Estimating the urban heat-related mortality burden due to greenness: a global modelling study

Background Heat exposure poses a substantial public health threat. Increasing greenness has been suggested as a mitigation strategy due to its cooling effect and potential to modify the heat–mortality association. This study aimed to comprehensively estimate the effects of increased greenness on heat-related deaths. Methods We applied a multistage meta-analytical approach to estimate the potential reduction in global heat-related deaths by increasing greenness in the warm season in 2000–19 in 11 534 urban areas. We used the enhanced vegetation index (EVI) to indicate greenness and a random forest model to predict daily temperatures in counterfactual EVI scenarios. In the factual EVI scenarios, daily mortality and weather variables from 830 locations in 53 countries were extracted from the Multi-Country Multi-City Collaborative Research Network and used to assess heat–mortality associations. These associations were then extrapolated to each urban area under both factual and counterfactual EVI scenarios based on meta-regression models. Findings We estimated that EVI increased by 10% would decrease the global population-weighted warm-season mean temperature by 0·08°C, EVI increased by 20% would decrease temperature by 0·14°C, and EVI increased by 30% would decrease temperature by 0·19°C. In the factual scenario, 3 153 225 (2·48%) of 127 179 341 total deaths could be attributed to heat exposure. The attributable fraction of heat-related deaths (as a fraction of total deaths) in 2000–19 would decrease by 0·67 (95% empirical CI 0·53–0·82) percentage points in the 10% scenario, 0·80 (0·63–0·97) percentage points in the 20% scenario, and 0·91 (0·72–1·10) percentage points in the 30% scenario, compared with the factual scenario. South Europe was modelled to have the largest decrease in attributable fraction of heat-related mortality. Interpretation This modelling study suggests that increased greenness could substantially reduce the heat-related mortality burden. Preserving and expanding greenness might be potential strategies to lower ambient temperature and reduce the health impacts of heat exposure.