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 and lab simulations that tested police decisions on when to shoot. They found something in common over centuries: Extreme weather, very hot or dry, means more violence.

"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.

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 by 11% to 14%.

For the United States, the formula says that for every increase of 5.4 degrees Fahrenheit (3 Celsius), the likelihood of violent crime goes up 2% to 4%.

Temperatures in much of North America and Eurasia are likely to go up by that amount 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.

Hsiang said that whenever the analyzed studies looked at temperature and conflict, the link was clear, no matter where or when.

In one study, police officers in a psychology experiment were more likely to choose to shoot someone in a lab simulation when the room temperature was hotter, Hsiang said. He also pointed to the collapse of the Mayan civilization that coincided with periods of historic drought about 1,200 years ago.

There is a good reason why people get more aggressive in warmer weather, said Ohio State University psychology professor Brad Bushman. His work was analyzed by Hsiang. 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.

Experts who research war and peace were split in their reaction to the new study.

"The world will be a very violent place by mid-century if climate change continues as projected," said Thomas Homer-Dixon, a professor of diplomacy at the Balsillie School of International Affairs in Ontario.

But Joshua Goldstein, a professor of international relations at American University and author of "Winning the War on War," found faults with the way the study measured conflicts. He said the idea runs counter to a long trend to less violence.

"Because of positive changes in technology, economics, politics and health," conflict is likely to continue to drop, although maybe not as much as it would without climate change, he said.