Population Exposure Projections to Intensified Summer Heat

Population aging, an increasing share of the elderly to the total population, can exacerbate heat vulnerability to intensified heat hazards. However, the aging impacts on increases in exposure of the elderly to unprecedented hot summers (UHSs) could be variable in the warming climate. Here, we present considerable contribution of age structure changes to increases in the elders' exposure to intensified UHSs in countries with a high percentage of youth population, namely, China, India, and Brazil, by analyzing combined scenarios of climate change with population structure. In the aforementioned regions, increases in exposure to stronger UHSs relevant to rapid aging will exceed at least half of the warming impact, which primarily causes frequent occurrence of stronger UHSs. Conversely, the United States and Europe, which have already entered an aged society, show a negligible aging impact. These results suggest that the future evolution of a society's age structure is an important constraint for projecting population exposure to strengthened summer heat. Plain Language Summary Population aging can worsen social heat tolerance for unprecedented hot summers (UHSs), which will get stronger by future warming. We show that aging will induce considerable increases in the elder's exposure to stronger UHSs in countries with a high percentage of youth population, such as China, India, and Brazil. In these regions, the growth of elderly exposed to stronger UHSs due to rapid aging will exceed at least half of the warming-induced increase. In contrast, the impacts of aging is negligible in already aged societies, namely, United States and Europe. Future policy to reduce heat vulnerability of society to intensified summer heat should consider the future evolution of a society's age structure, especially in countries with younger populations.