Cold Periods Caused Population Crashes in China Over the Past Millenium

It is conventional wisdom in the climate change debate that warming is detrimental to society. One example of this harm is a postulated reduction in agricultural output due to global warming that is followed by population decline due to lack of food. In the present study, Lee and Zhang (2010) examine this hypothesis using historical data from China. China is a good test case because it has been a well-populated, and primarily agricultural, country for millennia.

Data on Chinese history, including temperature, wars and rebellions, epidemics, famines, and population for the past millennium were examined. Over the study interval of 911 years, it was found that nomad migrations, rebellions, wars, epidemics, floods, and droughts were all higher in cold periods. All of these factors tended to act to disrupt population growth or cause mortality. Overall, 5 of 6 population contractions, with losses of 11.4 to 49.4% of peak population, were associated with a cooling climate. The 6th cool period evinced a great reduction in growth rate during a cool phase, but not a collapse. None of the population contractions were associated with a warming climate.

Prior theory had suggested that population crashes were either due to pure Malthusian cycles (overpopulation leading to a crash) or to bad governance, but this new work suggests that agrarian societies such as preindustrial China tend to increase in population to a maximum level, but then cold periods reduce agricultural production, thereby promoting multiple types of conflict and disruption that ultimately lead to population contraction. Given these finding, it would thus appear that cold episodes are more dangerous to human society than warm episodes, at least within the scope of these historical fluctuations.