Proc. Natl Acad. Sci. USA http://dx.doi.org/10.1073/pnas.1104268108 (2011)

Climatic shifts were the ultimate cause of humanitarian crises in pre-industrial Europe, according to a team of investigators based in China. The researchers analysed how 14 variables — describing agricultural production, demography and the economy — varied in relation to one another in early modern Europe, between 1500 and 1800. That period encompasses the region's 'golden' and 'dark' ages, as well as mild and cold phases of the Little Ice Age.

David Zhang of the University of Hong Kong, and his colleagues, used five criteria to assess the causal relationships between their many data sets. Variables such as agricultural production and per-capita food supply showed an immediate response to temperature changes, whereas social disturbances, such as war, migration and famine, tracked the per-capita food availability trend, but with a response lag of several years.

Some details of their project are startling. For example, the average height of Europeans closely followed the temperature, and the number of wars increased 41% in the cold phase of the Little Ice Age. The authors argue that many historical changes typically attributed to social factors actually have their roots in climate changes.