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.