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 Fahrenheit or so increases the chance of conflict between groups- 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 Fahrenheit, 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 that 5.4 degrees by about 2065 because of increases in carbon dioxide pollution, according to a separate paper published in Science on Thursday.
The same paper sees global averages increasing by about 3.6 degrees in the next half-century. So that implies essentially about 40 percent to 50 percent more chance for African wars than it would be without global warming, said Edward Miguel, another Berkeley economist and study co-author.