A new study by Neil Pederson and four co-authors presents a 1,112-year tree-ring-based self-calibrating Palmer Drought Severity Index (scPDSI) reconstruction for the Central Mongolian warm-season, using 107 living and dead Siberian pines growing on the Khorga lava flow. The reconstruction identifies fifteen years from 1211 to 1225 CE as being unprecedented for their consistent wetness in this region. The authors note that these years coincide with the dramatic imperial expansion of the Mongol empire under Chinggis Khan (Genghis Khan) from 1206 to 1227 CE. This led to the eventual usurpation of the Song (Chinese) empire by the later decades of the thirteenth century.
That the study is the product of an interdisciplinary collaboration is welcome, as is the presentation of an explicit mechanism by which the exceptional climate of these years could have contributed to the initial Mongol expansion. By potentially enhancing grassland productivity and consequent carrying capacity in the Mongolian steppe, specifically the Orkhon Valley region, the persistent wetness (and notable warmness) of 1211 to 1225 may have helped sustain the initial stages of imperial expansion through enhanced resource availability.
The study has received considerable media coverage, but awaits a substantial response from scholars specializing in human-environmental relations in disciplines ranging from geography and anthropology to archaeology and history. The study is likely to be criticized by some as insufficiently acknowledging the pervasive complexity of human-environmental interactions, in which any given outcome (be it imperial expansion or collapse) is the product of a chaotic interplay between evolving environmental pressures and potentials, an array of social and political factors, individual agency (consider here the abilities, choices and personal motivations of Chinggis Khan) and chance.
It might be fairly claimed that such wide-ranging considerations can rarely be treated in detail within a single letter-length study, but the presentation of new environmental data and the formulation of hypotheses must occur within a framework that explicitly acknowledges and shows understanding of the above reality. This is crucial if efficient use is to be made of newly available environmental data to advance a full understanding of the major developments of the human past, in which the role of environment has for too long suffered a lack of such data and often languished in the doldrums of environmental determinism and reductionism.
Pederson N, Hessl AE, Baatarbileg N, Anchukaitis KJ, Di Cosmo N, 2014. Pluvials, droughts, the Mongol Empire, and modern Mongolia. Proceedings of the National Academy of Sciences, early edition, doi: 10.1073/pnas.1318677111