La Jornada: Trees mitigate urban warming, but not enough in the poorest places

Washington. Trees offset almost half of the warming caused by pavement and buildings in the world’s cities, but they are not providing enough cooling in the hottest and poorest cities, where it is needed most as the planet’s temperature rises, according to a study.

If averaged across all cities in the world, tree cover – by providing shade and releasing water vapor – cools an average of 0.15 degrees Celsius, according to a study published in Nature Communications.

Without these trees, the world’s cities would warm by an average of 0.31 degrees Celsius due to the urban heat island effect, in which dark roofs and pavement absorb heat. This warming mechanism caused by humans is different from climate change caused by the burning of fossil fuels.

The researchers developed their analysis of the nearly 9,000 large cities on Earth by measuring temperatures in segments of approximately 150 blocks. That allowed them to capture cooling effects in cities and neighborhoods, so that, for example, the trees in New York’s Central Park would not be blamed for cooling more urbanized areas miles away in the Bronx.

Some 185 million people living in 31 of the largest cities already experience an average cooling provided by tree cover of at least 0.3 degrees Celsius. But the study’s lead author, Rob McDonald, a scientist at The Nature Conservancy, noted that the poorest and hottest large urban centers – which need it most – are not getting as much relief from extreme heat, which can cause death by confusing the brain, causing organ failure and overloading the heart.

Meteorologists measure the temperature difference between the center of a metropolis and a nearby rural area to determine the urban heat island effect. Scientists in this study used a combination of weather station measurements, satellite data and computer models to study the cooling provided by trees.

In 20 cities with at least three million inhabitants, residents feel 0.05 degrees Celsius. cooling due to trees. In four cities –Dakar, Senegal; Jeddah, Saudi Arabia; Kuwait City and Amman, Jordan – tree cover is so minimal that the more than 15 million people who live there experience virtually no cooling from the trees.

At the other end of the spectrum, McDonald examined cities where tree cover cools by at least 0.25 degrees Celsius. Nearly 40 percent of cities in rich countries get that level of cooling, but just 9 percent in poorer nations get that level of relief from trees, the study said.

Inequality

The list of places that get colder is topped by Berlin and includes Atlanta, Moscow, Washington, Seattle and Sydney, which have the most trees. For example, Atlanta has 64 percent of its land area under a tree canopy, McDonald said. Affluent areas of North America have larger plots, individual properties and residents with more political influence, which helps grow more trees and provide more cover, Chris Greene explained.

“There is this inequality,” McDonald said. “When you look at cities on a global scale, there are many, especially in developing countries, that have very low tree cover, and that’s why I think the air temperature cooling figure was a little lower than we expected.”

Thomas Crowther, an ecologist at King Abdullah University of Science and Technology in Saudi Arabia, said every little bit helps. He is in a region where cities benefit almost from cooling by tree cover, often because water is a scarce resource.

“As up to 75 percent of the human population moves into urban environments, these buffering effects of urban vegetation are going to be vital,” Crowther noted. “But we must reverse the devastating inequalities in the distribution of urban trees, so that their benefits can be experienced by low- and middle-income communities, who are often the most vulnerable to the effects of extreme temperatures.”

The study’s authors stated that cities, especially poorer and hotter ones, can and should do more to increase tree cover. But due to limitations in the availability of water, land and suitable species, in addition to the worsening of climate change, they would at most reduce future urban warming by 20 percent. “Trees will not save us from climate change. Climate scenarios show a much warmer world and tree cover can only help so much.”

Still, planting trees has benefits beyond reducing heat. In 2019, Crowther and Jean-Francois Bastin suggested in a study published in the journal Science plant a billion new trees – in addition to the 3 billion already growing on the planet – to absorb carbon dioxide, not so much for its cooling effect.

“Planting trees does help combat climate change in multiple ways, but this strategy is not nearly enough to slow climate change to a significant degree,” said Jonathan Overpeck, dean of environment at the University of Michigan. “Only by moving away from fossil fuels in favor of renewable energy and battery storage can we hope to stop the climate change that is wreaking havoc across the planet.”