Climate change significantly impacts human health, with heat-related illnesses posing the most immediate threat. The world's temperature continues to rise, exceeding pre-industrial levels. Studies have shown a substantial contribution of human-induced climate change to warm-season heat-related mortality. Cold temperatures also contribute significantly to mortality. While projections indicate increasing heat-related deaths and decreasing cold-related deaths with warming, these studies typically overlook the impact of population aging. The global population aged 65 and over is projected to increase substantially, and older adults are highly vulnerable to temperature extremes due to compromised thermoregulation, chronic conditions, and social isolation. This study aims to quantify the global impact of population aging on future temperature-related mortality at various levels of global warming, considering both heat and cold extremes. This is crucial for developing effective public health strategies to mitigate the combined threats of climate change and an aging population. The importance of this research lies in providing a more comprehensive and accurate assessment of future health risks associated with climate change, informing adaptation and mitigation strategies.

Existing research demonstrates a strong link between temperature extremes and mortality, with cold-related deaths currently outnumbering heat-related deaths globally. Projections consistently show a rise in heat-related mortality with global warming, while cold-related mortality is projected to decrease in many locations. However, most studies have not explicitly considered the effect of the aging population. Several regional and local studies have suggested that population aging may significantly amplify the impact of temperature extremes on mortality. A review of existing literature revealed limited global-scale analyses using standardized approaches and a lack of explicit estimates for the contribution of aging in future climate scenarios. This study addresses these gaps by employing a comprehensive methodology and a global dataset to quantify this effect.

This study utilized data from the Multi-country Multi-city (MCC) Collaborative Research Network, comprising daily meteorological and mortality data from 800 locations across 50 countries. A two-stage time-series analysis was employed. The first stage involved using quasi-Poisson regression with distributed lag nonlinear models (DLNMs) to estimate location-age-specific temperature-mortality associations. The second stage utilized multivariate random meta-regression to pool city-specific estimates, accounting for heterogeneity. Future temperature projections were obtained from 18 general circulation models (GCMs) from CMIP6, bias-corrected and downscaled for each location. Population aging was estimated using SSP8.5 and SSP3-7.0 scenarios from the SSP Database, projecting changes in the percentage of the population aged 65 and older. Two scenarios were considered: a 'climate-only' scenario assuming constant population demographics from 2015, and a 'climate-population' scenario incorporating projected population changes. The impact of population aging was quantified as the difference in temperature-related mortality fractions between these scenarios. Empirical confidence intervals were calculated to account for uncertainty in both temperature-mortality relationships and GCM variability.

The study projected increases in heat-related mortality fractions of 0.5%, 1.0%, and 2.5% at 1.5°C, 2°C, and 3°C of global warming, respectively, under the climate-population scenario. A substantial portion (20-25%) of these increases was attributable to population aging. While climate change alone predicted a decrease in cold-related mortality, population aging largely offset this, resulting in a net increase of 0.1%-0.4% at 1.5-3°C of warming. Overall, population aging significantly amplifies both heat and cold-related mortality burdens. The largest increases in combined temperature-related mortality were observed in Vietnam, Kuwait, French Guiana, Philippines, and Martinique. Decreases were found in Colombia, Moldova, Estonia, and Japan. The analysis revealed that population aging plays a dominant role in future cold-related mortality, while its contribution to heat-related mortality is smaller, yet still significant. Sensitivity analyses using the SSP3-7.0 scenario yielded similar results.

The findings highlight the crucial role of population aging in shaping future temperature-related mortality, emphasizing the need to integrate demographic projections into climate change impact assessments. The consistent increase in overall temperature-related mortality across all warming levels, even with decreases in cold-related mortality from climate change alone, underscores the significant influence of population aging. This study contributes to the understanding of the difference in projected health risks between 1.5°C and 2°C of global warming, a key knowledge gap identified in previous IPCC reports. The results emphasize the inadequacy of current climate action to limit warming to 1.5°C and stress the necessity for ambitious mitigation efforts and targeted adaptation strategies. The amplified impact of population aging on mortality underestimates the overall burden of temperature-related mortality and necessitates addressing this factor in future research and policy decisions.

This study provides the first global assessment explicitly accounting for population aging in projections of future temperature-related mortality at different global warming levels. The results clearly demonstrate the significant and consistent contribution of population aging to increasing mortality burdens from both heat and cold, irrespective of the level of global warming. Future research should focus on refining adaptation strategies to account for population vulnerability, explore subnational population aging projections, and fill existing data gaps in regions like South Asia and Africa, to gain a more comprehensive understanding of the impact of these combined challenges on human health.

The study acknowledges several limitations. It did not account for potential population adaptation to heat, primarily to isolate the effects of population aging and climate change. Sub-national population aging projections were unavailable for all countries. The analysis relied on primarily urban locations, potentially affecting the generalizability of findings to rural areas. Data limitations hindered the assessment of temperature-related mortality in some regions of South Asia and Africa, leading to potentially conservative estimates of the impact of population aging. Future international collaborations could address these data gaps.