Findings from what investigators call "the first comprehensive synthesis" of 60 different studies linking violence to climate change suggest that as global temperatures increase, so will wars, rape, and murder. The new study, conducted by researchers from the University of California, Berkeley, Princeton University, and the National Bureau of Economic Research in Cambridge, Mass., is published in the current issue of the journal Science.
According to the paper, the three researchers took what they call a "rigorous approach" to their selection of the studies that formed the basis of their work. They reviewed data from 60 primary studies, representing the efforts of over 190 researchers from around the world, that analyzed 45 different occurrences of violent conflict ranging over a period from about 10,000 BCE to the present. Their analysis revealed a strong historical correlation between societal violence and hotter temperatures. The authors looked at both interpersonal violence and intergroup conflict in the context of temperature and rainfall.
Although it is impossible to conduct a true controlled experiment on the effect of higher temperatures on human conflict, the authors used studies that quantitatively analyzed the amount of conflict in a given population just before and after a known change in climate had occurred. Consequently, inferences were based on how a population responds to climatic conditions that vary over time and a longitudinal analysis used to estimate the causal effect of climate on humankind's inclination to violence.
"The magnitude of climate's influence is substantial," say the authors. "For each one standard deviation change in climate toward warmer temperatures or more extreme rainfall, median estimates indicate that the frequency of interpersonal violence rises four percent and the frequency of intergroup conflict rises 14 percent."
In comparison to the effect of rainfall, the researchers found that temperature was a more reliable predictor of societal unrest. One study, for example, found that police officers were more prone to draw their weapons as the temperature in the training room was increased. According to co-author Solomon Hsiang, an assistant professor of public policy at UC Berkeley, in comments made to Bloomberg News, the officers felt more threatened in a hot room.
"Imagine you're in a country, there are some protesters, and some policemen who are supposed to be maintaining order. Their response to that protest may change based on environmental conditions," Hsiang said.
The researchers do not conclude that climate change is the sole, or even main, factor causing violent behavior. However, they do suggest that rising temperatures have a substantial impact on the incidence of human conflict that appears to extend across the planet, throughout history, and at every level of society.
Coauthor Edward Miguel, a professor of environmental and resource economics at UC Berkeley, explained in an interview with Living on Earth that a two degree Celsius rise in temperature in the United States would be associated with something like a five or six percent increase in violent crime. That adds up to many thousands of additional violent crimes in the US each year, he noted.
While the authors acknowledge the need for more research to illuminate the relationship between violence and climate change, they caution that disregarding the potential effect of human-caused global warming on societal conflict is "a dangerously misguided interpretation of the available evidence."
