WASHINGTON – As the world gets warmer, people are more likely to get hot under the collar, scientists say. A massive new study finds that aggressive acts like committing violent crimes and waging war become more likely with each added degree.
Researchers analyzed 60 studies on historic empire collapses, recent wars, violent crime rates in the United States, lab simulations that tested police decisions on when to shoot and even cases where pitchers threw deliberately at batters in baseball. They found a common thread over centuries: Extreme weather — very hot or dry — means more violence.
The authors say the results show strong evidence that climate can promote conflict.
“When the weather gets bad, we tend to be more willing to hurt other people,” said economist Solomon Hsiang of the University of California, Berkeley.
He is the lead author of the study, published online Thursday by the journal Science. Experts in the causes of war gave it a mixed reception.
The team of economists even came up with a formula that predicts how much the risk of different types of violence should increase with extreme weather. In war-torn parts of equatorial Africa, it says, every added degree or so increases the chance of conflict — rebellion, war, civil unrest — by 11 percent to 14 percent. For the United States, the formula says that for every increase of 5.4 degrees, the likelihood of violent crime goes up 2 percent to 4 percent.
Temperatures in much of North America and Eurasia are likely to go up by 5.4 degrees by about 2065 because of increases in carbon dioxide pollution, according to a separate paper published in Science on Thursday.
When the Intergovernmental Panel on Climate Change updates its report next year on the impacts of global warming, it will address the issue of impacts on war for the first time, said Carnegie Institution scientist Chris Field, who heads that worldwide study group. The new study is likely to play a big role, he said.
People often don’t consider human conflict when they think about climate change, which is “an important oversight,” said Ohio State University psychology professor Brad Bushman, who wasn’t part of the study.
There’s a good reason why people get more aggressive in warmer weather, Bushman said. Although people say they feel sluggish when they are hot, their heart rate and other physical responses are aroused and elevated. They think they are not agitated, when in fact they are, and “that’s a recipe for disaster,” Bushman said.