Higher temperatures fuel gun violence. Now we know how much is from climate change.

This story was originally published by The Trace, a nonprofit newsroom covering gun violence in America.​ This year, Philadelphia is on track to have the lowest number of homicides since 1963. But over Fourth of July weekend, a heat dome stretching from Maine to the Gulf of Mexico descended, leaving 165 million people facing major or extreme heat. Cities including Boston, Baltimore, and Raleigh saw record temperatures, and readings in Philadelphia topped 100 degrees for three consecutive days. The sweltering city also saw a busy weekend for gun violence. Between July 2 and July 4, the Gun Violence Archive recorded nine shootings, leaving three dead and nine wounded. “Those who commit gun violence are in a vulnerable population that have limited economic ability to make sure that they have air conditioning,” said Marion Campbell, executive director of Eddie’s House, a Philadelphia nonprofit that provides violence prevention and social services to such populations. “When you get hot you get irritated, OK! I’m just going to keep it real. You get irritated and your inability to manage your frustration could be heightened.” Research confirms what Campbell and others see on the ground: When temperatures rise, gun violence increases. What was previously unknown was the effect of climate change on gun violence. A new finding shared with The Trace by Climate Central, a nonprofit focused on climate change research and communication, estimates that approximately 2,000 shootings between 2021 and 2025 in the 100 U.S. cities with the most gun violence are attributable to high temperatures made more likely by climate change. In fact, the baking July Fourth weekend in Philadelphia was at least twice as likely to occur due to climate change, according to Climate Central. And this is only the beginning. As temperatures continue to rise, even more upward pressure on gun violence is likely. “Across all of these cities, we see that in aggregate, climate change is raising the risk of gun violence,” said Kristina Dahl, vice president for science at Climate Central. “Short of really swift and deep cuts to heat-trapping emissions globally, we expect the planet to continue to warm,” Dahl said. The theory that increased temperatures correspond with increased violence is well established, said John Roman, a distinguished senior fellow at the University of Chicago’s NORC social research center, whom The Trace asked to review the Climate Central analysis. It can be confusing for observers, because this summer has seen record high temperatures but record low gun violence in many cities. But that doesn’t mean that climate change isn’t having an effect, Roman said. “I think if we still had the same climate we had in 1980, I think we’d see even lower violence than we see today,” he said. Understanding how Until now, researchers haven’t pinned a number on the effect of climate change on violence. “We’re talking about close to 2000 shootings that we can attribute to climate change’s influence on high temperatures,” Dahl said. “So, that’s still a lot of people who are being exposed to this kind of traumatic event because of climate change.” No individual shootings were attributed to climate change. There are multiple stressors and contributing factors, and shooters themselves would never say they committed violence because it’s unusually hot outside. Instead, Climate Central researchers looked at the aggregate relationships between climate change, temperature, and gun violence for each of the 100 cities studied. They started by identifying the relationship between outside temperature and gun violence. This is complicated, given a 95-degree summer day is sweltering in the humid northeast, but barely noteworthy in Phoenix. So each city got its own gun violence model, an equation based on historical values that gives back the expected number of shootings for any given temperature. Read Next Next they applied the model to find the number of shootings that would have occurred if cities had seen cooler temperatures without climate change. To simulate that cooler world, researchers used 22 climate models from leading scientists around the globe, combined with real-world temperature records, to calculate hypothetical daily temperatures for each of the 100 cities. Comparing these temperatures to actual recorded temperatures returned estimates that climate change has made the typical day in those cities about 2.4 degrees Fahrenheit hotter, ranging from about 1.7 F in San Francisco and Miami to 3.8 F in Fresno, California. Once researchers know the gap, they can use a city’s temperature model to estimate the number of shooting incidents associated with it. Added up over five years and 100 cities, the difference comes to about 1,950 shootings, roughly 1.7 percent of the 116,281 total shootings recorded in the Gun Violence Archive for the cities during that time period. In Philadelphia, the gap was about 2.7 degrees F, associated with an estimated 149 additional shootings from 2021 to 2025. “Estimated” is key. The simulated temperatures are estimates, as are the number of shootings associated with them. And climate is just one of many facets that affect the rate of shootings in a city. “There are a lot of factors that go into a city’s gun violence incident rate. Temperature is just one,” Dahl said. However, “we can actually parse out the signal of climate change in this data,” she said. In other words, 149 additional shootings in