Modeling Decreased Intensity and Mortality of the 2003 European Heatwave with Nature-based Solutions of Evaporative Cooling

Nature-based solutions offer significant potential to mitigate the impacts of urban heatwaves if urban trees and their soils can capture unused stormwater and create evaporative cooling. This study employed the i-Tree Cool Air soil-vegetation-atmosphere transfer model to evaluate the effects of increasing neighborhood tree cover to a minimum of 30% in all neighborhoods of 10 Italian cities during the extreme summer of 2003. The analysis introduced a heatwave degree day (HWDD) metric to quantify reductions in heatwave intensity and duration, which were mapped alongside excess mortality attributed to heatwaves in the baseline scenario. Results reveal that transitioning from the average baseline tree cover of 8.2% to 30% would decrease HWDDs by 32.5%, with reductions varying from 15.8% in Cagliari to 84.1% in Bologna. Correspondingly, excess mortality among adults aged 65 and older would decline by 29.3%, sparing an estimated 574 lives from the 1962 killed by the 2003 heatwaves. The study also highlights spatial variability in mortality reductions, reflecting neighborhood-specific differences in tree cover, developed area, and population density. Enhanced tree cover improved ecosystem services, with a median annual increase in value of $11 million per city, generated by reductions in air pollution (53%) and stormwater runoff (33%), and increases in carbon sequestration (14%). This research underscores the transformative impact of urban greening in mitigating heatwave risks and highlights its utility for informing urban planning policies aimed at climate adaptation and public health.