Climate change is now well established as a constant background debate along with most of us feeling guilty about our contribution to pollution in general. The world climate has fluctuated previously, but detailed records of temperature and social events are lacking as soon as we look back further than a few hundred years.

Zhang et al in the Proceedings of the National Academy of Science have turned their attention to the “Little Ice Age” and the “General Crisis of the 17th Century” (1). The Little Ice Age was a period of low temperatures that lasted from 1560 to 1660 and the General Crisis refers to the mess that Europe made of its socio-political organization with revolution and war all over the place.

The authors poured over the records of the times for the period 1500 to 1800 and plotted their data for temperature, grain yields, wars, plagues, and famines and crunched the numbers in a truly heroic fashion looking for Granger causal relations.

Many of us would rather watch paint dry, or wait for politicians to make a good economic decisions, rather than get deep into statistics, but Granger got his Nobel Prize for trying to show with statistical trends how one economic factor causes another, so we need to listen, while remembering that fitting the Granger condition doesn’t, necessarily, mean that there isn’t another cause playing in the game.

To cut to the chase, the record is that as the climate got worse, grain yields got bad, people got hungry and restive. They migrated, rebelled, fought wars and in the poor conditions typhoid, dysentery and plague spread. The authors computers stirred this big mix and came up with climate fluctuation satisfying the Granger causality conditions for the good times and bad.

To put it simply, if the sun is shining and the population is well fed and content, they are less likely to run around and fight with their neighbor for their piece of bread. Not a good prospect for the coming century, I fear.


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