In Response to “Temperature and Violent Crime in Dallas, Texas: Relationships and Implications of Climate Change”

To the editor:

We were interested to read Gamble and Hess’s study finding that the daily incidence of violent crime in Dallas increased with temperatures up to 90°F (32.2°C), but decreased above this threshold. On this basis, their abstract surprisingly concludes that “higher ambient temperatures expected with climate change…. are not likely to be accompanied by markedly higher rates of violent crime” (p .239). This conclusion contrasts with the findings of previous studies.^1–3

Unfortunately, the authors did not attempt to actually estimate the magnitude of future warming that would be sufficient to bring about a decrease in aggregate annual violent crime, which will differ from the inflection point of the relationship between daily temperature and violent crime. We therefore used the piecewise regression model reported by the authors in order to investigate how annual aggravated assault incidence in Dallas is likely to be affected by changes in mean temperature. We focus on aggravated assault given that this was the crime for which a marked effect of temperature was reported. Temperature data for Dallas International Airport in 1999 was collected from the NCDC⁴. 1999 was used as a reference point being the last year in the series investigated by Gamble and Hess.

The simulation was conducted as follows. For each of a range of hypothetical annual temperature anomalies from −5 to +20°F, the annual anomaly was added to the actual mean temperature in each day of 1999 to obtain an annual series of daily temperatures. The piecewise regression model was then used to obtain the predicted number of assaults for each day of the series. These were then summed over the course of the year.

Our simulation suggests that the mean temperature in Dallas would have to increase by around 13°F (7.2°C) before subsequent temperature increases would begin to reduce annual aggravated assaults. At this point, the model predicts an extra 146 annual aggravated assaults per 100,000 population in comparison to a world with zero warming. Before this point, temperature increases would continue to increase assaults. Notably, a temperature increase of 13°F would be substantially greater than the warming likely by the end of the 21^st century on the basis of regional climate projections for central North America.⁵ As such, the inflection point in the temperature-violence relation appears to occur at too high a temperature to be of much comfort for those concerned with the implications of climate change for human violence in the medium term.

However, it is important to note that this analysis provides only conditional predictions about how many extra assaults are likely to arise in Dallas given a particular magnitude of warming, in comparison to an identical Dallas without this warming. The world of the future will be different from today’s in many ways other than simply being warmer. An unconditional forecast of future violent crime rates would need to take into account multiple predictors of crime, as well as temporal trends unrelated to global warming—such as the decreasing trend in violent crime in Dallas over the last two decades.⁶