Future age-specific exposure to heavy rainfall disasters under climate and demographic change

Although previous studies assessed the risk of heavy rainfall disasters under future climatic and demographic changes, comprehensive research on multiple rainfall-induced hazards remains limited. Knowledge on age-specific differences in exposed populations remains limited in mountainous countries, where floods and slope failures intensify, as well as in regions experiencing population decline and demographic aging. In this study, we used the Japanese version of the shared socioeconomic pathways (JSSPs) to evaluate future changes in populations exposed to fluvial flooding, pluvial flooding, and slope failure under combined climate and demographic changes. At the national scale, the exposed population change rate in 2050 and 2100 was consistently highest under JSSP5-8.5 and lowest under JSSP3-7.0. The population aged 65 years and older exhibited higher rates of increase in exposure than the other age groups, particularly in fluvial and pluvial flood exposure. Given the low likelihood of voluntary relocation among older adults, these findings highlight the need to implement non-relocation-based structural countermeasures. These findings deepen our understanding of future heavy rainfall disaster risks under population decline and can inform the development of effective and sustainable disaster risk reduction strategies.