National News

A Rise in Murder? Let’s Talk About the Weather

Posted September 21, 2018 7:48 a.m. EDT

The national murder rate reached a modern low in 2014, capping a quarter-century decline. Then it rose across the United States in 2015 and 2016. Why gun violence increased in those two years remains somewhat of a mystery, and no single factor is likely to explain it, but there is one potential contributor that is rarely talked about: weather.

Temperatures across the country were higher on average in those two years than they were in 2014. The relationship between temperature and crime has been discussed for decades. Fewer people are murdered in America during colder months than during warmer ones.

In the case of 2015 and 2016, it may be a small part of the story, but the broader correlation of weather and crime merits more research. Let’s look at 10 cities that make daily shooting data available and then compare shootings in each city with the day’s high temperature according to the National Oceanic and Atmospheric Administration.

For each of the 10 cities in recent years, it’s possible to calculate the average number of shooting victims per cold day (defined here as under 50 degrees Fahrenheit), per pleasant day (50 to 84) and per hot day (85 and up).

In Philadelphia, for example, there were 2.6 shooting victims per day on average when it was cold, 3.4 on pleasant days, and 4.4 on hot ones.

On average, about twice as many people are shot in Northern cities like Chicago, Milwaukee and Detroit when it’s hot versus when it’s cold (only nonfatal shooting victim data was available for the latter two). In Southern cities like Atlanta and New Orleans, the effect exists but is weaker.

Shootings generally increased in nine of the 10 cities as temperatures rose. San Francisco, the exception, has comparatively very temperate weather, averaging just 2.5 cold days per year and 10.6 hot days between 1990 and 2017.

Why the rise in shootings during warmer weather? Mostly because people are outside more. (There’s also some evidence that hot weather increases irritability and anger, although the question of causality regarding crime is hotly debated). This can be seen by looking closely at gun violence in Philadelphia, a city that provides data on whether a victim was inside or outside. Outdoor gun violence increases in the city as the temperature rises, while there’s virtually no change for indoors.

The rise in murder from 2014 to 2016 was not limited to a handful of cities, and neither were higher average temperatures. Murder was up in 2016 in each of the 20 cities that reported the most murders nationally in 2014 and in 37 of the 50 most populous cities overall.

Nearly all of the 20 cities with the most murders in 2014 experienced fewer cold days and/or more hot days in 2015 and 2016 than they averaged per year over the preceding 25 years. Only Los Angeles and San Antonio had more cold days in 2015 and 2016 than they averaged from 1990 to 2014, and only Chicago and San Antonio had fewer hot days than average.

The effect of warmer weather on rising gun violence in 2015 and 2016 in any individual city might have been small, but all told across the nation, it might have been significant. If you look at national murder data from 2000 to 2016 and compare it with the average annual temperature across the United States per year, you could predict a national rise or fall in murder using just the average annual temperature in 11 of 16 years.

In general, it makes sense not to make too much of fluctuations in crime. Murders are relatively rare, so even minor changes in any number of factors (including randomness) can have a major effect on a city’s murder count. Warmer weather can’t really explain big increases in murder in Chicago or Baltimore in 2015 and 2016 — or in Orlando, where the Pulse nightclub mass shooting took place. Many young people are outside in the summer because school is out, not because of warm weather. And temperatures rose at the same time that murder fell drastically and consistently nationwide after peaking in the first part of the 1990s.

But ignoring the connection seems unwise given another trend: climate change. Researchers have suggested that crime and social disorder could rise along with further temperature increases.

Extreme heat can cut both ways, of course. It’s plausible that if temperatures become too high, people will be more likely to stay inside, particularly if air-conditioning continues to spread, as it has in the last few decades. But it hasn’t necessarily spread evenly to all communities.

According to Jennifer Doleac, an associate professor of economics at Texas A&M who studies gun violence, “to the extent that there are disparities in which communities are able to escape the heat, that’s going to have disproportionate impacts on low-income communities.”

She pointed to a recent paper by Joshua Goodman at Harvard that examined the relationship between heat, air-conditioning and student test scores. She said: “You can imagine a similar story about increasing violence in communities where there is less reliable air-conditioning. That’s another way in which disadvantaged communities may become even more disadvantaged.”

The FBI will release its Uniform Crime Report data for 2017 on Monday, and indications are that murder will have fallen slightly. This would match a slight drop in the national average annual temperature last year, although it’s not clear what role, if any, weather might have played.

Gun violence in Chicago has also fallen over the last year and a half, with declines for 16 straight months. Murder is down 20 percent so far this year in the city. But the weekend of violence Aug. 3 through 5 — when at least 74 people were shot and 12 were killed — was a reminder of how grim it can get in Chicago during the summer. That suggests the need for more research and better data on shootings in U.S. cities to understand how weather affects gun violence.