Trees Are the Not-So-Secret Weapon in Keeping Cities Cool

We've long sung the praises of urban canopies and their unmatched ability to scrub the air, mitigate flooding, elevate moods and cool overheated cities. But per that last attribute, it's never been clear how many trees are needed to make soaring daytime temperatures drop on a single city block and stay cool overnight.

In a new study, researchers from the University of Wisconsin-Madison conclude that 40 percent is the magic number when considering a canopy's ability to provide relief from the heat. A minimum of 40 percent of a single city block's impervious surfaces (sidewalks, streets, buildings, etc.) must by shaded by a tangle of branches and leaves for there to be any discernible difference in temperature.

Picture yourself walking down a block of a stifling summer afternoon that's 30 percent shaded by trees. Over a quarter — not shabby as far as urban tree coverage goes. Thirty percent coverage means there will be a few individual shady spots to pause and wipe the sweat from your brow before lurching ahead.

But for true relief — relief than comes in the form of temps that are as much as 10 degrees Fahrenheit cooler compared to areas lacking adequate tree coverage — you'll need at least 40 percent tree coverage. The reason is simple: by shading impervious surfaces that absorb heat during the daytime and release it at night, trees can help a superlatively leafy city block maintain a temperature that's markedly cooler around the clock than neighboring blocks with fewer trees and more sun-baked pavement. Trees also transpire, or give off water vapor when they absorb carbon dioxide, which adds to the overall cooling effect.

Based on what we know about the myriad benefits of urban trees, it's safe to assume that people living on blocks with at least 40 percent coverage are a little less testy and enjoy summertime electric bills lower than the residents of neighboring blocks who, in the absence of an ample number of temp-lowering trees, are forced to crank the AC to full blast.

Published in the journal Proceedings of the National Academy of Sciences, the study zeroes in on the areas "where we live our daily lives within the city" as its lead author, Carly Ziter, explains in a University of Wisconsin new release.

Navigating a city's 'heat archipelagos' by bike

The urban heat island effect, the phenomenon in which rural outlying areas of a city are dramatically cooler than the asphalt-laden urban core, has been well-observed and documented. But as Ziter and her colleagues explore, temperature variations are a complex matter as there are much-cooler spots within urban heat islands. Depending on tree cover, these individual microclimates can potentially even be cooler than the sylvan rural outskirts of a city. The term "heat archipelago," which considers differences in temperature on a neighborhood-by-neighborhood or block-by-block basis, better describes the situation.

"We knew that cities are warmer than the surrounding countryside, but we found that temperatures vary just as much within cities," says Monica Turner, a professor of Integrative Biology at University of Wisconsin–Madison and a co-author of the study. "Keeping temperatures more comfortable on hot summer days can make a big difference for those of us who live and work there."

How widespread cool pockets are within a city depends on the number of individual blocks that are almost — or more than — half covered by an arboreal sun-blocking system.

Given that the urban heat effect is often observed using satellites that measure ground temperature but not air temperature, the heat archipelago-navigating team decided that hyper-local air temperature readings were required to better understand block-by-block heat variations based on tree coverage. As Ziter explains, ground temperature measurements are "not getting you quite as close to what people are actually feeling."

As UW-Madison News goes on to detail, deploying individual air temperature sensors at the scale desired for the study just wasn't in the cards financially, so Ziter took to her bike with a single portable weather station in tow.

In the summer of 2016, it wasn't uncommon to see Ziter biking around the city of Madison with a small weather station strapped to the back of her bike. In all, she biked ten different transects of the city multiple times during different times of day. The sensor on her bike marked her location and took an air temperature reading every single second as she rode, resulting in real-time data every five meters.

In total, Ziter biked roughly 400 to 500 miles around Madison all the while collecting a "massive amount" of data that she and her colleagues later analyzed, eventually coming to the conclusion that 40 percent is the minimum amount of tree coverage needed on a block to enjoy maximum cooling benefits.

Madison is 28 percent covered by tree canopy according to a 2018 joint study conducted by UW-Madison, UW-Extension, the Wisconsin Department of Natural Resources and the U.S. Forest Service. This is just slightly below the state average of 29 percent coverage for urban areas. Green Bay had the highest percentage of coverage at 33 percent while Milwaukee, with 26 percent, had the lowest of the four cities included in the study. In total, urban tree cover in the Badger State provided staggering economic benefits including $47 million in pollution removal and $78 million in reduced energy costs.

Cities need to help push kind-of leafy blocks over the edge

Based on her team's findings, Ziter believes city planners and others with the power to impart positive change should focus less on making already leafy blocks even more tree-heavy and more on planting trees in areas that are under — but relatively close to reaching — the 40 percent coverage threshold.

There are plenty of city blocks that are safely already there. There are, however, likely even more blocks that are almost there. Going well over 40 percent threshold might boost real estate values and increase a block's verdant charm, but this won't necessarily result in wildly cooler temps when compared to a slightly less shady block that hovers closer to the threshold. In other words, if you're at or over 40 percent coverage, you're all good.

At the same time, Ziter stresses that residential blocks with percentages of tree coverage that are nowhere near 40 percent shouldn't be neglected given that these are often undeserved areas where residents can reap the greatest benefit from canopy cover. "We want to avoid advocating for policies that are simply ‘rich get richer,'" she explains. Ziter also urges cities to think beyond parks and embark on tree-planting campaigns in the places where they're often needed most: on the streets where people live (although sometimes where they aren't always wanted.)

"It's not really enough to just kind of go out and plant trees, we really need to think about how many we're planting and where we're planting them," says Ziter. "We're not saying planting one tree does nothing, but you're going to have a bigger effect if you plant a tree and your neighbor plants a tree and their neighbor plants a tree."