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Global urbanization drastically reshapes the land atmosphere interactions, biogeochemical cycles, and ecosystems in the integrated earth system. Urban trees are used extensively for providing desirable ecosystem services, especially to mitigate elevated thermal stress in cities. Urban trees differ markedly from their natural counterparts in the physiology due to their close interactions with anthropogenic stressors. Here we present the cooling capacity of urban trees in response to thermal extremes in cities of the contiguous United States. The cooling capacity is quantified as the surface cooling rate, which is expressed as the negative ratio of land surface temperature (LST) changes to fractional tree cover (FTC) changes, i.e., -Delta LST/Delta FTC. Results show that the surface cooling rate is dominated by plant transpiration up to 1.336 degrees C per percentage of FTC in heat waves; its temperature dependence remarkably resembles the thermodynamic liquid-water-vapor equilibrium. Urban trees also exert pseudo cooling effect in cold waves in contrast to the anthropogenic heat emission. The average surface cooling rate in cold waves is 0.022 degrees C per percentage of FTC, which is much smaller than that in heat waves (0.202 degrees C per percentage of FTC). The enhanced cooling capacity of urban trees will enable their provision of better ecosystem services to the urban environment for projected future increase of extreme heat.