My Berkeley friends have written a
nice OP-ED available here discussing the ample past set of studies documenting a correlation between climate conditions and violence.
The serious paper is posted here. Does the past predict our future? Is this a law of physics? As I have argued again and again, urbanization and improved access to cooled buildings will nullify this result. The authors are likely to counter that one Harvard Ph.D thesis documents similar effects in the United States.
Matthew Ranson's study is available here. Since you don't have time to read it, let me help you.
He creates a county/year/month panel data set covering the years 1960 to 2009 and studies whether there is a climate/violence relationship by using within county month by month variation in climate conditions. According to his results reported in Table 3, violent crime rises when it is over 90 degrees F and declines when the temperature is less than 10 degrees. This suggests that climate change will raise violence in the U.S.
According
to the Census, there were 23,000 homicide victims in the United States in 1980 and even though the U.S population grew between 1980 and 2008 --- by 2008, there were 16,200 homicide victims. So, the homicide rate fell from 10.2 per 100,000 to 5.4 per 100,000 people. That's progress!
By way of contrast, 51,000 people died in car crashes in the 1980 in the U.S while 37,000 died in car crashes in the U.S in 2008.
(source) So, homicide rates are declining faster than car crash deaths.
My point is that Ranson's correlation could be a causal effect but the secular trend swamps his blips.
A richer nation with better information technology and active police force can activate efforts on hotter days to disperse population and keep the peace. An implicit Lucas Critique lurks in this work. As Lucas taught us back in 1976, when government changes the rules of the game --- people re-optimize. A variant of the
Lucas Critique is the "Kahn Critique" that states that when mother nature changes the rules of the game, that local government reoptimizes and this effects the reduced form correlation between violence and climate conditions.
Local government will take action because it anticipates the Ranson effect so that his prediction vastly over-states the increased death risk. The key issue here is what is the marginal cost of extra policing effort on hot days? These guys could work extra hard on those days and kick back and take it easy on off days. So, these reduced form researchers need to explore what is the marginal productivity of the police and how this varies on hot and colder days. Without such structural estimates, it is impossible to know whether local governments can completely offset the effects that the "historical correlation" scholars have uncovered. A richer nation will have the resources to deploy the extra cops even if the marginal cost of effort is high. This is another example of how income shields us from climate change.
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