Projected temperature-related mortality attributable to the urban heat island effect in the Tokyo Metropolitan Area stratified by age and geographical location

The urban heat island (UHI) effect, intensified by climate change, is reshaping the distribution of temperature-related health risks by amplifying heat exposure while offsetting cold-related impacts. However, current assessments of the UHI's future health impacts remain incomplete. Therefore, we estimated future UHI-attributable mortality in the Tokyo Metropolitan Area (TMA) considering the combined effects of climate change and different demographic scenarios through the end of the 21st century. Age-specific temperature-mortality relationships were examined for populations aged <65 and ≥65 years, and UHI impacts were quantified by contrasting urban temperatures with non-urban counterfactuals. Under a high-emission pathway (SSP5-RCP8.5), the UHI effect is projected to increase heat-related mortality from 4,100 (2010) to 11,270 (2090s), while decreasing cold-related deaths from 17,454 (2010) to 12,655 by the 2090s. Population aging also significantly amplified heat-related mortality among older adults (≥65 years), even though younger populations (<65 years) also showed a notable heat-attributable mortality risk. Spatial analyses further revealed stronger declines in avoided cold-related mortality and sharper increases in heat-related mortality owing to the UHI effect in densely populated urban cores. These findings underscore the growing health burden of UHIs under climate change and highlight the need for targeted, age- and location-specific adaptation strategies to protect vulnerable populations.