Trees cut extra city heat by half, but not quite fairly, study says

Trees cut extra city heat, but unevenly, according to new global study

Trees are cooling many of the world’s cities, but not uniformly, according to a study published in Nature Communications that examined how urban vegetation affects temperatures. The research concluded that, on average, tree cover counters nearly half of the additional heat that darker pavement and buildings store and re-emit—an effect researchers call the urban heat island.

In the study’s global citywide average, trees reduce temperatures by an average of 0.27 degrees Fahrenheit (0.15 degrees Celsius). The scientists calculated that, without those trees, city conditions would be about 0.56 degrees Fahrenheit (0.31 degrees Celsius) warmer on average due to the urban heat island effect.

The researchers said their work separates vegetation cooling from broader climate change driven by fossil-fuel burning. They also said their analysis helps avoid crediting cooling in the wrong place: the team studied Earth’s nearly 9,000 large cities by looking at temperature impacts for segments covering about 150 city blocks, rather than treating a city as a single, blended measurement. That method, the study said, helps prevent an example such as New York’s Central Park being credited with cooling effects across distant parts of the Bronx.

About 185 million people across 31 of the large cities studied already experience at least half a degree Fahrenheit (0.3 degrees Celsius) of cooling from tree cover, the study found. But lead author Rob McDonald, a scientist at the Nature Conservancy, said cities that are poorer and hotter—where extreme heat can be most dangerous—are getting less cooling relief than the global average suggests. He tied the shortfall to the health risks of high heat, saying the hotter cities are dealing with temperatures that can kill by confusing the brain, shutting down organs, and overworking the heart.

The study also reported wide differences between cities in how much cooling trees provide. In 20 cities with at least 3 million people, residents feel less a tenth of a degree Fahrenheit (0.05 degrees Celsius) from cooling trees, the researchers said. In four cities—Dakar, Senegal; Jeddah, Saudi Arabia; Kuwait City; and Amman, Jordan—the study found tree cover is so limited that more than 15 million people in those places get essentially no cooling benefit from trees.

On the other end, McDonald identified cities where tree cover cools at least 0.45 degrees Fahrenheit (0.25 degrees Celsius). Nearly 40% of cities in wealthy nations met that benchmark, while just under 9% of cities in the poorest countries did, the study said. The places that cool the most, the study reported, include Berlin, along with Atlanta, Moscow, Washington, Seattle and Sydney.

McDonald also pointed to an inequality in how tree cover is distributed across cities worldwide. “There’s this inequality,” he said. “When you look at cities globally, there are many, many cities, especially in developing countries, that have very low tree cover, and so I think the air temperature cooling number was a little less than we expected.”

Chris Greene of the University of Dalhousie, who was not part of the study, said higher tree cover in wealthy areas can reflect differences such as larger lot sizes, individual ownership, and residents with more political influence, factors that can help trees grow and spread. Greene’s remarks described how urban planning and local power can shape how much shade and cooling vegetation provides.

Other experts said even small amounts of added urban trees can help, but they warned that hot, dry places face constraints that can blunt the benefits. Thomas Crowther, an ecologist at the King Abdullah University of Science and Technology, said water limits can be a key issue in regions where cities have nearly no cooling from tree cover. He said every little bit helps, adding that cities will need to overturn what he called devastating inequities in the distribution of urban trees so that the benefits reach low- and middle-income communities that he said are often most vulnerable to extreme temperatures.

The authors said cities, especially poorer and hotter ones, can and should do more to increase tree cover. But McDonald said those changes have limits. He said that, because of constraints such as water availability and land, and because of the continuing warming driven by climate change, cities could reduce future urban heating by at most 20%.

“Trees won’t save us from climate change,” McDonald said, emphasizing that climate scenarios point to a much warmer world and that tree cover can only help with a portion of it. Jonathan Overpeck, dean of the University of Michigan environment school and not part of the study, made a similar point about the broader climate response. He said planting trees can fight climate change in multiple ways, but that it is not nearly enough on its own to slow climate change to a significant degree.