Wars, murders, and other acts of violence will likely become more commonplace in coming decades as the effects of global warming cause tempers to flare worldwide, a comprehensive new study warns.

The research, detailed in this week's issue of the journal Science, synthesizes findings scattered across diverse fields ranging from archaeology to economics to paint a clearer picture of how global warming-related shifts in temperature and rainfall could fuel acts of aggression.

Though scientists don't know exactly why global warming increases violence, the findings suggest that it's another major fallout of human-made climate change, in addition to rising sea levels and increased heat waves.

"This study shows that the value of reducing [greenhouse gas] emissions is actually higher than we previously thought," said study first author Solomon Hsiang, an economist at Princeton University in New Jersey. (Related: "Global Warming Making People More Aggressive?")

Leveling the Field

To perform their analysis, Hsiang and his colleagues sifted through hundreds of studies published across a number of fields, including climatology, archaeology, economics, political science, and psychology.

"[As economists], we were way out of our comfort zone," Hsiang said. "It's been quite an interesting experience. I've never done anything like this before."

The team eventually settled on 60 studies on subjects related to climate, conflict, temperature, violence, crime, and more, and reanalyzed those studies' data using a common statistical framework. An analogy would be converting currencies from different European countries into the euro so that meaningful comparisons could be made.

They did this to account for the fact that different parts of the world experience different variabilities in temperature and rainfall. For example, an increase of 2°F (1.1°C) might not be a big deal in the United States, where temperatures can vary widely, but it might be unusual for a country in Africa.

When the team converted the data and compared them, the results were striking: They found that even relatively minor departures from normal temperatures or rainfall amounts substantially increased the risk of conflict on a variety of levels, ranging from individual aggression, such as murder and rape, to country-level political instability and international wars.

The study data covered all major regions of the world and different time spans as well, from hours and years to decades and centuries. Across the data, the researchers found similar patterns of human aggression fueled by climate factors.

Examples included spikes in domestic violence in India and Australia, increased assaults and murders in the United States and Tanzania, ethnic violence in Europe and South Asia, land invasions in Brazil, and police using force in the Netherlands.

Ancient Insights

The effect wasn't limited to just modern societies, either. Among the research Hsiang and his team looked at was a study that linked increased political instability and warfare in the ancient Maya civilization around A.D. 900 to prolonged droughts brought about by global warming-related climate shifts in lands near the Pacific Ocean. (Related: "Why the Maya Fell: Climate Change, Conflict—And a Trip to the Beach?")

"That's when the classical period of Mayan civilization ends," said study co-author Edward Miguel, a professor of economics at the University of California, Berkeley.

Another study linked the 14th-century collapse of Cambodia's ancient Khmer civilization, which built the temple of Angkor Wat, to decades of drought interspersed with intense monsoon rains.

"Archaeologists can actually observe how [Khmer] engineers were trying to adapt," Hsiang said. "They were trying to keep up with the climatic changes, but in the end, even though they were the most sophisticated water engineers in the region at the time, it still seemed too much."

Hsiang says his team included these historic case studies in their analysis in order to understand how populations adapted—or didn't—to the kinds of gradual climate changes that climatologists predict for the future. But he thinks there are also lessons to be learned from the past.

"A lot of the civilizations that were nailed by climatic shifts were the most advanced societies in their region or on the planet during their day, and they probably felt they could cope with anything," he said.

"I think we should have some humility [and] recognize that people in the past were very innovative and they were trying to adapt to these changes as well."

Why Does Warming Make People Mad?

Brad Bushman, a professor of communication and psychology at Ohio State University who specializes in human aggression and violence, called the study "impressive."

"The convergence of findings across so many different disciplines increases your confidence that you've got a pretty reliable effect here," said Bushman, who was not involved in the research.

"Hopefully, this study will increase awareness that climate change spans many different domains of human activity, including conflict." (See "6 Ways Climate Change Will Affect You.")

While the new study helps strengthens the case for climate change influencing human aggression, it was not designed to address the question of why it does.

Other scientists have speculated on possible mechanisms. For example, Bushman thinks dramatic changes in temperature and rainfall are unpleasant and naturally make people more cranky. "When people are in a cranky mood, they're more likely to behave aggressively," he said.

Another theory is that too much or too little rain can negatively affect a country's agriculture and lead to economic ruin.

"When individuals have very low income or the economy of the region collapses, that changes people's incentives to take part in various activities," study first author Hsiang said. And "one activity they could take part in is joining a militant group."

The team thinks researchers will eventually discover that multiple mechanisms are at play simultaneously.

Hsiang compared modern scientists studying the relationship between climate and aggression to medical doctors in the 1930s who knew that smoking and lung cancer were linked but had not yet uncovered the mechanism.

"It took decades, but people did eventually figure out what was going on, and that helped us design policies and institutions to help mitigate the harmful effects [of smoking]," Hsiang said.

Similarly, co-author Miguel said, pinning down the mechanisms behind how global warming affects aggression will be the "next key frontier" for this area of research.