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War and Peace in China: The Roles of Drought and Sweet Potatoes
Volume 17, Number 22: 28 May 2014

In an important article published in The Economic Journal, Ruixue Jia of the Institute for International Economic Studies at Stockholm University in Stockholm, Sweden, writes that "in recent years, historians and climatologists have found evidence of a link between weather-induced economic decline and conflict for ancient Egypt (Fagan, 2009), the Classic Mayan culture (Diamond, 2006; Yancheva et al., 2007) and the Qing and Tang Dynasties in China (Hinsch, 1988; Yancheva et al., 2007)," after which she goes on to describe her contribution to the subject by employing data from 267 prefectures that had been obtained over more than four centuries, which allowed her to investigate two important questions about historical China: "to what extent did weather-induced bad harvests cause civil conflict? And to what extent did the historical introduction of (drought resistant) sweet potatoes mitigate these effects?"

"Historical China," in Jia's words, is "a good testing ground for the link between weather shocks and civil conflict, as there is detailed information on abnormal weather conditions and the occurrence of peasant revolts" - her proxy for civil conflict - "at the prefecture level going back to the 15th century," which data indicate there was a peasant revolt in 0.22% of all prefecture-years. However, when she focused on prefecture-years when there was an exceptional drought, she says "there was a peasant revolt in 0.58% of prefecture-years," such that "a peasant revolt at the prefecture level was almost three times more likely in a drought year."

But what about floods, since they also decrease the local supply of crops? Jia found that the price effect of droughts was nearly three times that of floods, and that droughts thus had more severe negative effects on local food production, consistent "with historians' argument that droughts were the most important natural disasters driving historical peasant revolts," citing Xia (2010).

So how do sweet potatoes enter the picture? Jia says that "sweet potatoes survive bad weather better than wheat and rice and also provide more calories per unit of land" and that they also "can be grown on land that is inappropriate for wheat and rice and are not grown during the same season," citing Pomeranz (2000) and Cao (2004), with the result that "the availability of sweet potatoes translates into greater agricultural production, especially in bad weather conditions."

To rigorously determine the implications of these facts, Jia collected data on the adoption and diffusion of sweet potatoes across different provinces or collections of prefectures, finding that before the introduction of sweet potatoes "there was a peasant revolt in 0.78% of prefecture-years with an exceptional drought," but that "after the introduction of sweet potatoes, there was a peasant revolt in only 0.26% of prefecture-years with an exceptional drought."

So what does any of this have to do with CO2? A perusal of the materials listed under the heading of Growth Response to CO2 with Other Variables (Water Stress - Agricultural Crops) in our Subject Index reveals that there are several different ways by which atmospheric CO2 enrichment enables crop plants to better tolerate the adverse effects of drought. And these phenomena should thus lead to a weakened ability of drought to foster civil unrest and, in the worst of circumstances, warfare.

As an added bonus, atmospheric CO2 enrichment also enhances the biomass production of sweet potatoes (Ipomoea batatas), by approximately 34% for a 300-ppm increase in the air's CO2 content, as has been demonstrated by six different experiments whose results are summarized at on our website, and where a seventh study indicates that a 600-ppm increase in the atmosphere's CO2 concentration may have the potential to boost sweet potato biomass production by almost 150%.

Yes, it is indeed true that there are situations where "by small and simple things are great things brought to pass."

Sherwood, Keith and Craig Idso

Cao, L. 2004. Meizhou liangshi de chuanru chuanbo jiqi yingxiang yanjiu (The adoption, diffusion and impacts of American crops), Guin Nongye (Historical and Modern Agriculture), Vol. 3, pp. 95-103.

Diamond, J. 2006. Collapse: How Societies Choose to Fail or Succeed. Penguin Group. New York, New York, USA.

Fagan, B. 2009. Floods, Famines, and Emperors: El Niņo and the Fate of Civilizations. Basic Books, New York, New York, USA.

Hinsch, B. 1988. Climatic change and history in China. Journal of Asian History 22: 131-159.

Jia, R. 2014. Weather shocks, sweet potatoes and peasant revolts in historical China. The Economic Journal 124: 92-118.

Pomeranz, K. 2000. The Great Divergence. Princeton University Press, Princeton, New Jersey, USA.

Xia, M. 2010. Zhongguo lishishangde hanzai jiqi chengyin (Droughts in historical China and their causes). Guangming Daily, 21 April 2004.

Yancheva, G., Nowaczyk, N., Mingram, J., Dulski, P., Schettler, G., Negendank, J., Liu, J., Sigman, D., Peterson, L. and Haug, G. 2007. Influence of the intertropical convergence zone on the east Asian monsoon. Nature 445: 74-77.