Interplay of environmental and socio-political factors in the downfall of the Eastern Turk Empire in 630 CE

The collapse of the Eastern Turk Empire (ETE, ca. 584-630 CE) in 630 CE marked the rise of Tang China as the paramount power on the Silk Road. It was followed by the Tang defeat of the Western Turk Empire in 659 and opened a phase of Chinese expansion into central Asia. Climate-induced environmental changes as well as economic and political consequences are mentioned in medieval Chinese records as major factors in the ETE collapse. The role of cooler temperatures has also been discussed in current scholarship. Here, we re-evaluate this question by assessing the available historical sources in the light of a global network of 16 tree-ring chronologies for this period, which reveal distinct summer cooling in the ETE heartland between 626 and 632 CE. Reconstructed peak cooling of up to - 3.4 A degrees C in 627 and 628 CE (relative to the 1961-90 mean climatology) coincided with heavy snowfall and severe frost events in the territory of the ETE. A strong sulfate spike in Greenland ice cores that has been dated circa 626 CE is implicated in the abrupt surface cooling. We argue that the climatic perturbation and associated reduction in vegetation growth and livestock mortality are relevant in understanding the causes of the fall of the ETE but these indirect drivers must be evaluated within a comprehensive analysis of political relations within both the Turk and the Tang leadership. Our study underscores and contextualizes the vulnerability of past nomadic societies to small and episodic climate fluctuations, particularly when coupled with concurrent socioeconomic, political, and demographic changes.