Satellite‐based evidence highlights a considerable increase of urban tree cooling benefits from 2000 to 2015

Tree planting is a prevalent strategy to mitigate urban heat. Tree cooling efficiency(TCE), defined as the temperature reduction for a 1% tree cover increase, plays animportant role in urban climate as it regulates the capacity of trees to alter the sur-face energy and water budget. However, the spatial variation and more importantly,temporal heterogeneity of TCE in global cities are not fully explored. Here, we usedLandsat-based tree cover and land surface temperature (LST) to compare TCEs at areference air temperature and tree cover level across 806 global cities and to exploretheir potential drivers with a boosted regression tree (BRT) machine learning model.From the results, we found that TCE is spatially regulated by not only leaf area index(LAI) but climate variables and anthropogenic factors especially city albedo, withouta specific variable dominating the others. However, such spatial difference is attenu-ated by the decrease of TCE with tree cover, most pronounced in midlatitude cities.During the period 2000–2015, more than 90% of analyzed cities showed an increas-ing trend in TCE, which is likely explained by a combined result of the increase in LAI,intensified solar radiation due to decreased aerosol content, increase in urban vaporpressure deficit (VPD) and decrease of city albedo. Concurrently, significant urbanafforestation occurred across many cities showing a global city-scale mean tree coverincrease of 5.3 ± 3.8% from 2000 to 2015. Over the growing season, such increasescombined with an increasing TCE were estimated to on average yield a midday sur-face cooling of 1.5 ± 1.3°C in tree-covered urban areas. These results are offeringnew insights into the use of urban afforestation as an adaptation to global warmingand urban planners may leverage them to provide more cooling benefits if trees areprimarily planted for this purpose.