The Heat of Battle

By Eric Wagner

When the Nobel committee handed Al Gore and the Intergovernmental Panel on Climate Change the 2007 Peace Prize, some critics were perplexed. What, they wondered, did the fight against climate change have to do with peace? A new study in Proceedings of the National Academy of Sciences helps answer these questions by finding that, in the past, climate change and war have gone hand in hand.

While environmentalists have long argued that climate change will lead to political unrest, no one had actually examined how previous temperature fluctuations related to changes in societies’ behavior. The researchers behind the PNAS study, led by David Zhang from the University of Hong Kong, took records of food prices, population, and war between the years 1400 and 1900 and then compared them with climate data from the same span. They uncovered a cyclical pattern: a drop in temperatures coincided with higher food prices, declining populations, and an increased number of wars.

Global cooling, their hypothesis goes, led to decreases in rainfall, which made the land less fertile. The result: food shortages, skyrocketing food costs, and sometimes famine. Communities fell into disarray, fomenting broad social unrest. Armed conflicts surged. The number of wars in years with the coolest average temperatures—1450, 1650, and 1820—was nearly twice that of the mild eighteenth century. When temperatures rose again, relative calm was restored.

Global cooling might seem irrelevant at a time when temperatures are rising, but the researchers note that it is the magnitude of the temperature change that matters, not the direction of it. They also recognize that war and its causes can be quite complex, often resting on factors that are independent of climate. But climate change, they write, can tip the scales by making already strained social conditions worse.

Even so, societies may now be better positioned to avoid such a fate, in part because social institutions are more robust than they once were. Back in 1400, no one had heard of the United Nations, the World Health Organization, or other groups built to alleviate social stress. The researchers hope that, with these organizations in place as the temperature changes this time around, people will be more likely to play nice.

Zhang, D.D. et al. 2007. Global climate change, war, and population decline in recent human history. Proceedings of the National Academy of Sciences 104(49):19214-19219.

Photo: ©Stewart Behra/iStock.com

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