A new tool developed by ecologists will help urban planners cool cities more efficiently.

Planting trees is a common cooling method, but it relies primarily on guesswork and isn't backed by precise science.

In a study published in Proceedings of the National Academy of Sciences, a team of researchers from China and New York studied the relationship between trees and temperature and found a way to predict effective cooling on a citywide scale.

"Trees are good at cooling because they pump a lot of water from the ground into the air, and when that water evaporates at the leaf surface, it absorbs a vast amount of heat," said Steward Pickett, co-author and urban ecologist at Cary Institute of Ecosystem Studies.

"That's just the physics of evaporation. The shade provided by trees also helps with cooling," he added.

The researchers chose four cities with different climates — Beijing and Shenzhen in China and Baltimore and Sacramento in the U.S. — and analyzed their satellite imagery and temperature data.

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Currently, the effectiveness of tree cooling is typically studied in small areas like a neighborhood or a single street. But Pickett said urban planners and decision-makers need a method for extrapolating this to form strategies for larger places.

"They're asking, 'How much tree canopy do we need for the whole city? What happens when we scale it up?' And that information hasn't been available," he said.

Pickett and his co-authors said their findings "[make] it possible to predict cooling effects at the whole-city scale, offering a valuable tool for managers to set urban tree canopy goals to reduce extreme heat."

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This new scaling tool may help bring more greenery to urban areas, which benefits residents and the environment in multiple ways. A study from Georgia State University found that planting trees and other vegetation near city highways can dramatically lower air pollution from cars.

Trees are an asset to cities, and some researchers have studied just how much they are worth. Forest Research found that urban trees provide billions of dollars in services every year, such as creating oxygen, removing carbon pollution from the air, and providing food and shade.