While New York City works to protect itself against rising sea levels and storm surges and California withers under devastating drought, another threat may be imminent in a warmer, more chaotic world: increased crime.
The link may seem tenuous, but the idea is nothing new. A paper published in The Quarterly Journal of Economics in 1917 drew a link between climate change, agriculture, and the fall of Rome. And years of intense drought have been implicated in the collapse of the once-mighty Mayan civilization.
"We have more and more evidence that climate affects all sorts of things that we know are related to conflict," Marshall Burke, an assistant professor of environmental Earth system science at Stanford, told VICE News.
In a review of 55 studies across archeology, economics, and other disciplines, Burke, along with two colleagues, found a clear causal relationship between climatic factors, such as temperature, and human conflict.
'It gets hot and people just sort of freak out.'
Their analysis, published in the National Bureau of Economic Research, found that hot days can be dangerous. But it's not simply a matter of setting the thermostat to 90 degrees. It's the increase in temperature that's important.
"We're not comparing San Francisco to Nigeria," Burke told VICE News. "We're just comparing Nigeria in a normal year to Nigeria in a hot year."
The effect is more significant on group-level crimes, like riots and civil wars, than on individual crimes such as road rage or rape. For every unit of deviation from the norm, the analysis found, individual conflict increases by two percent. For group-level conflict, that increase jumps to 11 percent, a number that can translate to real damage. In tropical Africa, where temperatures tend to stay relatively consistent, a one degree Celsius (1.8 degree Fahrenheit) increase in temperature makes for 20 percent more group crime.
What the study doesn't tease out is why climate change leads to conflict. Multiple factors are likely the trigger for violent behavior, said Burke. In laboratory studies, heat has been found to have a detrimental effect on the human psyche, a phenomenon that plays out in real-world scenarios.
One study found that when police training simulations were conducted under hotter conditions, the officers were more likely to draw and fire their weapons. A 2014 study that looked at 30 years' worth of data from nearly 3,000 U.S. counties found that increasing temperature had a "remarkably linear" effect on violent crimes, Burke and his co-authors authors write. That study predicted 22,000 murders, 180,000 rapes and myriad other climate-induced crimes between now and the end of the century.
"It gets hot and people just sort of freak out," Burke told VICE News. "They're more aggressive, they're worse at making decisions."
But a human aversion to abnormally hot days doesn't explain the whole story. Conflict rates were also affected by rainfall, but only in areas that are economically reliant upon agricultural crops, which can be destroyed by unusually heavy or sparse rains. The only study in the analysis to examine weather disasters found that more thefts occur in the typhoon-prone Philippines in years following high winds, and that higher temperatures led to increased murders and other crimes.
That study, though, found that theft rates actually decreased in years following heavy rains, a point that belies the complexity of linking climate and conflict.
Idean Salehyan, an associate professor of political science at University of North Texas, calls a view that climate change is necessarily responsible for an increase in conflict "simplistic." Salehyan's research on the effects of water scarcity has found that it can increase some types of crime while decreasing others, including civil war and insurgency.
"I think they've shown that temperature has a more robust relationship to conflict, but what is it about temperature that should matter?" Salehyan told VICE News. "Why do people fight over scarce resources? Where do they fight? All of that is ignored."
Burke doesn't dispute that his team's analysis doesn't explain the mechanisms behind climate-caused crime. He likens the results of their analysis to wet roads and car accidents: A crash is ultimately caused by a driver's error or mechanical failure, but slick streets increase the odds of something going awry.
Nuance is necessary for shaping policies that could alleviate the pressures that ultimately lead to conflict, Salehyan said, such as increasing access to irrigation and refrigeration in places that need it. Otherwise, a more tumultuous climate could mean a more violent and aggressive world.
"If we don't adapt," Burke told VICE News, "then our results suggest that we should see more of these different types of conflict around the world."
Follow Laura Dattaro on Twitter: _[@ldattaro_](https://twitter.com/ldattaro)