A surprising new link between winter temperatures and crime rates has been revealed by researchers at the University of Colorado, Boulder, suggesting warmer, milder winters could result in higher incidences of violent crimes. The usual trajectory is that crime peaks in summer and usually drops in fall and winter, but as winter temperatures get warmer and warmer, the researchers note some parts of the US could see higher rates of violent crimes like assault and robbery.

“During mild winters, more people are out and about, creating the key ingredient for interpersonal crimes: opportunity,” said lead author Ryan Harp in a statement. 

Working in collaboration with NOAA, the CIRES team analyzed historical climate data against violent crime rates compiled by the FBI from over 16,000 cities since 1979. They then measured the relationship by aggregating cities into regions in order to see bigger-scale fluctuations and offset individual changes in city characteristics, such as policing methods or demographics. Their results are published in Advancing Earth and Space Science: GeoHealth.  

Over the last four decades, the researchers found a “large and statistically significant” relationship between crime and temperature. So strong, they say, that the relationship between crime and increasing winter temperatures in the northeastern part of the country could explain more than half of the changes in annual crime rates – that same relationship nearly diminished in the summer months.

Crime rates vary for all sorts of reasons and the study does note limitations worth mentioning. Because it’s a region-wide study, it did not address pockets of changes from within those regions, and it additionally lacked in “directly accounting for various socioeconomic variables, such as income inequality or age demographics of an agency’s covered population.” Regardless, the authors say this wintertime correlation is surprising.

“It’s highly unusual to find correlations this high in big, messy data sets, especially spanning disciplines like climate and health or sociology. The initial disbelief forced us to recheck our work more than a couple of times,” said researcher Kris Karnauskas.

The authors explain the relationship using the Routine Activities Theory, suggesting interpersonal crime rates are driven by a combination of variables, including a motivated offender, target, and absence of someone who could prevent it. Simply put, nicer weather means more people are out and about versus cooped up inside during winter. They also note Temperature-Aggression Theory – that people act more aggressively in extreme heat – could play a role.

Does that mean human-caused climate change could increase crime? Potentially.

“This study is significant because it broadens our thinking on connections between climate and human health, to encompass a very real and dangerous threat to our bodily safety an, therefore, health," concluded Karnauskas.