For more than a century, scientists have interrogated the link between seasonal weather and crime, consistently finding that warm summer temperatures yield more violence. Higher heat tends to fray nerves and shorten tempers, inspiring aggression, which is why riots tend to happen in the warmer months.

Scientists in Colorado, however, recently explored what happens to crime when winters become warmer. Their study found that milder winters — which are occurring more frequently as a result of climate change — caused regional crime rates to rise over the past several decades. The researchers think crime increases during a warmer-than-normal winter because people don’t hunker down inside to stay warm. Instead, they go out.

“People are out of their homes more,” said Ryan Harp, a doctoral candidate in the University of Colorado Boulder’s department of atmospheric and oceanic sciences, and the study’s lead author. “Basically, more pleasant weather increases the chance that people will go out to restaurants, cafes, parks, whatever it may be, and that a potential perpetrator and potential victim will cross paths without a guardian present.” By “guardian,” he means someone in a position to prevent or stop a crime, such as a police officer.

Winter was particularly warm in 2018. In February, the average temperature in the Midwest was 10 degrees F above normal. Source: NOAA/NWS

Harp stressed, however, that the paper, which appears in the journal GeoHealth, doesn’t look directly at the influences of climate change, but simply how crime totals for each month varied with the temperature for that same month. The direct influence of climate change is on their future agenda.

“Though it’s easy to make a connection between our study and climate change — in fact, taking a look at that is one of our next projects — we only looked at how the year-to-year swings in temperature connected with the year-to-year swings in crime for a particular location,” Harp said. “Both temperature and violent crime go up and down each year, but all that we were interested in was whether or not they do so together.”

Global warming is shifting the temperature distribution, making extreme cold less likely and extreme heat more likely. Source: Environmental Protection Agency

The answer appears to be yes, although it skirts the notion that hot temperatures drive aggressive behavior. Rather, it suggests that weather changes peoples’ daily activities, spurring more crime. Scientists say three factors lead to crime during a mild winter: a motivated offender, a suitable target and no one there to stop it.

Surprisingly, the study found that warmer winter temperatures influence crime more than warmer summer temperatures. “Warmer winters matter more than warmer summers, as a warmer summer matters less to people’s behaviors than a warmer winter,” Harp explained. “For instance, if the temperature is 20 degrees warmer than normal in the winter — for example, 20 degrees to 40 degrees — it’s likely to affect people’s day-to-day activities. But if the same temperature difference happened in the summer — for example, 70 degrees to 90 degrees — it’s unlikely to have as much of an effect.”

The researchers also found that the relationship between crime hikes and warmer winters was strongest in regions that typically have the harshest winter, such as the Midwest and Northeast. “This is likely because these places are more susceptible to changes in temperature than warmer places,” Harp said. “For instance, a 20-degree-above-average winter day means a lot more to New Yorkers than to Texans, since the average winter day in Texas is warmer than it is in New York.”

Source: Pexels

The scientists collected seasonal temperature data from NOAA and crime data from the FBI covering 16,000 cities across five regions — the Northeast, Southeast, South Central, West and Midwest — from 1979 to 2016. They based their analysis on violent and property crime, including homicide, rape, robbery and aggravated assault, for the former, and larceny, burglary and motor vehicle theft, for the latter.

“Once we had the data, we did some quality control on the crime totals for each city and established our long-term baseline for each agency’s data,” Harp said. “Then we could compare the crime that actually happened in a given month against the baseline, or what we would expect to happen.”

Normalized January temperature and violent crime in the Northeast over time. Warmer temperatures are linked with more violent crime. Source: GeoHealth

They then combined all of the data for a given region’s cities. “The region boundaries were established by finding areas whose climate fluctuated together in a similar manner,” he said. “Finally, since both climate and crime vary on a seasonal cycle, we pulled out data for each individual month and compared it against the rest of the data for that month. So, we only compared January data against January data for a specific region, and so on. Then, finally, we analyzed all of our monthly results for every region to get the whole picture.”

“It’s a reach to say that a warmer winter will definitely mean that a single city will see higher than expected crime, since there’s so much randomness in month-to-month crime totals for one particular place,” Harp said. “So even if it is a warmer winter month, the natural fluctuations in crime may overpower any effects of warmer temperature.”

But, “when we take a step back and look at regional totals, that’s when we do see this strong tie between warmer winters and increased crime, and we expect this relationship to be consistent,” he added. “One of the biggest takeaways here might just be that climate impacts our lives in many more ways than we usually discuss, including our health and our safety.”

Marlene Cimons writes for Nexus Media, a syndicated newswire covering climate, energy, policy, art and culture.