As the East Coast of the United States was pounded by a hurricane over the weekend, mere days after an earthquake had cracked monuments and upset lawn furniture from Virginia Beach to Baltimore, Mother Nature was once again front-page news across the country. So it was fortuitous that last week's issue of the scientific journal Nature included a much-talked-about article linking the wrath of nature to the wrath of man. "Climate Shifts Cause War" and "First Proof that Climate Is a Trigger for Conflict," the headlines suggested.
In the paper, Princeton University researcher Solomon Hsiang and colleagues argue -- as paraphrased by a Nature news article -- that "tropical countries face double the risk of armed conflict and civil war breaking out during warm, dry El Niño years than during the cooler La Niña phase." El Niño and La Niña (collectively known as ENSO, for the El Niño Southern Oscillation) are the warm and cool parts of the variation in temperatures that occurs every few years in the Pacific Ocean. In different parts of the tropics, El Niño can cause conditions ranging from floods to droughts -- in turn potentially linked to lower agricultural output and other risks. Hsiang and his co-authors looked at data on the timing of ENSO and civil conflict in the tropics and concluded that as many as one in five civil wars worldwide over the last 60 years may be related to El Niño.
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Given that climate change is likely to be associated with warmer, drier tropical regions, the study's findings led numerous commentators to warn that the world's future could be increasingly violent. Thankfully, the study -- for all its careful design and academic interest -- provides little evidence that human-induced climate change will have any such effect. The nature of the relationship between the weather and violence in the past remains open to question, and the study itself suggests reasons why we'd expect any impact to decline in the future.
The paper is the latest in a line that has linked climate with violence. The Malleus Maleficarum, a 15th century Catholic treatise on witchcraft, has a whole chapter on how witches "Raise and Stir up Hailstorms and Tempests, and Cause Lightning to Blast both Men and Beasts," as economist Emily Oster notes in her study of the link between bad weather and witch burnings. During the Little Ice Age in the mid-centuries of the last millennium, witch burnings increased as the climate got cooler; as many as 1 million people were killed. In 2007, University of Hong Kong geographer David Zhang and colleagues from around the world looked at data covering global temperatures and warfare dating from 1400 to 1900 and estimated that the number of wars worldwide per year was almost twice as high in cold centuries as it was in warm centuries. This was, they suggested, because cold weather caused declining food yields and rising food prices, which brought with them famine and political instability.
Further south, the usual concern is with heat and drought rather than cold and overcast weather, so the plausible relationship between climate and warfare is different. In 2009, University of California, Berkeley, economist Marshall Burke and colleagues looked at temperature and conflict data from Africa and found a positive association between warmth and war. They went as far as to argue, "When combined with climate model projections of future temperature trends, this historical response to temperature suggests a roughly 54% increase in armed conflict incidence by 2030, or an additional 393,000 battle deaths if future wars are as deadly as recent wars."
But is the relationship between climate and violence really that clear? First off, even when rainfall and temperature patterns were directly included in Hsiang and colleague's statistical analysis, the association between El Niño years and civil violence remained. In other words, whatever the impact of El Niño on violence, it apparently isn't connected to its effect on precipitation levels or high temperatures in tropical countries. Perhaps, the paper suggests, El Niño's impact on violence is due to the timing of the rainfall, or altered wind patterns, or humidity, or cloud cover -- but those theories are (so far) untested.
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