Rising heat wave trends in large US cities

Exposures to dangerously high temperatures are a public health threat expected to increase with global climate change. Heat waves exacerbate the risks associated with heat exposure, and urban residents are particularly vulnerable to threats of heat waves due to the urban heat island effect. To understand how heat waves are changing over time, we examine changes in four heat wave characteristics from 1961 to 2010, frequency, duration, intensity, and timing, in 50 large US cities. Our purpose in measuring these trends is to assess the extent to which urban populations are increasingly exposed to heat-related health hazards resulting from changing trends in extreme heat. We find each of these heat wave characteristics to be rising significantly when measured over a five-decade period, with the annual number of heat waves increasing by 0.6 heat waves per decade for the average US city. Additionally, on average, we find the length of heat waves to be increasing by a fifth of a day, the intensity to be increasing 0.1 A degrees C above local thresholds, and the length of the heat wave season (time between first and last heat wave) to be increasing by 6 days per decade. The regions most at risk due to increasing heat wave trends must plan appropriately to manage this growing threat by enhancing emergency preparedness plans and minimizing the urban heat island effect.