Philadelphia is an estimate, but the direction is clear: Climate change is pushing up the number of shootings. Who’s most vulnerable In any given city, not all neighborhoods are equally vulnerable to the effects of increased temperatures. Instead, the burden of climate change’s effects will likely fall on neighborhoods where gun violence is already high. The leafy green New York City where the movie characters Harry and Sally strolled is real. But just three miles away from the Upper West Side of Manhattan lies the Bronx’s Mott Haven neighborhood, a place that sees more than its share of gun violence. The median income is about $100,000 less, and there is 40 percent less green space, according to city health department data. Pavement traps and concentrates heat, raising the surface temperature about 5 degrees on the average day — a gap that can literally mean life or death. Neighborhoods like Mott Haven are known as urban heat islands, and nationally, they disproportionately affect residents who are not white. Mott Haven itself is 97 percent Black and Latino. Read Next At a recent community meeting, residents gathered to organize around the stifling effects of heat. About 84 percent of households in the neighborhood have air-conditioning units, leaving 16 percent without access, the health department data shows. But even for those with air conditioners, the cost can be prohibitive. One mother brought a monthly electric bill of $1,000. “You can’t run your AC all day,” said Soledad Alvarado, a longtime Mott Haven resident who is a climate justice fellow at the Waterfront Alliance. Instead, residents use it just to survive the hottest hours. When residents leave home, they find little relief. Major bus routes are on streets with little shade. Underground subway stations are essentially heat traps and can be up to 18 degrees hotter than the street-level temperature. Playgrounds — covered in asphalt, not grass — become unbearable. For workers who work outside, heat stress accumulates all day. Currently the city has no requirement that work stop, no matter how extreme the temperature is. These conditions are not an accident, but rather reflect the historical realities of how cities decided to invest — or not invest — in neighborhoods, said Dahl of Climate Central. “We know that the hottest parts of cities tend to be places where there hasn’t been an investment in the sort of natural environments that would help to keep the environment cooler,” Dahl said. “Trees and parks that can really create a lot of shade and help cool the city down are less likely to be present.” “This is one example among many, many, many,” she said, “in which climate change falls hardest on the people who have the fewest resources to cope with it.” When this heat burden falls on disadvantaged communities, it can be more likely to translate into increased violence, said Kimberly Meidenbauer, an assistant psychology professor at Washington State University who leads an environmental neuroscience lab there. Financial strain, resource scarcity, discrimination, and exposure to violence put the body in a stressed state known as allostatic load. When the stressors are chronic, she said, it becomes difficult for the human body to calm back down. “In the context of heat, we’re talking about an additional stressor. Well, if your body’s already dysregulated and under a lot of stress it’s just probably not going to be able to handle an additional stressor quite in the same way,” she said. Science of why Why would heat lead to shootings, when most people have been miserably hot without harming anyone? Scientists have two main explanations, and they aren’t mutually exclusive. The first has to do with human physiology. It begins when heat throws our bodies into disequilibrium, Meidenbauer said. The hypothalamus region of the brain detects when the body is too hot and starts trying to move heat from the core of the body to the periphery by initiating sweating, and expanding veins and vessels near the surface of the skin. “Thermoregulation is pretty effective at keeping our bodies in a safe range, but it is also uncomfortable,” she said. Read Next That discomfort, in turn, ushers in negative emotional states like irritability, hostility, and frustration, while simultaneously impairing parts of the brain that normally help us keep our emotions in check. Research is also starting to point to the possibility that our brains aren’t good at placing blame for stress where it belongs, she said. Trapped in a hot subway car, a person might blame their stress on a crying baby instead of the heat. What’s worse, this state also seems to change how a person understands the emotional states of others. “When you’re hot, you actually perceive others as being more hostile and aggressive, so it actually alters your perception of the intentions and threat of other people, which we think also might be driving, in part, what we see here,” Meidenbauer said. Scientists are still working on showing definitively that heat as a stimulus causes aggression toward others, she said. In the Washington State University lab, they put study participants in an infrared sauna and asked them to play a video game against a (non-existent) player in a different room that allowed stealing from opponents as part of game play. Warmer temperatures were associated with the “impulsive aggression” of stealing, she said. The other possible cause, known as routine activities theory, originated in the 1970s and suggests that violence increases with temperature because people