On the heels of another year of higher-than-average temperatures and alarming increases in shootings, a new study examines the link between two of the biggest issues threatening American cities: climate change and gun violence. 

The paper, published on December 16 in The Journal of the American Medical Association, found that abnormally high daily temperatures in U.S. cities were associated with higher levels of shootings. The relationship was most significant in the Northeast and Midwest, regions where hotter weather is seasonal, and where cities experience high levels of residential segregation and environmental racism. 

Overall, the study found that nearly 7 percent of all shootings could be attributed to days hotter than a city’s median temperature. In the Northeast and Midwest, nearly 10 percent of all shootings occurred on days with elevated temperatures, compared to regions like the Southeast, where less than 3 percent of shootings were attributed to high heat.

“The analytic technique that we used let us really disentangle if it’s just hotter temperatures in the summer or is today hotter than it should be for this season, and what is the risk of that daily temperature on firearm violence,” said Dr. Vivian H. Lyons, an epidemiologist at the University of Washington who led the study.

To conduct their study, researchers from the University of Washington and Boston University analyzed data from the 100 most populous U.S. cities with the highest number of shootings between 2015 to 2020. They used data from the Gun Violence Archive, which tracks shootings through media reports, and the National American Land Data Assimilation System Phase 2, which tracks daily temperatures. The researchers looked at shooting incidents and not at individual firearm injuries, meaning that multiple people could have been injured in each instance of firearm violence included in the dataset.

Most shootings during the survey period took place in moderate heat, compared to extreme weather, but the hottest days had the highest incidence of gun violence. Shootings that were attributed to heat occurred most frequently during the summer. but there were still heat-attributable shootings that took place on hotter days during non-summer months.

The findings reinforce concerns that climate change has the potential to worsen gun violence, especially among people who live in disadvantaged communities, where environmental factors can contribute to extreme heat. While temperatures are rising across the nation, metropolitan areas — known as urban heat islands — retain heat and are significantly warmer than the suburban and rural areas that surround them. In under-resourced urban communities, with little green space or few trees to provide shade, for example, the effects can be compounded. 

“These bigger structural issues are what is forcing people outside of their homes and getting them in situations where conflict arises in their local neighborhoods,” said Dr. Daniel Semenza, a sociologist at Rutgers who studies gun violence. “This study points out that this is a much bigger issue, and if we’re talking about climate change in a longer term, that there have to be bigger investments in cooling and homes.”

The researchers hope their findings can inform city leaders of the importance of implementing intervention strategies that account for the effect of heat on firearm violence, like deploying violence interrupters on days that are hotter than normal. 

“This study is starting to hint at an alignment between those kinds of interventions that focus on gun violence prevention with very similar interventions intended to address environmental injustices and climate vulnerability,” said Dr. Jonathan Jay, a professor at the Boston University School of Public Health and one of the study’s authors. “Over time, we start to see these as really closely aligned missions that can be mutually reinforcing.”

Semenza added that interventions at the city level can be fairly inexpensive. Increasing green space by planting trees or limiting asphalt has been linked to a decrease in crime and shootings, especially in communities that suffer from disinvestment. A recent study conducted by the University of Pennsylvania showed that relatively low-cost renovations of homes, like installing new windows, led to a 13 percent reduction in gun assaults. 

The new study suggests the need for year-round heat adaptation strategies, especially as the residual effects of climate change become more profound and are distributed less evenly throughout the country.

The researchers said that the regional differences seen in the results could be attributed to a host of reasons, including underlying rates of firearm violence, gun ownership norms, and historical redlining. 

The Reverend Vernon K. Walker, program director of CREW, which prepares communities for climate resiliency in Boston, is all too aware of the dangers of extreme heat. His organization works in neighborhoods like Roxbury and Chinatown, which are both predominantly minority communities, to help residents prepare for the summer heat through workshops and the distribution of energy-efficient air conditioners and cooling kits. 

“The communities that we focus on are communities that have been historically redlined and disinvested,” said Walker. “We focus on getting resources to these communities because they are the most vulnerable.”