Fraction-dependent variations in cooling efficiency of urban trees across global cities.

• The CE-TCP relationship is nonlinear globally, with CE exhibiting an initial sharp decline followed by a gradual reduction as TCP rises. • The nonlinear CE-TCP relationship is more pronounced in tropical and arid climates compared to other regions. • There is a higher CE and cooling potential of mitigating urban population heat exposure in areas with fewer trees. Investigating the relationship between cooling efficiency (CE) and tree cover percentage (TCP) is critical for planning of green space within cities. However, the spatiotemporal complexities of the intra-city CE-TCP relationship worldwide with distinct climates, as well as the differing impacts of consistently increasing tree cover within urban regions on cooling potential, remain unclear. Here we used satellite-derived MODIS observations to investigate the CE-TCP relationship across 440 global cities during summertime from 2018 to 2020. We further investigated the impacts of enhancing tree cover by a consistent amount in different urban locales on the reduction of population heat exposure among specific age groups. Our results demonstrate a nonlinear CE-TCP relationship globally – CE exhibits an initial sharp decline followed by a gradual reduction as TCP rises, and this nonlinearity is more pronounced in tropical and arid climates than in other climate zones. We observe that 91.4% of cities experience a greater reduction in population heat exposure when introducing the same amount of TCP in areas with fewer trees than in those with denser canopies; and heat exposure mitigation is more prominent for laborers than for vulnerable groups. These insights are critical for developing strategies to minimize urban heat-related health risks. [ABSTRACT FROM AUTHOR]