The most amazing graph I've seen all month: magnetization of lake sediments and the collapse of Chinese dynasties

This is from the innocuously titled paper Influence of the intertropical convergence zone on the East Asian monsoon published in Nature (2007) by Yancheva et al.

Quote from abstract: "Here we present high-resolution records of the magnetic properties and the titanium content of the sediments of Lake Huguang Maar in coastal southeast China over the past 16,000 years, which we use as proxies for the strength of the winter monsoon winds. We find evidence for stronger winter monsoon winds before the Bølling–Allerød warming, during the Younger Dryas episode and during the middle and late Holocene, when cave stalagmites suggest weaker summer monsoons. We conclude that this anticorrelation is best explained by migrations in the intertropical convergence zone. Similar migrations of the intertropical convergence zone have been observed in Central America for the period AD 700 to 900, suggesting global climatic changes at that time. From the coincidence in timing, we suggest that these migrations in the tropical rain belt could have contributed to the declines of both the Tang dynasty in China and the Classic Maya in Central America."

Click to enlarge

From the last two paragraphs of the paper's discussion:
The role of climate and environmental change in the success or failure of societies is a matter of intense debate. It would be simplistic to imagine that all episodes of societal change are driven by climatic events, especially in an advanced and complex society such as dynastic China. Nevertheless, we note that, on the basis of our new Huguang Maar data, major changes in Chinese dynasties occurred when the winter monsoon was strong (Fig. 3). The anti-correlation between winter and summer monsoon strength indicated by com- parison of the Huguang Maar data with the cave records would suggest that dynastic transitions tended to occur when the summer monsoon was weak and rainfall was reduced. Dynastic changes in China often involved popular uprisings during phases of crop failure and famine, consistent with a linkage to reduced rainfall. The Tang dynasty has been described as a high point in Chinese civilization, a golden age of literature and art. The power of the dynasty began to ebb in the eighth century, starting with a defeat by the Arab army in AD 751. Rebellions further weakened the Tang empire, and it fully collapsed in AD 907. 
It is intriguing that the rise and collapse of the Classic Maya coincided with the golden age and decline of the Tang dynasty in China. Comparison of the Ti records from Lake Huguang Maar and the Cariaco basin reveals similarities, including both a general shift towards drier climate at about AD 750 and a series of three multi-year rainfall minima within that generally dry period (Fig. 3), the last of which coincides with the final stage of Maya collapse as well as the end of the Tang dynasty. Given these results, it seems possible that major circum-Pacific shifts in ITCZ position catalysed simultaneous events in civilizations on opposite sides of the Pacific Ocean.

Related: Human migration and the green Sahara, three other paleoclimate papers on social collapse

h/t Peter

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