I read a lot of mystery stories.  Old ones, new ones, ones set in times past and ones set it the future.  A common thread is it is summer and a heat wave so crime is up.  You hear this every spring from the Boston Police and the Mayor – we need to prepare for the combination of hot weather and school being out.  So is this just an urban myth?  Maybe not.  A new study to be published in the journal, Science, was summarized in Sunday’s New York Times.

But researchers are now quantifying the causal relationship between extreme climate and human conflict. Whether their focus is on small-scale interpersonal aggression or large-scale political instability, low-income or high-income societies, the year 10,000 B.C. or the present day, the overall conclusion is the same: episodes of extreme climate make people more violent toward one another.

In a paper published this month in the journal Science, we [MARSHALL BURKE, SOLOMON HSIANG and EDWARD MIGUEL] assembled 60 of the best studies on this topic from fields as diverse as archaeology, criminology, economics, geography, history, political science and psychology. Typically, these were studies that compared, in a given population, levels of violence during periods of normal climate with levels of violence during periods of extreme climate. We then combined the results from those studies that concerned modern data in a “meta-analysis,” a powerful statistical procedure that allowed us to compare and aggregate findings across the individual studies.

We found that higher temperatures and extreme rainfall led to large increases in conflict: for each one standard deviation change in climate toward warmer temperatures or more extreme rainfall, the median effect was a 14 percent increase in conflict between groups, and a 4 percent increase in conflict between individuals.

Global mean surface temperature difference from the average for 1880–2009 (Photo credit: Wikipedia)

The study went beyond the present day, back to the collapse of several civilizations:  The Maya, Angkor Wat and the Akkadian in Syria.  Climate appears to have played a role in the collapse of each.

Our findings held at very high levels of statistical confidence. To illustrate the consistency of the results: of the 27 quantitative studies we looked at that examined a link between temperatures and modern conflict, all of them found that higher temperatures were associated with more violence. This sort of pattern in the results was extremely unlikely to happen by chance. (Imagine trying to get 27 “heads” in a row when flipping a coin.)

What explains the strong link between climate and conflict? Different mechanisms are most likely operating in different settings, but the two most important factors appear to be aggression and scarcity. The aggression factor is intuitively easy to understand (again, recall summer in the city), and it probably underlies the finding that anomalously hot months have significantly higher crime rates in cities in the United States. As for scarcity, the logic is only slightly more complex. In low-income countries largely dependent on agriculture — like those in much of sub-Saharan Africa and parts of Asia and Latin America — when the rains fail and temperatures scorch, crops wilt and die. This leaves many people dangerously close to the edge of survival, which can lead to social strife and even civil war.

So besides gearing up for more shootings, stabbings and homicides on the summer streets, what are the implications?  The study concludes

Our findings help us better understand both the past and the present, but they are particularly important for what they imply about the future. Many global climate models project global temperature increases of at least 3.6 degrees Fahrenheit (2 degrees Celsius) over the next half century. Our results imply that if nothing changes, this rise in temperature could amplify the rate of group conflicts like civil wars by an astonishing 50 percent in many parts of the world — a frightening possibility for a planet already awash in conflict.

…

Decision makers must show an understanding that climate can fundamentally shape social interactions, that these effects are already observable in today’s world and that climate’s effects on violence are likely to grow in the absence of concerted action. Our leadership must call for new and creative policy reforms designed to tackle the challenge of adapting to the sorts of climate conditions that breed conflict — for instance, through the development of more drought- and heat-resistant agricultural technologies.

As we contemplate intervention in Syria and look at the increasing and never-ending violence in Africa, we also need to ask ourselves some questions about violence here at home.  Will global warming lead to increased domestic violence?  What do we do with this knowledge and all the guns on the streets?  And will anyone pay any attention?

Related articles

• Rising temperatures and flaring tempers linked, says study (thehindu.com)
• Climate strongly affects human conflict and violence worldwide, says study (eurekalert.org)
• Cool heads likely won’t prevail in a hotter, wetter world (sciencedaily.com)
• Scientists say global violence could rise with global warming (upi.com)