Street trees: great anywhere, but especially in the most crowded cities. Regis Duvignau/Reuters

Trees clean and cool the air, but just how much depends on where you are, a new report finds.

Plant a tree in a city, and it pays off in dividends. You’ll get carbon sequestered, pollutants and rainfall absorbed, a provision of oxygen, shade and cooling, and psychological boosts to boot. Especially as climate change worsens heat waves (already the world’s leading weather-related cause of death), and as growing urban populations generate more harmful fine particulate matter, trees are one of the single best infrastructure investments cities can make, and an emerging body of scientific literature proves it.

In fact, a major new report by the Nature Conservancy concludes that trees are essentially the only cost-effective solution addressing both deteriorating air quality and rising urban temperatures. Some of the world’s largest cities could dramatically improve public health by those standards by investing just $4 per capita in their canopies, it finds. Crunching some numbers on how additional street trees (coniferous or leafy—palms don’t count here) could reduce pollution and heat inside the world’s 245 largest cities, the report shows that the residents of ultra-dense, ultra-populated, and ultra-polluted metropolises of Southeast Asia would see especially high ROIs, since the trees’ benefits would spread to so many people per square mile, and since material costs are comparatively affordable.

(The Nature Conservancy)

In Beijing, for example, levels of PM2.5—microscopic particles emitted by cars, factories, and heating systems that are easily breathed into human lungs and are estimated to cause 3.2 million deaths per year globally—have been known to exceed 600 micrograms per cubic meter in multiple locations. (The World Health Organization has declared a “safe” daily average of PM2.5 to be 25 micrograms per cubic meter.) Tree leaves can absorb anywhere between 7 to 24 percent of these particles in a range of roughly 100 meters, the Nature Conservancy reports.

The new study estimates that for an annual additional investment of $2.9 million in street trees, 2.2 million Beijing residents could see a reduction in PM2.5 greater than 1 microgram per cubic meter per 24-hour period. Most people would see a far greater reduction, exceeding 10 micrograms per cubic meter. And more than 2 million people would also feel a reduction of 1.5° C (2.7° F) in summertime air temperatures.

(The Nature Conservancy)

Other dense, highly polluted cities in the global south, such as Jakarta and Hong Kong, would see similarly high returns-on-investment. Tree planting could be even more cost-effective in poorer cities like Dhaka and Karachi.

In North American and European cities, average ROI tends to be a little lower since the air is cleaner by global standards. But targeted plantings can still serve up meaningful, localized reductions in harmful pollutants, the report finds. For example, in Los Angeles—a city challenged by heat, drought, and regular temperature inversions that trap pollution near the ground—the median ROI of tree planting is fairly moderate by global standards. But, the authors write, there are a handful of denser neighborhoods, such as in central Los Angeles, Santa Monica, and Long Beach, where a boost to the canopy would be especially effective. For an additional annual investment of $6.4 million in street tree planting, they estimate that more than 400,000 people could enjoy summertime temperatures cut by at least 1.5° C and a reduction in PM2.5 greater than one microgram per cubic meter.

(The Nature Conservancy)

Even without accounting for the numerous additional benefits urban trees provide, these reductions demonstrate trees’ potential to reduce mortality rates connected to heat waves and particle pollution, says Rob McDonald, the report’s lead author and the lead scientist for the Global Cities program at the Nature Conservancy. Still, greening up a city shouldn’t be mistaken for a complete answer to sweltering heat waves and unbreathable air. “Tree planting is not a solution for an entire planet trying to reduce greenhouse gas emissions,” he says.

Instead, trees should be thought of as surprisingly powerful tools for cities as they’re dealing with climate-related health concerns. “Cities often think about tree planting budgets totally separately from their health budgets,” he says. “We want cities to see the link between the two.” That’s especially critical as urban development replaces green space: The report states that 26 percent of the cities in consideration saw a decline in forest cover between 2000 to 2011.

A full ranking and analysis of the cities can be found here. Below is an interactive that contains a mapping tool which lets readers explore which neighborhoods in their cities could see the greatest benefits from a larger urban forest.

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