A while ago I noticed a Mother Jones blog entry highlighting a recent article in the journal Human Ecology titled Climate Change and War Frequency in Eastern China over the Last Millennium by David Zhang, Jane Zhang, Harry Lee, and Yuan-qing He. Wiggins over at Opposed Systems Design noticed it too. I got a copy of it and read it. I will first summarize the contents, and then add a few words of my own.
Zhang et al have a data set from the "Tabulation of Wars in Ancient China, dates 800 BC to 1911 AD. They looked at 899 wars between 1000 AD to 1911 AD. They divided China into "North" and "South" based on climate. The temperature data comes from compiling the five "most influential" climate series data sets. They then compared temperature fluctuations and frequency (and type and location) of wars.
They found that "wherever they occurred rebellions are always significantly correlated with temperature change," (407) as the temperature change leads to food shortages, famine and (relatively) heavier taxation by the state. Wars in the "South" region are also significantly correlated with temperature anomalies, despite the fact that the "North" region's climate is more vulnerable to temperature change. The authors believe this is because when climate changes affected the "North" region, northern Chinese migrated down into the "South" region, especially as China was frequently ruled by Northern tribes such as the Mongol and Manchu.
Essentially the authors look at the temperature data and see a cyclical pattern between warm and cold. During the warm period, agriculture expands and China is able to support a large population. During the cold period, agriculture contracts and China's population contracts via famine and resource wars. Economic stress forces tribes outside "civilized" China to attack China's borders, peasants rebel when they can no longer afford to pay taxes (the paper describes scenes from the Ming dynasty of Chinese eating bark, and when there was no more bark, digging up corpses and eating them - and then taxes on top of that).
The authors found "Almost all of the dynastic changes occurred in cold phases, with the exception of the Yuan dynasty, which collapsed 8 years after the end of a cold phase... largely a result of power struggles among different rebel groups"(412). They conclude that "it was the oscillations of agricultural production brought by long-term climate change that drove China's historical war-peace cycles" (413).
Basically, when a cold cycle hit, the amount of available food decreased in both North and South China. In South China, peasants rebelled at taxation, leading to war. In North China, tribes from the north who also felt pressure due to reduced food invaded. The Chinese dynastic state had enough food to feed northern tribes but not the whole of China. The state was weakened through peasant rebellions in the South, and tribes from north of China invaded. Those with power soon became those with food. In time, the climate cycle restarted, the northern tribes because the sitting dynasty, the climate warmed up, agriculture expanded, and China grew used to having more food just in time for another cooling period and more warfare.
Wiggins, at Opposed System Design, didn't have access to the paper, but from its summary, assumes "Zhang would be optimistic about the likelihood of peace in a warming future." The point of Zhang's article was not that cold periods lead to war and warm periods to peace. The point was that when societies adapt to a certain amount of resources, anything that leads to constraints on those resources will lead to conflict. Thus, current global climate change will most likely lead to conflict, even though it is warming rather than cooling. Human civilization has adjusted to climate of a hundred years ago. Any sudden shift that leads to a constriction on resources, whether its oil, arable land, housing, or water, will lead to conflict over that resource.
The logic is the same as if you have 3 toddlers with 3 GI Joes, and then took away two of them (the GI Joes, not the toddlers). You would then have 3 toddlers, each accustomed to having their own GI Joe, with only one GI Joe between them. What are the chances they would learn to share?
David D. Zhang, Jane Zhang, Harry F. Lee, Yuan-qing He. Climate change and war frequency in eastern China over the last millennium. Human Ecology. Volume 35, Number 4, August, 2007, Pages 403-414.