A new study reveals that trees are countering nearly half of the urban heating caused by pavement and buildings worldwide, but the cooling effect is significantly weaker in hotter, poorer cities that need it most. Published in Nature Communications, the research found that tree cover—through shade and water vapor release—cools cities by an average of 0.27 degrees Fahrenheit (0.15 degrees Celsius). Without trees, urban areas would warm by 0.56 degrees Fahrenheit (0.31 degrees Celsius) due to the urban heat island effect, where dark surfaces absorb heat.
Inequitable Cooling Across Cities
The study analyzed nearly 9,000 large cities, dividing them into segments of about 150 city blocks each. This granular approach allowed researchers to isolate local cooling effects. Approximately 185 million people in 31 larger cities experience at least half a degree Fahrenheit of cooling from trees. However, lead author Rob McDonald of the Nature Conservancy notes that poorer, hotter urban centers receive far less relief, exacerbating heat-related health risks such as brain confusion, organ failure, and heart strain.
Minimal Tree Relief in Hot, Dry Poor Places
In 20 cities with populations over 3 million, residents feel less than a tenth of a degree Fahrenheit (0.05 degrees Celsius) of cooling from trees. Four cities—Dakar (Senegal), Jeddah (Saudi Arabia), Kuwait City, and Amman (Jordan)—have such sparse tree cover that over 15 million people derive virtually no cooling benefit. Conversely, nearly 40% of cities in wealthy nations achieve at least 0.45 degrees Fahrenheit (0.25 degrees Celsius) of cooling from trees, compared to under 9% in the poorest countries. The most cooled cities include Berlin, Atlanta, Moscow, Washington, Seattle, and Sydney, with Atlanta boasting 64% tree canopy cover.
Root Causes of Inequality
Chris Greene of Dalhousie University, not involved in the study, attributes the disparity to larger lot sizes, individual ownership, and greater political influence in wealthier North American areas, fostering more tree cover. McDonald emphasizes the global inequality: many developing-world cities have very low tree cover, resulting in less cooling than expected.
Limited Potential of Trees Against Climate Change
While planting trees offers benefits beyond cooling—such as carbon sequestration—the study warns that even maximum tree cover would only reduce future urban heating by 20% due to water, land, and species constraints. McDonald states, “Trees won’t save us from climate change.” Thomas Crowther of King Abdullah University of Science and Technology notes that in arid regions like his, urban vegetation is vital as 75% of humanity moves to cities, but equity in tree distribution must be addressed. Jonathan Overpeck of the University of Michigan adds that only transitioning from fossil fuels to renewable energy can significantly halt climate change.
