Study Finds That Urban Cooling Remains Weakest In Poorer, Hottest Regions Of The WorldTrees are countering nearly half the urban heating from pavements and buildings in the world’s cities, but they’re not doing enough cooling in hotter, poorer cities where it’s needed the most as the world warms, a new study found.When averaged out over all the world’s cities, tree cover — by giving shade and releasing water vapour — cools an average of 0.15°C, according to a study in ‘ Nature Communications ’ .Without those trees, the world cities would warm at twice the rate due to the urban heat island effect, where dark roofs and pavement absorb heat. The human-caused warming mechanism is distinct from climate change caused by the burning of fossil fuels.Researchers built their analysis of nearly 9,000 large cities by measuring temperatures in 150 city sections each. That allowed them to capture cooling effects for cities and neighbourhoods so that trees in New York’s Central Park, for example, weren’t credited for cooling more built-up areas miles away in the Bronx.
Significant swathes of population in 31 of the larger cities already feel an average cooling from tree cover of at least 0.3°C. But study lead author Rob McDonald, a scientist at the Nature Conservancy, said poorer and hotter large urban centres that need it the most aren’t getting as much relief from higher heat, which can kill by confusing the brain, shutting down organs and overworking the heart.The scientists used a combination of weather station measurements, satellite data and computer models to see the cooling trees provided.In 20 cities, residents feel less than five-hundredths of a degree from cooling trees. In Dakar, Senegal; Jeddah, Saudi Arabia; Kuwait City and Amman, Jordan, there ’ s such minimal tree cover the people living there essentially have no cooling from trees.On the other end of the spectrum, McDonald looked at cities where tree cover cools at least 0.25°C — nearly half of cities in wealthy nations get that much cooling, compared to not even 10% in the poorest countries.The list of places that cool the most is topped by Berlin and includes Atlanta, Moscow, Seattle, Sydney and Washington, DC, which have more trees. For example, Atlanta has 64% of its land area under tree canopy, McDonald said.Wealthy areas in North America have larger plots and residents with more political clout, all of which contribute to larger tree cover, said Chris Greene of University of Dalhousie in Canada, who wasn’t part of the study. “There’s this inequality,” McDonald said.Thomas Crowther, an ecologist at the King Abdullah University of Science and Technology in Saudi Arabia, said every little bit helps. He’s in a region where cities have nearly no cooling from tree cover, often because water is at a premium.“As up to 75% of the human population shifts towards living in urban environments, these buffering effects of urban vegetation are going to be vital,” said Crowther.The study’s authors said that cities, especially poorer and hotter ones, can and should do more to increase tree cover. But because of limitations in availability of water, land and proper species, combined with worsening climate change, at most they’d reduce future urban heating by 20%, McDonald said.“Trees won’t save us from climate change,” McDonald said. “The climate scenarios are showing a much warmer world and there’s only so much of that that tree cover can help with.”
