Background: While many studies have projected future health impacts under climate change scenarios; limited research has quantified the impact of population aging on these projections. We aim to assess the impact of population aging on future temperature-related mortality under both climate and population changes in a multi-country multi-city setting. Methods: We collected daily data on meteorology and mortality from 729 cities in 42 countries from the MCC Collaborative Research Network. We estimated the city-specific temperature-mortality associations in two-stage time-series analyses using quasi-Poisson regression with distributed lag nonlinear models and multivariate meta-regression. We then obtained age-specific associations and bias-corrected temperatures from five general circulation models to estimate temperature-attributable fractions of mortality in the baseline (2010-2019) and future (2090-2099) under two climate-only scenarios (Representative Concentration Pathway (RCP4.5 and RCP8.5) and two combined climate-population scenarios (RCP4.5-Shared Socioeconomic Pathway (SSP)2 (low aging scenario) and RCP8.5-SSP5 (high aging scenario)), assuming no changes in population vulnerability. We quantified the impact of population aging as the difference in the change in attributable fractions (future minus baseline) between the RCP-specific pairs of climate-only and climate-population scenarios.Results Preliminary results show that population aging will lead to an overall increase of 13.2% (95% CI: 5.0 to18.9) in temperature-related mortality fractions in 2090-2099 compared with 2010-2019 under RCP4.5-SSP2. Under RCP8.5-SSP5, population aging will result in a substantial increase of 21.8% (-36.7 to 45.7) mortality fractions globally. Significant increases in mortality fractions due to population aging are found in most countries in North America, Europe, East Asia, and Australia, whereas positive but imprecise changes are observed in most countries in Central and South America, South Africa, Middle-East Asia, and South-East Asia. Conclusions: Our preliminary findings suggest that the progressive aging of the population will constitute an important driver for a larger impact of non-optimal temperatures under a warmer future climate.