Spatiotemporal trends in human vulnerability to the heat across the United States

While human vulnerability to excessive heat has been well documented, relatively few studies have examined long-term trends in vulnerability to heat events. In this research, we examine temporal trends in mortality associated with heat waves, defined using three different definitions of heat wave, for the largest 51 metropolitan areas of the US, over a 36-year period (1975-2010). Regardless of the definition of heat wave, an overall decline in heat vulnerability is seen over the period. While in the first years of the study, 18 to 26 metropolitan areas showed statistically significant increases in mortality on heat wave days, by the final decade of the study period, this had decreased to 6 to 7. Within this narrative, however, examining individual metropolitan areas shows greater variability within the downward trend. Several contributing factors to the variability were observed, including the occurrence of an extreme heat wave affecting the overall heat wave-mortality relationship, and the frequency of heat events over a given period. These broad decreases in heat vulnerability, while encouraging, should be viewed in a cautionary sense. With society aging, there will be a greater number of highly susceptible individuals in the future; further adaptation gains are difficult in many places as air conditioning is now available in most homes in the US. Further, increased use of air conditioning has been associated with a stronger heat island; which, moving forward, is likely to occur alongside a greater number of heat events.