The hotter it gets, the more likely we are to kill each other. Murder rates go up in heat waves; in some countries, civil war is also more likely. In training exercises in hot weather, police are more likely to pull out a gun and fire. As the climate changes and extreme heat becomes more common, it makes sense that violent crime will become more common at all scales of destruction–from bar fights to rape to mass riots.
A new working paper released by the National Bureau of Economic Research analyzes 55 studies about climate change and violence and comes to the same conclusion. “Hotter than average temperatures do cause an increase in conflict,” says Stanford University researcher Marshall Burke, the lead author of the paper. “And that’s what we find over and over again across different types of conflicts across different settings.”
The paper looked at studies about violence of all kinds, says Burke–“everything from aggravated assault and murder, and even mundane things like violence in baseball games, all the way up to large group conflicts, things like riots and civil wars.”
Besides rising temperatures, climate change is also linked to drought, which the researchers found to be another cause of violence in places that depend on local farming. At the other extreme, heavy rains like monsoons can cause violence in some areas.
All of the studies examined look at changes within particular regions. Nigeria might be hotter and more conflict-prone than Norway, but there are so many differences it doesn’t make sense to compare the two. Instead, the included studies look at how a country like Nigeria changes over time as the climate varies.
The effects are different depending on location and are more significant for group crimes than interpersonal ones. In Africa, if the temperature rises by one degree Celsius, there could be a 20% increase in group violence like civil wars. In the U.S., a similar temperature increase might lead to a 1% increase in crimes like assault and murder.
The paper doesn’t look at why climate affects violence–just the fact that it does. “We do know that the increase in temperature caused more conflict, but we can’t say definitively what the mechanism is,” Burke explains. “So we see that as a big research agenda going forward, to try to understand exactly what’s going on.”
There’s enough evidence that it makes sense for society to start planning for an angrier, more aggressive future. “We actually should expect perhaps substantial increases in the amount of conflict,” Burke says.