A new study finds that rising global temperatures could lead to an increase in violence and may even make waging war more likely, scientists say, as climate change continues to warm the planet faster than experts anticipated.
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 -- coincided with 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, who was the lead author of the study, published online Thursday by the journal Science.
A Stanford University report released on Aug. 1 showed that climate change is on pace to occur 10 times faster than any other change recorded in the past 65 million years.
The report showed that extreme weather events – like heat waves, droughts, and heavy rainfall – are expected to become more severe and more frequent. Scientists said that by the end of the century, if current emissions of greenhouse gases remain unchecked, temperatures over the northern hemisphere will be 5 to 6 degrees Celcius warmer than today’s averages.
“In this case, the hottest summer of the last 20 years becomes the new annual norm,” the report read.
While most people don’t connect climate change with human conflict, Ohio State University psychology professor, Brad Bushman, whose work on crime and heat was analyzed by Hsiang, says there are psychological factors behind the phenomenon.
Bushman told Al Jazeera that the human brain is hard-wired for situations like fight or flight, hot and cold temperatures – though heat is harder to regulate. For example, when it’s cold you can put on more layers, but not every house has central air conditioning.
When people get hot and have few options of how to handle the rising temperatures, their fight or flight response is automatically triggered. According to Bushman, heat increases arousal -- how sensitive and responsive the brain is to stimuli -- in humans.
“When people are highly aroused, attention level decreases. There’s an upside-down ‘U’ relationship between arousal and wise thinking.”
But since not everyone is aware of the arousal brought on by higher temperatures, people often mistake the origin of their arousal to be an event other than the heat – making violent interactions more likely.
“When arousal (or heat) becomes too high, deliberative cognitive thought goes out the window,” Bushman said.
Some experts in the causes of war were skeptical about the link between heat and violence. Joshua Goldstein, a professor of international relations at American University and author of "Winning the War on War," said the idea of hotter tempers brought on by hotter temperatures is only one factor in conflict, and that it runs counter to a long and large trend to less violence.
But Hsiang said that whenever the analyzed studies looked at temperature and conflict, the link was clear, no matter where or when.
He says that his team's analysis demonstrates a dire pattern that will only worsen as the earth continues to warm. In the U.S., for example, for every increase of 5.4 degrees Fahrenheit, the likelihood of violent crime increases by between 2 percent and 4 percent.
In war-torn parts of equatorial Africa, the study says, every added degree or so Fahrenheit increases the chance of conflict between groups -- rebellion, war, civil unrest -- somewhere in the range of 11 percent to 14 percent.
And because of increases in carbon dioxide pollution, according to a separate paper published in Science on Thursday, temperatures in much of North America and Eurasia are likely to increase by 5.4 degrees by about 2065.
The same paper sees global average temperatures increasing by about 3.6 degrees in the next half-century. That implies essentially about 40 percent to 50 percent more chance for African wars than there would be without global warming, said Edward Miguel, another Berkeley economist and study co-author.
The Berkeley analysis examines about a dozen studies on collapses of empires or dynasties, about 15 studies on crime and aggression and more than 30 studies on wars, civil strife or intergroup conflicts.
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.
In another study, baseball pitchers were more likely to retaliate against their opponents when a teammate was hit by a pitch on hotter days. Hsiang pointed to the collapse of the Mayan civilization that coincided with periods of historic drought about 1,200 years ago.
Renee Lewis contributed to this report. Al Jazeera and wire services