http://www.pnas.org/content/108/42/17296.full

Abstract
Recent studies have shown strong temporal correlations between past climate 
changes and societal crises. However, the specific causal mechanisms underlying 
this relation have not been addressed. We explored quantitative responses of 14 
fine-grained agro-ecological, socioeconomic, and demographic variables to 
climate fluctuations from A.D. 1500–1800 in Europe. Results show that cooling 
from A.D. 1560–1660 caused successive agro-ecological, socioeconomic, and 
demographic catastrophes, leading to the General Crisis of the Seventeenth 
Century. We identified a set of causal linkages between climate change and 
human crisis. Using temperature data and climate-driven economic variables, we 
simulated the alternation of defined “golden” and “dark” ages in Europe and the 
Northern Hemisphere during the past millennium. Our findings indicate that 
climate change was the ultimate cause, and climate-driven economic downturn was 
the direct cause, of large-scale human crises in preindustrial Europe and the 
Northern Hemisphere.

Apparent bottom line - humans do worse when climate cools.  Lessons/targets for 
GE?  Avoiding global warming is one thing, but don't think I want to be around 
for the next ice age either.
-Greg

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