In a warmer world it might be best not to leave your windows open. As temperatures rise, so do crime rates, suggesting climate change will lead to millions of extra offences in the coming decades. However, factors such as better policing may keep a lid on the problem.
The link between temperature and crime has been researched for years, and criminologists agree that warm days see more offences. Nobody is sure why. Snow and closed windows in colder weather may deter some crimes, while warmer weather may increase social interaction and thus the likelihood of some offences. Hotter days may also affect people's physiology, making them more aggressive.
In theory, climate change should make this worse. To find out how much, economist Matthew Ranson of social policy think tank Abt Associates in Cambridge, Massachusetts, studied monthly figures from the FBI's Uniform Crime Reports database, which collates crimes recorded by police for nearly 3000 US counties between 1980 and 2009. He combined them with daily weather information to figure out how the rates of crimes vary with the maximum daily temperature.
Ranson then used a mathematical model to predict how climate change would affect crime rates in each county, using his crime-temperature relationships and the average daily maximum temperature changes predicted by 15 climate models. He used a modest climate scenario in which the world warms by 2.8 °C by 2100.
The model predicts relatively small increases in crime rates, of 0.5 to 3.1 per cent depending on the type of crime. But that amounts to a lot of extra crimes. Between 2010 and 2099, his model predicts climate change will lead to an extra 22,000 murders, 180,000 rapes, 1.2 million aggravated assaults, 2.3 million simple assaults, 260,000 robberies, 1.3 million burglaries, 2.2 million cases of larceny and 580,000 vehicle thefts – and that's just in the US.
"It is a step forward in understanding the relationship between weather and various social phenomena, in this case crime," says Neil Adger of the University of Exeter, UK. He says Ranson's daily weather data allows a fine-grained analysis of social impacts.
However, the relationship between temperature and crime is not entirely settled, says John Simister of Manchester Metropolitan University in the UK, who has studied the link between thermal stress and violence. He favours a linear relationship, but says that some researchers think offending rates will fall once temperatures rise above a particular threshold, perhaps because it is simply too hot to go out. If that's true, the effects of climate change will be less severe than Ranson's model predicts.
The 21st century may not see the increases in crime that Ranson's data predict, says criminologist Ellen G. Cohn of Florida International University in Miami. Other factors, such as better policing or better rehabilitation, might mean the overall crime rate still falls.
Ranson agrees that the thermal effect on crime is likely to be masked. "There are many other factors that affect crime, ranging from the economy to culture to changes in socio-economic factors," he says. But the iniquitous effects of a warmer world will still be operating in the background. "There has been a strong historical relationship between temperature and crime and that is likely to continue in the future," he says.
"Climate change is going to affect our lives in a variety of ways," says Ranson. "It is going to affect the social fabric of the places where we live."
Journal reference: Journal of Environmental Economics and Management, doi.org/rpj