First Bernie Sanders said "climate change was directly related to the growth of terrorism." Then Prince Charles said drought was the root cause of the Syrian conflict and current refugee crisis. Pundits everywhere leaped at the opportunity to say both claims were wrong. Who is right?
We have been studying the possible link between climate and violence for years, and we'd like to clarify what the most up-to-date science can and cannot say on this topic.
In general, conflicts are complex events — the result of many instigating factors, not just one. When we notice that violence occurs at the same time as a major climatic event, like the war in Syria erupting at the same time as a drought, we don't ask, "Was the climactic event the cause of the conflict?" Rather, we ask "Was the climatic event a cause of the conflict?" It might be that you need both ethnic rivalries and a drought to create a conflict, and missing either key ingredient might lead to a more peaceful outcome.
Another caveat: We can't predict that a particular conflict will or will not happen. Instead, we can assess the risk that violence might occur in response to changes in the climate. The situation is similar to assessing the risk of a car accident. Nobody ever says "If you drive to the store now, you will get into an accident." Instead, we might say "If you drive to the store during this rainstorm, you are more likely to get into an accident than if you wait until the rain stops."
We have studied many types of violence — including sports violence, murder, gang violence, riots and civil wars. What we find time and again, around the world and throughout human history, is that climatic events are a cause of social conflict. They are not the only cause, but in places where there is a risk of violence because of non-climate factors, climate changes can amplify this risk.
Data from multiple countries show that high temperatures make small-scale personal conflicts more likely, probably because heat affects individuals' propensity for aggressive behavior. For instance, research shows that hot days are associated with more rape and murder in the United States and more domestic violence in Australia.
Similarly, high temperatures and, in agrarian populations, extreme rainfall (either too much or too little) amplify the risk of large-scale conflict between groups of people — probably because the economic impact of these events alters the political landscape. Multiple studies have shown that hotter than average temperatures substantially increase the likelihood of both local-level group violence and full-blown civil war in Sub-Saharan Africa.
These findings are controversial. Some researchers have claimed that our conclusions are biased. Further studies, however, have verified our work. The link between climatic change and violence is remarkably consistent and reproducible.
That said, little is known about how terrorism — and in particular, carefully planned strikes such as the November Paris attacks — might be related to climate change. To the extent that terrorism is a tactic used in political civil conflicts, terrorism may fluctuate with the climate. Similarly, if terrorist groups find havens in war-torn regions, such as northern Syria, then the fact that climatic changes amplify the risk of civil war could mean that they also amplify the risk of terrorism. But we don't really know.
As for the Syrian conflict, what's indisputable is that it's unfolding against a backdrop of unprecedented warming and a multiyear drought in the region. We also know that high temperatures and drought increase the risk of civil war, and that climate change worsened the drought. Putting two and two together, it's entirely possible that climate change made Syrian violence more likely than it otherwise would have been. Nevertheless, we cannot prove that climate change caused this specific conflict.
Coming back around to Bernie Sanders and Prince Charles, our verdict is their assertions are reasonable, but not yet supported by strong scientific evidence.
Our broader view, though, is that Sanders and Charles were right to highlight the large social risks associated with a warming climate. Right now we are turning the knobs on the global climate with limited understanding of what this will do to our children, our grandchildren or us. Future research might or might not prove that climate change caused recent terrorist incidents or the Syrian conflict. But there's already plenty of evidence that climate change could lead to a more violent world.
Solomon Hsiang is the chancellor's associate professor of public policy at UC Berkeley. Marshall Burke is assistant professor of earth system science at Stanford University.
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