are outside more and thus more likely to encounter potential sources of conflict. An example of this is the ubiquity of shootings at high school sporting events. These shootings are almost never planned, they occur because games bring together a large number of people, some of whom are carrying guns. Roman said he sees evidence for this behavior in the data itself. Gun violence starts going up at relatively moderate temperatures, and keeps increasing — up to a point. Then at extremely high temperatures, there is a signal that gun violence goes down again, he said. “If it’s 95, I don’t care how mad you are at somebody else, you’re gonna wait until it’s not 95 to go out and do something about it,” Roman said. When it’s really hot, he said, “people go inside.” This also points to an unexpected way climate change could change gun violence patterns, he said, because it won’t just increase very hot days, it will also increase the number of days per year that clear the minimum temperature threshold. “The more days you have that are warm enough to be outside, the more people are exposed to violence, and so now it isn’t that July is super hot and people are madder or shorter fuse. It’s that April and March and October and November are much nicer to be out and about,” he said. Stereotypes Back in Mott Haven, community members said they worry that connecting temperature and gun violence could stir up old stereotypes about heat and its effect on Black and brown people. “People who are not from here, they might see a cluster of people outside in the Bronx and think that something violent is going to happen,” she said, but she wants people to know that the time-honored tradition of hanging out outside in warm weather can be both peaceful and joyful. This summer’s outdoor celebrations around the World Cup and the Knicks’ victory parade showed people of all races and backgrounds could be outdoors, together, in the city, peacefully, she said Indeed, climate, race, and violence have been bogusly linked throughout history. Read Next Climatic determinism, the idea that hot climates produce hot-tempered, less civilized people, was used for centuries to justify slavery and colonialism. In 1967, civil unrest occurring in 128 cities in protest of racial discrimination, police mistreatment, and economic oppression, was given the minimizing moniker of “long hot summer.” Before that summer, the FBI warned that racial unrest had become “a summer phenomenon” driven by hot weather, and it pointed to Communist agitation. “Temperature is only one of many factors that go into why someone might pull out a gun on a hot day,” Dahl said. “There’s a whole constellation of supports and services and policies that could help to alleviate some of the other stressors people are facing that might lead to these kinds of decisions.” Solutions Many cities give residents access to cool spaces, like libraries, swimming pools, and community centers so their bodies can at least take breaks from extreme heat. New York City operates hundreds of these heat shelters during heat emergencies. The mayor’s office also recently announced vans that can travel to older residents’ homes and pop-up cooling centers that can be set up at job sites to give relief to outdoor workers. Still, resources are limited. The city’s first-come, first-served program that gives air conditioners to the most vulnerable households, people over 60, young kids, and people with heat-sensitive conditions, stopped taking applications before summer even started, earlier than any previous year. And the program does nothing to help families pay for running the units once they have them. Another idea might be to warn people about what to expect, Meidenbauer said. “So much of our public messaging right now is about ‘on hot days, stay cool and watch out for signs of heat exhaustion and heat illness,’” she said. “We also know that heat affects us psychologically. So telling people to look out for changes in how they’re feeling, whether they’re thinking clearly, and disrupted sleep due to nighttime heat.” Longer-term solutions involve addressing the underlying causes of heat islands. Back in Philadelphia, a 2018 study found that removing trash and adding grass to randomly selected vacant lots was associated with about a 29 percent reduction in nearby gun violence in neighborhoods below the poverty line. Now, the city is undertaking a massive greening project called Tree Plan, which is city-funded but run by the Pennsylvania Horticultural Society. The goal: 30 percent tree canopy citywide. Some neighborhoods have as little as 2 percent tree cover and are up to 22 degrees hotter than leafier areas. Such efforts are needed because models predict warming will only get worse. “We know when it gets hot outside people get angrier, and when people get angrier and there’s guns present, it’s more likely to end up violent,” said Adam Garber, executive director of CeaseFirePA, a nonprofit advocacy group. And, he said, “We know it’s getting hotter every year because of climate change.” Climate Central and The Trace collaborated on this story and the analysis behind it. As experts on climate science and gun violence, respectively, we each wanted to explore how climate change is affecting violence, and we knew that neither of our organizations could do so without the input and expertise of the other. Over the course of the project, we have met regularly to define and refine the analytical approach, identify places to dig into with on-the-ground reporting, and review results.