Hydroclimate variability was a factor in the collapse of several ancient agricultural societies including the Maya, Ancestral Puebloan and Harappan. However, less is known about the role of hydroclimate in affecting complex, nomadic societies and empires. Several massive land empires developed on the steppe of Arid Central Asia (ACA) during the first and second millennia, including the 7th and 8th century Uyghur and the 13th century Mongol empires. Using the rings of long-lived Siberian pine trees, we developed two annually dated reconstructions of past hydroclimate variability for ACA for the last 2060 years. We compared drought and pluvial events in the reconstructions to the NCAR CESM Last Millennium climate model ensemble. We also used these hydroclimate records as inputs to an ecosystem model, DLEM 3.0, to produce long reconstructions of both grassland and livestock productivity. Here we ask: what was hydroclimate, grassland productivity and livestock productivity like during the rise and fall of nomadic empires? We hypothesize that empire development, expansion, and innovation occurred under moderate and stable climate conditions while entrenchment, retraction, collapse, and migration of refugees occurred under severe, protracted drought. We hypothesize that moderate and stable climatic conditions may provide a favorable environment—given the presence of other social and economic variables— for political change towards more centralized and stronger polities, and occasionally empires, while severe and protracted droughts may lead to economic retraction, centrifugal tendencies, and thus to social and political disorder, eventually causing famines and migrations, depending on prevailing political conditions, both internal and external.
Results/Conclusions
Nomadic steppe empires of the first and second millennia experienced a range of hydroclimate conditions, including severe, protracted droughts as well as warm pluvial conditions. Modeled time series of grassland and livestock productivity reflect this variability. Our review of the Uyghur and the Mongol periods indicate that environmental variability acts as a lens whose aperture opens to allow for greater opportunity, innovation, and connectivity during optimal, stable climate conditions. When environmental conditions deteriorate, the environmental lens closes, reducing options for the human enterprise. These studies demonstrate that tree-ring reconstructions, when integrated with ecosystem and climate models, can be tied to historical events and reflect environmental variables directly related to the human enterprise.