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Climate Change and Large-Scale Human Crises
Zhang, D.D., Lee, H.F., Wang, C., Li, B., Pei, Q., Zhang, J. and An, Y. 2011a. The causality analysis of climate change and large-scale human crisis. Proceedings of the National Academy of Sciences USA: 10.1073/pnas.1104268108.

The authors note that early paleo-temperature reconstructions suggested that "massive social disturbance, societal collapse, and population collapse often coincided with great climate change in America, the Middle East, China, and many other countries in preindustrial times (Bryson and Murray, 1977; Atwell, 2001; deMenocal, 2001; Weiss and Bradley, 2001; Atwell, 2002)." They also say it has been shown more recently that "climate change was responsible for the outbreak of war, dynastic transition, and population decline in China, Europe, and around the world because of climate-induced shrinkage of agricultural production (Zhang et al., 2005, 2006, 2007a,b; Lee and Zhang, 2008; Lee et al., 2009; Lee and Zhang, 2010; Tol and Wagner, 2010; Zhang, 2010; Zhang et al., 2011b)."

What was done
In a study designed to provide still greater support for this general relationship, Zhang et al. (2011a) "examined the climate-crisis causal mechanism in a period [AD 1500-1800] that contained both periods of harmony and times of crisis," the most prominent of the latter of which was the General Crisis of the 17th Century (GCSC) in Europe, which was marked by widespread economic distress, social unrest, and population decline. This they did by examining linkages between temperature data and climate-driven economic variables that defined the "golden" and "dark" ages in Europe and North America.

What was learned
The seven scientists were able to demonstrate, in their words, that "climate change was the ultimate cause, and climate-driven economic downturn was the direct cause, of large-scale human crises in pre-industrial Europe and the Northern Hemisphere." In addition, they say it was cooling that triggered the chain of negative responses in variables pertaining to physical and human systems. Initially, for example, they found that agricultural production "decreased or stagnated in a cold climate and increased rapidly in a mild climate at the multi-decadal timescale," while the time course of crisis development was such that "bio-productivity, agricultural production and food supply per capita (FSPC) sectors responded to temperature change immediately, whereas the social disturbance, war, migration, nutritional status, epidemics, famine and population sectors responded to the drop in FSPC with a 5- to 30-year time lag." Thus, the dark ages they delineated by these means were AD 1212-1381 (the Crisis of Late Middle Ages) and AD 1568-1665 (the GCSC), whereas the golden ages were the 10th to 12th centuries (the High Middle Ages), the late-14th to early 16th centuries (the Renaissance), and the late-17th to 18th centuries (the Enlightenment).

What it means
Several centuries of European and Northern Hemispheric data reveal that warming and warmth beget human wellness, while cooling and cold produce human misery.

Atwell, W.S. 2001. Volcanism and short-term climatic change in East Asian and world history, c. 1200-1699. Journal of World History 12: 29-98.

Atwell, W.S. 2002. Time, money, and the weather: Ming China and the 'great depression' of the mid-fifteenth century. Journal of Asian Studies 61: 83-113.

Bryson, R.A. and Murray, T.J. 1977. Climates of Hunger: Mankind and the World's Changing Weather. University of Wisconsin Press, Madison, Wisconsin, USA.

deMenocal, P.B. 2001. Cultural responses to climate change during the late Holocene. Science 292: 667-673.

Lee, H.F., Fok, L. and Zhang, D.D. 2008. Climatic change and Chinese population growth dynamics over the last millennium. Climatic Change 88: 131-156.

Lee, H.F. and Zhang, D.D. 2010. Changes in climate and secular population cycles n China, 1000 CE to 1911. Climate Research 42: 235-246.

Lee, H.F., Zhang, D.D. and Fok, L. 2009. Temperature, aridity thresholds, and population growth dynamics in China over the last millennium. Climate Research 39: 131-147.

Tol, R.S.J. and Wagner, S. 2010. Climate change and violent conflict in Europe over the last millennium. Climatic Change 99: 65-79.

Weiss, H. and Bradley, R.S. 2001. Archaeology. What drives societal collapse? Science 291: 609-610.

Zhang, D.D., Brecke, P., Lee, H.F., He, Y.Q. and Zhang, J. 2007a. Global climate change, war, and population decline in recent human history. Proceedings of the National Academy of Sciences USA 104: 19,214-19,219.

Zhang, D.D., Jim, C.Y., Lin, C.S., He, Y.Q. and Lee, F. 2005. Climate change, social unrest and dynastic transition in ancient China. Chinese Science Bulletin 50: 137-144.

Zhang, D.D., Jim, C.Y., Lin, G.C.-S., He, Y.-Q., Wang, J.J. and Lee, H.F. 2006. Climatic change, wars and dynastic cycles in China over the last millennium. Climatic Change 76: 459-477.

Zhang, D.D., Lee, H.F., Wang, C., Li, B., Zhang, J., Pei, Q. and Chen, J. 2011b. Climate change and large scale human population collapses in the pre-industrial era. Global Ecology and Biogeography 20: 520-531.

Zhang, D.D., Zhang, J., Lee, H.F. and He, Y.Q. 2007b. Climate change and war frequency in Eastern China over the last millennium. Human Ecology 35: 403-414.

Zhang, Z., 2010. Periodic climate cooling enhanced natural disasters and wars in China during AD 10-1900. Proceedings of the Royal Society B: Biological Science 277: 3745-3753.

Reviewed 21 December 2011