Atmospheric health burden across the century and the accelerating impact of temperature compared to pollution

Atmospheric conditions significantly impact human health, with exposure to temperature extremes and air pollution posing considerable risks. Increased air pollution and non-optimal temperatures (both high and low) are directly linked to increased mortality. The Global Burden of Diseases (GBD) estimates that these factors cause approximately 6.5 million deaths annually, with about 60% attributable to ambient air pollution, primarily PM2.5. PM2.5's small size allows it to penetrate respiratory barriers and enter the circulatory system, causing cardiovascular disease and other health problems. While the individual impacts of air pollution and temperature on mortality have been extensively studied, simultaneous investigations of their projected combined effects remain limited. Human activities substantially influence both risk factors, with anthropogenic emissions altering atmospheric composition and climate. Projecting the evolution of air quality and surface temperature is crucial for assessing future health burdens. This study leverages Coupled Model Intercomparison Project (CMIP) simulations from Earth System Models (ESMs) to estimate current and future atmospheric health burdens by calculating the global mortality attributable to long-term exposure to both non-optimal temperatures and PM2.5.

Existing research demonstrates a strong link between increased air pollution and mortality. Similarly, studies have established a correlation between non-optimal temperatures and increased mortality rates. The GBD study provides comprehensive estimates of mortality attributable to these risk factors, highlighting the significant global health burden. While studies have individually examined the projected impacts of air pollution and temperature on mortality, integrated analyses considering both factors simultaneously have been scarce, especially at the global scale. Previous research has focused primarily on urban or regional levels. This study addresses this gap by utilizing CMIP simulations to provide a comprehensive global assessment.

This study estimates current and future atmospheric health burdens by calculating global mortality attributable to long-term exposure to non-optimal temperature and PM2.5. The methodology closely follows the GBD approach. Daily-specific mortality data from nine countries, representing a significant portion of the global population and temperature range, informed the cause-specific exposure-response functions. Output data from global numerical simulations within the CMIP6 framework (specifically, models CESM2-WACCM, GFDL-ESM4, and MIROC-ESM2-0) were used. These models provide daily near-surface temperature and monthly PM2.5 data for historical and future periods (1980-2099) under three climate scenarios (SSP1-2.6, SSP2-4.5, and SSP5-8.5). Data were downscaled and bias-corrected using methods like Climate Impacts Consortium's Climate Downscaling (ClimDown) package and Quantile Delta Mapping. Population data were obtained from SEDAC and IIASA, providing gridded population data and projections, along with age distribution. Cause-specific baseline mortality rates (BMR) were downloaded from the Institute for Health Metrics and Evaluation (IHME). Exposure-response functions (ERFs) from GBD2019, for both non-optimal temperature and PM2.5, were employed. The attributable fractions (AF) were derived from relative risks (RRs) established in various studies. The study used the M-BRT tool to generate ERFs for non-optimal temperature, differentiating by climate zone and cause-specific mortality. PM2.5 ERFs were age-specific for various diseases. Mortality estimates were calculated using equations (1) and (4), considering different diseases and age groups. The final results, at a resolution of 0.125 x 0.125 degrees, represent the total number of deaths attributable to non-optimal temperature or PM2.5. Confidence levels of mortality estimates were calculated using confidence intervals provided by GBD2019.

The study projects a substantial increase in global mortality attributable to non-optimal temperature and PM2.5 by the end of the century. Under the SSP2-4.5 scenario (a realistic medium scenario), total attributable mortality is projected to increase from 7.7 million in 2000 to 30.3 million in 2090. Non-optimal temperature accounts for a significant portion (36%) of this total in the SSP2-4.5 scenario. More pessimistic (SSP5-8.5) and optimistic (SSP1-2.6) scenarios yielded higher (44.3 million) and slightly lower (36.9 million) total mortality estimates, respectively. In all scenarios, temperature-related mortality increases more drastically than pollution-related mortality, surpassing the latter in various regions. While the mortality attributable to PM2.5 increases by a factor of approximately 7.5 in the SSP2-4.5 scenario, the increase attributable to temperature is higher (a factor of 6.7). Population growth and aging are identified as key drivers of the increasing mortality burden. In high-income countries, temperature-related mortality already exceeds that of pollution-related mortality today and this disparity is projected to significantly widen in the future. Regions like Central and Eastern Europe, South Asia, and East Asia are projected to experience particularly high mortality rates. The analysis also highlights that the largest increase in warm temperature attributable mortality is in tropical regions, but increases are also present in extratropics.

The findings highlight the accelerating impact of climate change on global mortality, exceeding the projected reduction in mortality from improved air quality. The significant increase in temperature-related mortality underscores the urgent need for stronger climate mitigation policies. While air pollution control measures are crucial, they alone will not suffice to address the escalating health burden from climate change. The study’s results are consistent with previous GBD estimates for the period 2000-2019, lending credence to the projections. The analysis of contributing factors reveals that population growth and aging are significant drivers of the increase in mortality across the century, suggesting potential for mitigation strategies targeting demographic shifts. The substantial regional variations in projected mortality highlight the need for context-specific adaptation and mitigation strategies.

This study projects a dramatic increase in global mortality attributable to temperature and air pollution by the end of the century, with temperature-related deaths significantly outpacing those from air pollution. These findings emphasize the urgent need for strong climate policies to prevent catastrophic future health consequences. Future research should focus on refining exposure-response functions, exploring regional variations in vulnerability, and investigating the potential effectiveness of combined mitigation and adaptation strategies.

The study's reliance on model projections carries inherent uncertainties related to climate and socioeconomic scenario assumptions. The accuracy of the mortality estimates depends on the accuracy of the underlying datasets used for population, mortality rates, and exposure-response functions. Regional variations in data availability and quality could also influence the precision of the projections. Further research is needed to refine the methodologies and integrate additional factors that might influence the complex interplay between climate change, air pollution, and human health.