Trees and urban heat: what EPA and two published studies report
What the US Environmental Protection Agency says trees do about heat islands, and what two papers in the Proceedings of the National Academy of Sciences measured about canopy cover and air temperature, each with its method and its limits as the authors state them.
Checked against the sources at the bottom of this page on October 9, 2026. Rules, fees and pay change: the source has the last word.
This page attributes every number to the named source and the way that source measured it. The studies cover different cities and methods, so their figures are not comparable and nothing here is a target for a place. The canopy target in the 3-30-300 guideline is on the 3-30-300 rule page.
What EPA says
The EPA heat islands page on the benefits of trees and vegetation says trees and other vegetation lower surface and air temperatures by shading and by evapotranspiration, the process in which water taken up by roots evaporates from the leaves and uses heat from the air. It cites a review of 308 studies that found, on average, urban forests were 3.0 degrees F (1.6 degrees C) cooler than urban non-green areas. It lists these further statements, each attributed in its references:
- EPA says parks and urban forestry can cut the energy demand of nearby buildings by 10 percent.
- EPA gives 15 to 27 percent of annual rainfall as the share urban trees can absorb, which reduces stormwater runoff.
- Tall dense roadside vegetation can lessen downwind pollutants by approximately 30 percent, as the page words it.
- One analysis cited by EPA estimates that raising tree cover by 10 percent would mean about 50 fewer deaths a year in Salt Lake City, Utah and about 3,800 fewer in New York City, New York.
- EPA cites one study in which city areas with less vegetation were hotter and had more residents on lower incomes and more Hispanic or Black residents.
EPA also points to Chapter 2 of its Reducing Urban Heat Islands compendium for costs, other factors and tools.
Two measured studies
| Paper | Method | Reported result |
|---|---|---|
| Ziter, Pedersen, Kucharik and Turner, Proceedings of the National Academy of Sciences, April 2019 | Bicycle-mounted sensor sampling air temperature every 5 m along 10 transects in one midsized Upper Midwest US city, with tree and impervious cover analysed at 10, 30, 60 and 90 m radii | Daytime air temperature varied by 3.5 degrees C on average (range 1.1 to 5.7). Temperature fell nonlinearly with canopy cover, with the greatest cooling when canopy exceeded 40 percent, and cooling was greatest at city block size, 60 to 90 m. Daytime warming from impervious cover was less than the cooling from canopy. At night, canopy effects were limited and the authors say reducing impervious surface remains critical. |
| Wang, Zhou, Pickett and Qian, Proceedings of the National Academy of Sciences, November 2024 | Cooling efficiency, the temperature reduction linked to a 1 percent increase in urban tree canopy, analysed across spatial scales from small units to whole cities | Cooling efficiency followed a power law as scale increased, in cities with different climates during summer daylight hours, and the form held under different summer weather. The authors propose the approach as a tool for managers setting canopy goals. |
The 2019 study sampled one city, and the 40 percent point is the abstract's finding for that city. The 2024 abstract states the power-law result and does not give a single canopy figure to aim for.
Sources
- US EPA, benefits of trees and vegetation (heat islands), read October 9, 2026
- US EPA, using trees and vegetation to reduce heat islands, read October 9, 2026
- Ziter and colleagues, Scale-dependent interactions between tree canopy cover and impervious surfaces reduce daytime urban heat during summer, PNAS, April 2019, abstract read on Europe PMC October 9, 2026
- Wang and colleagues, A scaling law for predicting urban trees canopy cooling efficiency, PNAS, November 2024, abstract read on Europe PMC October 9, 2026