Trees are an overlooked way to reduce heat and pollution in cities. In most places, public health officials don't talk to urban forestry officials, despite evidence that trees are as cost-effective as, say, taking diesel buses off the road, or painting roofs white.
A big new report elevates trees to their proper place, encouraging cities to think of trees for their health benefits, not just for beautification. Investing $100 million would benefit 77 million people with cooler cities and 68 million people with "measurable reductions in particulate matter pollution," says the analysis from The Nature Conservancy (TNC), sponsored by the C40 Cities Climate Leadership group.
"Trees are cost effective and deserve to be one of the tools in the toolbox as cities think about air quality and heat," says Rob McDonald, TNC's lead scientist for global cities. "Hopefully by making the case for trees, cities will prioritize them higher and try and set aside more funds." (Philanthropy would also help).
Particulate pollution (PM) leads to about three million early deaths a year, while heat waves kill about 12,000 people every year. Trees can reduce particulate concentrations by 7%-24% downwind, and cut summer air temperatures by two-four degrees Fahrenheit, according to studies reviewed by TNC.
The report maps 245 cities looking at opportunities to plant street trees and what the return on investment (ROI) would be in each case. The biggest returns are in South Asia and Africa, because pollution levels are high and trees and labor are relatively cheap. Karachi, Pakistan, comes top of the list for PM, while Dhaka, Bangladesh, could see the biggest returns on heat levels.
In North America, the biggest potential winners include Philadelphia and New York (for heat) and Los Angeles and Washington D.C. (for particulates). Less dense cities—Denver, say—have less potential because each tree planting affects fewer households.
McDonald cites Toronto as an example of a city that is taking trees seriously. As a result of climate change, it's forecast to see extreme summer temperatures of over 110 degrees by 2080, and the city has made a point of planting trees in vulnerable neighborhoods, like those with tall buildings where AC isn't common.
The report offers guidelines for cities, including what trees to plant. For particulates, that means species with larger surface area leaves that are sticky or hairy, or both—for example, Eastern red cedar, red maple and American elm. Each city gets a map showing where planting would have maximum benefit; there's a fourfold difference between the most and least optimal locations, McDonald says.
If the 245 cities planted trees to the maximum reasonable extent, the report estimates death reductions of 2.7% to 8.7%, or between 11,000 and 36,000 lives annually. Numbers like that mean we should stop looking at trees merely for their beauty and start considering them as health investments.