According to a new review of 55 separate studies, there is a meaningful connection between climate change and human violence.
The working paper, put out by researchers with the National Bureau of Economic Research, is what’s called a meta-analysis: a study of studies, in effect. After going through numerous analyses of the relationship between climate change and violence in various settings, the researchers settled on 55 of the most rigorous pieces of work. They then evaluated the picture painted by those studies, and worked to amalgamate their findings into a single statistical result.
They looked at conflicts between individuals — “domestic violence, road rage, assault, murder, and rape” — as well as conflicts between larger human groups — “riots, ethnic violence, land invasions, gang violence, civil war and other forms of political instability, such as coups.” The end result? The researchers determined that changes in drought and rainfall patterns, but especially increases in temperature, all have a meaningful link to increases in both forms of violence. “We find that deviations from moderate temperatures and precipitation patterns systematically increase the risk of conflict, often substantially, with average effects that are highly statistically significant,” the researchers wrote.
Statistical significance is technical term meaning that the researchers’ numbers are robust enough to point to a real phenomenon in the populations their meta-analysis covered. It doesn’t confirm exactly what is going on; there could still be a correlation-versus-causation problem between climate change and violence, for example. But it does mean Burke and his colleagues aren’t being fooled by random noise in the data.
The effects are different for different parts of the globe. Stanford researcher Marshall Burke, one of the study’s three co-authors, told Chris Mooney at the Washington Post that “for a degree Celsius of temperature increase (1.8 degrees Fahrenheit) … there could be a 20 percent increase in civil conflict in Africa.” In the United States, meanwhile, every one degree Celsius increase in warming should bring “a one percent increase in interpersonal conflicts.”
It’s important to note that what rising temperatures and climate change are doing here is increasing the odds of violence rather than causing it in a direct, if-this-then-that relationship. Steroids in baseball are a good analogy: they don’t cause any particular home run, but instead increase the likelihood that a batter will hit a long bomb. It works the same way with extreme weather, where global warming increases the chances and severity of events rather than directly causing any particular storm or drought.
The latest work by the United Nations’ Intergovernmental Panel on Climate Change also suggests climate change drives up inter-group conflicts by placing added stresses on societies: food shortages, water shortages, extreme heat, flooding, and the like. In turn, societies that suffer from more violent conflict are also more vulnerable to the damage climate change can do, and are less able to adapt.
Countries like Nigeria and Syria have been pointed to by experts as examples of violent conflict helped along by climate change. A 2011 report on Nigeria by the U.S. Institute for Peace explained how additional heat, reduced rainfall, and the resulting desertification damaged the Nigerian economy and placed added social and psychological stress on Nigerians themselves: “Evidence in and outside Nigeria suggests that alienated young people who lack resources and economic opportunity are more likely to join rebellions,” the report went on. “In the dusty streets of Borno’s state capital, for instance, the violent anti-establishment Islamic group Boko Haram attracts rafts of jobless young men, as do the Delta’s many militias and groups.”
Another recent and extremely granular examination of the risks climate change posed for the American economy also included increased crime rates for different parts of the country. Overall, the effect was modest, amounting to a likely cost of only $35 per person by 2100. But it also became significantly more pronounced on days over 95°F. “A growing body of rigorous quantitative research across multiple disciplines has found that weather, and in particular temperature, affects the incidence of most types of violent and non-violent crime in American cities and rural areas alike” according to the study.
Mooney also talked to Richard Larrick, a professor at Duke’s Fuqua School of Business, who was the lead author on a study looking into the effect of unusually hot days on how often Major League Baseball pitchers throw retaliatory beanballs at the other team’s batters. “Researchers in social psychology have studied the relationship between temperature and aggression for many decades,” Larrick said. Laboratory research in “tightly controlled” settings shows that Larrick “changes in temperature directly lead to more aggression.”
“Heat changes the way people feel and think, increasing anger and making thoughts of aggression increase,” Larrick continued.