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.
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, economistMarshall 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.
And these results regarding temperature and precipitation should come as no surprise given earlier studies on the climate-conflict link. In 2010, Halvard Buhaug, a researcher at the Peace Research Institute Oslo, re-examined Burke's earlier study of weather and war in Africa and concluded that it didn't stand up to further scrutiny. With more data, he argued, the link between rainfall, temperature, and violence disappeared -- a point accepted by Burke and his colleagues.
Second, Hsiang and his co-authors are careful to clarify that they don't think El Niño caused warfare, but rather that it was a contributing factor -- that in many cases, conflicts that would have broken out anyway may have occurred earlier owing to the effects of the El Niño cycle. That fits with the conclusions of a 2008 review of the evidence linking climate to conflict in the Journal of Peace Research, which suggested that any link is contingent on a range of factors from governance through wealth to land-use patterns and "claims of environmental determinism leading seamlessly from climate change to open warfare are suspect."
Indeed, saying the weather is responsible for civil war is like saying drought is responsible for famine. At most, weather can be an additional stressor to an environment already made combustible by human activities. For example, experts on the Shining Path insurgency in Peru or the Sudanese conflict might be surprised at the idea that these two conflicts are seen as prime examples of the impact of Pacific weather patterns on civil war, given that both have a whole range of causes (including poverty, twisted ideology, and a cruel and incompetent government and military response in the case of the Shining Path).
The considerable limits to climate determinism are clear from the ENSO paper itself. One way of understanding the results is to look at the risks to peace associated with El Niño. Between 1950 and 2004, the chance that a conflict did not begin in any given year in any country was 97 percent, according to the data. Take the results of the paper at face value: For countries affected by El Niño, a 1-degree Celsius rise in El Niño-related temperatures decreased the probability that a conflict didn't begin to 95.5 percent. Even if there is a link, there is a lot more to explaining war and peace than the weather -- not least, those 95 cases out of 100 in which nothing happened.
And in fact, Hsiang and colleague's paper contains some good news about our warmer future related to that other 95 percent: First, it suggests that the effect of El Niño on warfare declines as countries get richer. So Africa's last decade of rapid growth (with agriculture's share of the region's GDP falling from 22 to 13 percent between 1967 and 2009) should mute any future impact of climate on violence. In fact, temperatures may have been consistently hotter than average since 1990 in Africa, and precipitation consistently lower, but there has still been a drop-off in the number of civil wars ongoing since the mid-1990s and a dramatic fall in war deaths since that time.
Second, the analysis points to the relative importance of factors like geopolitics in explaining the outbreak of violence. The second-highest risk of civil war between 1950 and 2004, according to the paper, was in 1989 -- a La Niña year -- part of a dramatic peak in war risk that continued until 1994, and has gone unmatched before or since. That speaks to the impact of the end of the Cold War on civil conflict. The good news is that in the period since the mid-1990s, conflict risk has been on the decline as global cooperation to settle disputes has been on the rise. Even if climate cycles are a short-term influence on conflict, the long-term trends are dominated by factors other than the weather. The argument that we should reduce greenhouse gas emissions to slow climate change is beyond reasonable dispute -- but that it will make for a more pacific world is yet to be demonstrated.