Projected trends in high-mortality heatwaves under different scenarios of climate, population, and adaptation in 82 US communities

Abstract

Some rare heatwaves have extreme daily mortality impacts; moderate heatwaves have lower daily impacts but occur much more frequently at present and so account for large aggregated impacts. We applied health-based models to project trends in high-mortality heatwaves, including proportion of all heatwaves expected to be high-mortality, using the definition that a high-mortality heatwave increases mortality risk by ≥20 %. We projected these trends in 82 US communities in 2061-2080 under two scenarios of climate change (RCP4.5, RCP8.5), two scenarios of population change (SSP3, SSP5), and three scenarios of community adaptation to heat (none, lagged, on-pace) for large- and medium-ensemble versions of the National Center for Atmospheric Research's Community Earth System Model. More high-mortality heatwaves were expected compared to present under all scenarios except on-pace adaptation, and population exposure was expected to increase under all scenarios. At least seven more high-mortality heatwaves were expected in a twenty-year period in the 82 study communities under RCP8.5 than RCP4.5 when assuming no adaptation. However, high-mortality heatwaves were expected to remain <1 % of all heatwaves and heatwave exposure under all scenarios. Projections were most strongly influenced by the adaptation scenario- going from a scenario of on-pace to lagged adaptation or from lagged to no adaptation more than doubled the projected number of and exposure to high-mortality heatwaves. Based on our results, fewer high-mortality heatwaves are expected when following RCP4.5 versus RCP8.5 and under higher levels of adaptation, but high-mortality heatwaves are expected to remain a very small proportion of total heatwave exposure.

Fig. 1

Study communities. Each dot shows one of 82 study communities. Dot size indicates population at present or for a population scenario. Dot color shows, for projected populations, percent population increase compared to present

Fig. 2

Projected trends in high-mortality heatwaves ([a] frequency and [b] exposure) under different adaptation and climate scenarios. Each line connects three projections for an ensemble member, with points for projections for present-day (left) and for two scenarios of greenhouse gas forcing (RCP4.5 [middle] and RCP8.5 [right]). “Frequency” (a) indicates total number of high-mortality heatwaves projected for all 82 study communities. “Exposure” (b) indicates total expected exposure across all 82 communities in millions of person-days per year. Color corresponds to the adaptation assumption: none (red), lagged (green), or on-pace (blue). All projections are based on the bagging health-based model, and all projections for (a) are based on the SSP3 population projection