According to newly published U.S. Geological Survey research conducted in 8 large cities coast to coast, urban trees in hot and dry cities can amplify the cooling of local air temperature. 

The multi-year study was conducted in Baltimore, Los Angeles, Phoenix, Portland, Miami, Tucson, Denver and Las Vegas. Scientists placed 80-100 sensors on trees in each city and measured hourly air temperatures for three months during the summers of 2016-2019. 

“All trees have a cooling effect, but trees in hot, dry areas can have a greater impact than in humid cities,” said Peter Ibsen, USGS research ecologist. “Trees in areas like Las Vegas, Phoenix and Tucson are particularly effective at reducing heat.” 

Tree canopy is important in urban environments as it has implications for city planning, public health, and climate resilience. 

The team found that trees have a more pronounced cooling effect in hotter and drier regions, a contrast to buildings, which tend to have a warming effect in hotter and drier regions. Though in more humid cities like Baltimore, Portland, and Miami, tree cooling is stronger than warming coming from impervious surfaces, which highlights the potential for increasing local air temperature reductions by replacing impenetrable surfaces with tree canopy. 

Trees reduce heat in all studied cities, but their effectiveness was contingent on local water availability. Irrigation is crucial for maintaining trees’ cooling effects in all studied areas.  

Other surfaces had temperature changes more consistently across all regions. Flat surfaces like grass had a cooling effect consistently across the cities in the study, while paved areas had a warming effect consistently across the study.  

The study, Urban tree cover provides consistent mitigation of extreme heat in arid but not humid cities appears in the Oct.15 edition of Sustainable Cities and Society.   

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