm and persistently wet. This period, characterized by 15 consecutive years of above- We present a 1,112-y tree-ring reconstruction of warm-season, average moisture in central Mongolia and coinciding with the rise of self-calibrating Palmer Drought Severity Index (scPDSI) (17) Chinggis Khan, is unprecedented over the last 1,112 y. We propose (Supporting Information), a measure of aggregate water balance, that these climate conditions promoted high grassland productivity derived from 107 living and dead Siberian pine (Pinus sibirica) and favored the formation of Mongol political and military power. trees growing on a Holocene lava flow in central Mongolia (Fig. 2). Tree-ring and meteorological data also suggest that the early 21st- Trees growing on the Khorgo lava flow today are stunted and widely century drought in central Mongolia was the hottest drought in the spaced, occurring at the lower tree line on microsites with little last 1,112 y, consistent with projections of warming over Inner Asia. to no soil development. These trees are severely water-stressed, Future warming may overwhelm increases in precipitation leading and their radial growth is well correlated with both moisture to similar heat droughts, with potentially severe consequences for availability (scPDSI) and grassland productivity [Normalized Dif- modern Mongolia. ference Vegetation Index (NDVI)] (Fig. 3). Our reconstruction, calibrated and validated on instrumental June–September scPDSI paleoclimate | dendrochronology | human ecology | Anthropocene | (1959–2009) (Table S1), accounts for 56% of the variability in the coupled human natural systems Significance EARTH, ATMOSPHERIC, brupt climate changes have immediate and long-lasting con- AND PLANETARY SCIENCES Asequences for ecosystems and societies. Although studies have A 1,112-y tree-ring record of moisture shows that in opposition linked the demise of complex societies with deteriorating climate th – to conventional wisdom, the climate during the rise of the 13 - conditions (1 4), few, if any, have investigated the connection century Mongol Empire was a period of persistent moisture, between climate, surplus resources, energy, and the rise of empires. unprecedented in the last 1,000 y. This 15-y episode of persis- The rapid expansion of the Mongols under Chinggis Khan (also tent moisture likely led to a period of high grassland productivity, SCIENCE known as Genghis Khan) from 1206 to 1227 CE resulted in the contributing fuel to the Mongol Empire. We also present evi- SUSTAINABILITY largest contiguous land empire in world history (Fig. 1, Inset). dence that anthropogenic warming exacerbated the 21st-century The Mongol conquests affected the history of civilizations from drought in central Mongolia. These results indicate that ecosys- China to Russia, Persia to India, and even left a genetic finger- tems and societies in semiarid regions can be significantly print on the people of Eurasia (5). Although historians have affected by unusual climatic events at the decadal time scale. proposed climate as a possible factor in Mongol history (6), few paleoenvironmental data of the necessary temporal resolution are Author contributions: N.P. and A.E.H. designed research; N.P., A.E.H., and N.D.C. per- formed research; N.P., A.E.H., and K.J.A. analyzed data; and N.P., A.E.H., N.B., K.J.A., available to evaluate the role of climate, grassland productivity, and N.D.C. wrote the paper. and energy in the rise of the 13th-century Mongol Empire. The authors declare no conflict of interest. Lake sediment data from central Mongolia suggest that the This article is a PNAS Direct Submission. climate of the Mongol Empire may have been unusually wet (7), Data deposition: All tree ring data have been deposited in the International Tree Ring but the temporal resolution of these records is too coarse to Databank, www.ncdc.noaa.gov/paleo/treering.html. capture conditions during the 2 decades of rapid growth of the 1 N.P. and A.E.H. contributed equally to this work. Mongol Empire. Annual tree-ring records of past temperature 2To whom correspondence may be addressed. E-mail: [email protected] or Amy. from central Mongolia extending back to 558 CE document [email protected]. warm conditions during the 11th century, consistent with other This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. Northern Hemisphere records, but also indicate a subsequent 1073/pnas.1318677111/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1318677111 PNAS | March 25, 2014 | vol. 111 | no. 12 | 4375–4379 Downloaded by guest on September 24, 2021 Fig. 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). regional scPDSI (17) during the months when 73% of the annual Discussion rainfall occurs. Most energy exploited by human societies in 13th-century Mongolia Our reconstruction covers important climatic eras of the last was derived from the productivity of grasslands. Herbivores, be millennium including the Medieval Climate Anomaly (MCA), the they domesticated or wild, are closely tied to climatic conditions Little Ice Age, and the beginning of the Anthropocene (Fig. 2). via net primary productivity (19, 20), and net primary produc- The MCA was characterized by several severe and persistent tivity of Mongolia’s steppe is directly linked to growing season droughts in central Mongolia. The first half of the 900s was moisture availability (21). This relationship is reflected in the generally dry with an extended drought between 900 and 964 CE. strong positive correlation between our tree-ring chronology and Similarly, 1115–1139 CE and 1180–1190 CE were extremely dry, NDVI of the central Mongolian steppe (Fig. 3B). The warm and with periods of below-average reconstructed scPDSI. The 1180s consistently wet conditions of the early 13th century would have drought occurred during the turbulent early years of Chinggis led to high grassland productivity and allowed for increases in Khaan, a historical period characterized by warring tribes and domesticated livestock, including horses. factions on the Mongolian steppe (18). These droughts were fol- Dry climatic conditions from the 1180s to the early 13th cen- lowed by a wet period beginning in the early 13th century and then tury coincided with extreme political instability in Mongolia, a return to drier conditions until the late 14th century. From there, characterized by continuous warfare, the deterioration of estab- multiannual to decadal-scale variation in hydroclimate continued lished hierarchies, and a remaking of the political order that until the 20th century when climate was wetter than any other accompanied the rise of Chinggis Khan (22–24). Although the century since 900 CE. historical reasons for the