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Climate change may mean more crime

A new report predicts another human cost to hotter weather

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As the global climate changes around us, the potential effects tend to be discussed in familiar environmental terms: warmer average temperatures, changing rainfall patterns, rising seas.

But it can be difficult to imagine how exactly this altered climate will affect day-to-day life—how it might change the social fabric of, say, a relatively prosperous American city like Boston.

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Increasingly, researchers are trying to draw out those scenarios, including ones that could come from a rise of just a few degrees in temperature. And a new study by an economist in Cambridge suggests that, even without utterly upending the world as we know it, projected climate change could have very concrete social effects indeed.

Matthew Ranson, an environmental economist at Cambridge-based Abt Associates, a public policy research and consulting firm, predicts that over the remainder of this century, rising temperatures in the United States will lead to an additional 22,000 murders, 180,000 cases of rape, and 1.3 million burglaries, among other crimes.

These numbers may sound dramatic, but they’re based on existing crime data and broadly accepted projections for temperature change. Ranson also projects future costs, and estimates that a 5-degree rise in global temperatures over the next century—a middle-of-the-road climate change scenario from the Intergovernmental Panel on Climate Change, or IPCC—will end up costing society $38 billion to $115 billion across the country. He concludes that climate change could have a noticeable impact on how we deploy police and other resources to combat crime.

Though it’s the most comprehensive look at the future effects of climate change on crime specifically, Ranson’s study builds on decades of research on crime and temperature, and links it with new research on how climate change could destabilize societies in ways beyond melting polar ice and rising seas.

“I think that these studies as a whole are suggesting that violence and conflict are part of the larger costs of climate change,” Ranson says.

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It’s a familiar idea to city-dwellers that a long, hot summer can often lead to violence. So what happens if they’re longer and hotter? Ranson’s study, published online Feb. 4 in the Journal of Environmental Economics and Management, examines 30 years of monthly crime and weather data from every county in the United States. It confirms what previous studies have found: Many types of crimes spike in the summer months. But the study goes a step farther to look at how fluctuations from the norm affect crime—in other words, does a heat wave or an unseasonably cold spring day mean that crime rates change?

The answer, for the most part, is yes. “In a particular location and a particular month, if temperatures happen to be unseasonably warm, then crime rates happen to be higher,” he says. And when it’s unseasonably cold, crime rates drop. The picture is slightly different depending on the crime: Nonviolent crimes like burglaries go down when the weather is below 40 or 50 degrees, then remain steady as the temperature climbs above that. In contrast, he says, “for violent crimes, the hotter it is, the more crime.” Murders seem to taper off at temperatures over 100 degrees, while rapes pretty much rise with heat. And people who live in naturally warm climates aren’t immune to the effects of hotter days, he found.

Those hotter days are expected to become more frequent. The IPCC estimates average global air temperatures will rise from 2 to 11.5 degrees in the next century, and warm days and heat spells are very likely to increase. Ranson used 15 separate climate models developed at institutions around the world to predict how weather patterns will vary in future decades in each location. He calculated crime rates under each of these models, in order to capture uncertainty about future climatic conditions. He estimates that between 2010 and 2099, climate change will boost US crime rates by a few percentage points, and would require a 4 percent increase in police forces to compensate. For Suffolk County, Ranson estimates an average cost of these crimes of $600 per person over that time period (twice as high as the predicted cost nationwide, since large cities like Boston have higher crime rates). Ranson says it’s hardly a doomsday scenario, but it represents a substantial cost to society—and, of course, to crime victims in particular.

The study adds to a longstanding association between high temperatures and crime. Many but not all studies have found a similar relationship: For example, a recent study in Dallas found that murder rates climb with temperature, but drop on days above 90 degrees, perhaps because people spend more time inside with air conditioning. Boston Police Department spokesman Sergeant Mike McCarthy confirms that crime rates rise in the city in June, July, and August. “During the summer months the BPD focuses on particular crime hotspots and directs resources to those areas,” he says.

Why does greater heat cause more crime? One explanation is that warmer weather simply makes crime easier, with more potential victims on the streets, more social interactions that could lead to violence, and more windows left open. But psychology research also suggests that heat directly contributes to aggression. In laboratory studies in hot rooms, for instance, people are more likely to interpret social interactions as aggressive and have more hostile thoughts. A 2011 study analyzed decades of baseball statistics and found that on hot days, pitchers were more likely to hit a batter with a ball after one of their own teammates had been hit, a common retaliatory move. Richard Larrick, a psychologist at Duke University who led the study, believes that heat may change the way we interpret ambiguous events. “There’s something about the discomfort of being hot that makes you see hostility more,” he says.

It appears that that discomfort is about to get worse. Solomon Hsiang, a public policy researcher at University of California at Berkeley who has also looked at temperature and violence, says that the Ranson study’s comprehensive view, examining multiple crimes over 50 years for the entire country, helps to address one argument against the heat effect: that we’ll all just adapt. “The new study shows us that heat affects people in hot places as well in cold places,” he says, and its long view suggests that these effects haven’t diminished over time.

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Ranson’s study adds to an intriguing body of research documenting climate change’s impact on all kinds of human conflicts and strife. A paper published in Science last summer by Hsiang and colleagues analyzed studies of modern and historical data from around the world, and found a surprisingly universal relationship between rising temperatures and increasing conflicts or social unrest. The studies they looked at ranged from archaeological studies of long-term climate shifts to laboratory studies in hot rooms, and included conflicts as small as Larrick’s retaliating pitchers and as large as the collapse of civilizations. “What really struck us was that we actually found very similar results everywhere we looked,” Hsiang says. “It is almost like a law about how people respond to their environment.”

Their findings have been controversial; some scientists have questioned the paper’s statistical methods, while others have objected that the research amounts to “environmental determinism,” placing physical conditions ahead of political and social events that underlie violence. Hsiang emphasizes that he and his colleagues aren’t saying that hot weather directly causes violence or outweighs other factors in people’s decisions. But, he says, “the way we make those decisions can be subtly influenced by our environment.”

Cullen Hendrix, a researcher at the University of Denver who has studied the effects of rainfall fluctuations on social conflicts in Africa, says the story isn’t a simple one. “I’m relatively convinced that there are relationships between climatic factors and conflict,” he says. But there are several explanations for why those relationships exist, he says, and until we understand the underlying causes better, “we are not going to be able to design better policy measures.”

What we already know—both from current events like island communities threatened by rising seas, and from historical ones like the strife that arose from the Little Ice Age in the 17th century—is that climate change affects people in myriad ways, whether by altering their local economies, threatening their homes, or directly affecting their psychology. For now, researchers like Ranson hope that broadening the discussion to all the effects of climate change will give people more of a stake in slowing it. “Although increased crime is only a small part of the potential impacts of climate change,” Ranson says, “it’s a compelling example of how climate change could affect our daily lives.” Making rational, informed decisions about our future, he says, “will only be possible if we understand the many different costs and benefits of taking action or doing nothing.”

Courtney Humphries is a freelance writer in Boston.
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