by Solomon M. Hsiang Thursday, January 5, 2012
In 2007, 11 retired admirals and generals from the U.S. armed forces published a report arguing that global climate changes represented a major threat to global security. That same year, U.N. Secretary-General Ban Ki-moon argued in a Washington Post op-ed that the ongoing Sudanese civil conflict was, in part, attributable to climatic changes. By combining new techniques from climate physics and econometrics, my colleagues and I have found evidence that there is some truth in these statements. Indeed, the global climate can influence the outbreak of civil wars.
Archaeologists, historians, paleoclimatologists and political scientists have all addressed the question of how climate influences societies. They often conclude that climate-driven changes stress societies, leading to conflicts over resources that sometimes escalate to violence, political revolution, the breakdown of social institutions — and even the collapse of civilizations, as was arguably the case for the ancient Akkadian, Mayan and Angkor empires.
However, one important question still lingers: Did societies really experience conflict because of environmental change or did they just coincidentally have problems at the same time that the environment changed? If conflicts are randomly scattered in time, then we would actually expect some conflicts to occur during periods of environmental stress, even if the two are unrelated. Documenting that a historical conflict occurred alongside environmental change isn’t enough to show that the two are linked.
To verify whether the connection is systematic, my colleagues and I devised a new way to assess the question. In an ideal experiment, we would compare two societies that are identical in every way except in their climate. Unfortunately, this is impossible for a number of reasons, including that any two societies taken from different locations (and different climates) are different in many cultural ways. So we did the next best thing: We compared a society to itself over time to see if it becomes more or less likely to erupt in violence when the global climate changes.
To implement this “quasiexperiment,” we needed some type of global climate variation that changes very quickly. That’s because a society’s non-climate properties, such as institutions, culture, politics and economics, evolve slowly over time. So if we want to infer whether any violence that occurs is the result of the rapid climate change rather than these slow social changes, we need to repeatedly expose the population to an abrupt climate change and observe its response.
With this in mind, we turned to the rapidly changing El Niño-Southern Oscillation (ENSO), the single largest pattern of global climate changes that occurs on a year-to-year basis. ENSO is driven by random oscillations in the coupled circulation of the Pacific Ocean and its overlying atmosphere. During the El Niño phase, anomalously warm surface waters slosh across the equatorial Pacific, releasing large quantities of thermal energy into the atmosphere. This generates atmospheric waves that propagate around the planet, generally causing surface temperatures to rise and rainfall to decline for a large swath of the tropics.
Using data on conflicts that occurred between 1950 and 2004, we examined whether tropical countries were more or less likely to erupt into violence during El Niño years than they were during the cooler, wetter La Niña phase of ENSO. Our results were startling: The probability of new conflicts breaking out doubled when the global climate changed from La Niña to El Niño. In a La Niña year, the probability that a randomly selected tropical country began a new conflict was 3 percent, but this risk level rose to 6 percent in hotter and drier El Niño years. Using these results, we calculated that the onset of one in five civil conflicts in these countries since 1950 was influenced by ENSO.
Of course, correlation doesn’t always imply causation. However, careful examination of our data revealed a number of patterns that all point toward a causal link between ENSO and conflict — the odds of all of these patterns occurring exclusively by chance seem extremely unlikely. First, in a “placebo” set of countries that are too far from the equator to be strongly influenced by ENSO, we find no correlation between ENSO and the risk of conflict. Second, when we look at the months in which tropical conflicts start, we find that the additional conflicts that occur in El Niño years happen between June and December — the months when El Niño dominates. Finally, we find that a country’s conflict-sensitivity to ENSO is strongest for poor countries, a result that we would expect if either resource scarcity or weak government institutions exacerbated the social impacts of ENSO.
Do our findings say that future global warming will cause more conflicts? From a strict scientific perspective, the answer is no. Societies in the future may behave very differently from societies in the past, and expected anthropogenic climate changes are different from natural ENSO variations. However, the results should give us pause. We have been exposed to ENSO variations for thousands of years, and yet our modern global society manages these fluctuations so poorly that they double our risk of tropical civil wars. Future climate changes may be less abrupt than El Niño, but they may be larger in absolute magnitude and completely unprecedented.
I hope we can manage future changes better than we’ve managed ENSO in the past. Modern technology provides us with many tools that could be used to manage climate variability, but they must be applied thoughtfully and taken seriously by decision- makers. For example, it is now possible to forecast strong El Niño events up to two years in advance and to rapidly communicate such early warnings to populations spread over large geographic regions.
But if households, businesses, governments, relief organizations or their donors don’t take these warnings seriously, behavior will not change, and we’ll continue to see episodes of unanticipated climate changes, conflict and tragedy. This would be a shame. We possess all the technology we need to overcome both modern and future climate-induced crises. All we have left to do is take advantage of this opportunity.
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