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.
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