Tree canopy halves urban heat island effect globally but disproportionately benefits higher incomes and suburbs

Urban trees are proposed as a nature-based solution for urban heat island (UHI) mitigation. However, there has been no comprehensive global urban assessment of their ability to reduce air temperature (AT), with most multi-city estimates using instead land surface temperature. Here, we combine a global 1-km AT dataset with high-resolution land cover information to estimate the tree canopy cooling for 8,919 urban areas, housing 3.6 billion people. We find the tree canopy reduces summer AT by a population-weighted average of 0.17±0.03°C, but with substantial variation (0.0-2.7°C range). Models indicate tree canopy on average reduces wet bulb globe temperature (WBGT) 3.1x more than AT. Approximately 914 (805-1040 95% CI) million people experience a reduction of AT of 0.25°C or greater, with 203 (169-243) million experiencing at least 0.5°C reduction. Cooling mitigates an average of 48.6±1.3% of the UHI that would occur without urban tree canopy. However, cooling benefits are greater in areas where they are less needed: mesic climates, high-income countries, and suburban areas. Moreover, current cooling can only mitigate 10% (6.7-18%) of the median mid-century projected climate change increase. Expanding tree canopy in densely settled low-income urban areas will have the greatest value for local heat mitigation and climate adaptation.