To solve the modern problem of urban heat islands in a warming world, some cities are turning to an ancient solution: trees.

The Nature Conservancy, an environmental nonprofit, is helping 20 cities in the United States cool off this summer by planting trees. Trees promote cooling by providing shade to streets and buildings and through a process called evapotranspiration, which disperses moisture in the atmosphere.

A study of 245 cities around the world published by the Nature Conservancy in October 2016 found that urban trees can reduce summer temperatures by as much as 6.5 degrees F. Trees also absorb carbon dioxide and other air pollution, reducing the amount of fine particulate matter emitted by factories, power plants, and cars.

“Urban trees can save lives and are just as cost-effective as more traditional solutions like putting scrubbers on smokestacks or painting roofs white,” according to Robert McDonald, lead scientist for global cities at the Nature Conservancy.

Planting more trees will only amplify these benefits, the paper says, and it makes good economic sense, too. A study by the US Forest Service and the University of California, Davis last year found that every $1 spent on tree management in California returns $5.82 in benefits, by providing shade, soaking up pollution, retaining storm water, and increasing real estate values.

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And it’s not just the US using trees to beat the heat. Melbourne, Australia, hopes new trees can lower average temperatures 7 degrees by 2030. In Toronto, all new buildings must include a green roof. And in Singapore, a national Plant-A-Tree program encourages individual residents to get planting.

Planting trees is not the only way a city can cool off. Studies have shown that increasing the solar reflectiveness of roofs and paved surfaces can result in energy savings of as much as 25 percent.