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.
