On a 92-degree Saturday afternoon in Portland, Oregon, I went looking for shade in Cully Park, which was built on top of an old landfill and opened in 2018. The city included plenty of trees in the design—I mean, this is Oregon. But those trees are still slender saplings, each throwing enough shade for maybe a chihuahua. So the park’s designers also included two large metal canopies that protect a seating area from both winter drizzle and the summer scorchers that are becoming routine here. The tables were full of families regrouping after a soccer game; a couple of women chatted while sharing a bottle of juice. As I walked into the shade, I could feel my body go from a state of mild alarm to drowsy summer relaxation.
As the climate warms, our cities are getting hotter, and people who live in cities are suffering more heat-related illnesses, as well as losing opportunities to socialize and exercise outside. For years, conversations about how to solve that problem have focused on trees. Across the country, environmental groups and city governments are calling for more urban trees, advocating for canopy-cover equity, and launching initiatives to plant a million trees. You get the idea. Trees are indeed a wonderful and absolutely necessary part of cities, and they should be planted in many more places. The thing about trees, though, is that they must grow for years before they can provide meaningful shade. To get shade fast typically means erecting an awning, a shade sail, or a wall—it means building something. So where’s the million-awnings initiative?
Trees have dominated the conversation about city heat in part because the problem of city heat tends to be described in terms of the “urban-heat-island effect,” the idea that all hard surfaces in cities absorb and retain the heat of the sun more than green areas do, which raises cities’ ambient air temperature relative to the surrounding area. Trees do an excellent job of mitigating this problem, both by creating shade and by cooling the air when they release moisture from their leaves. But David Hondula, the director of the Office of Heat Response and Mitigation for Phoenix, Arizona, a city that knows a thing or two about heat, told me that he cares a lot less about the the average air temperature of the city than he does about something called “mean radiant temperature”—the average temperature of all the objects that transfer heat to a person, adjusted for distance. Preeminent among these objects is the sun.
Blocking the sun can lower how hot a person feels by 36 to 72 degrees Fahrenheit. That far outweighs the heat-island effect, which can raise temperatures up to 7 degrees Fahrenheit. (If 72 degrees seems like a dramatic temperature drop, the measurement is a testament to just how hot hard surfaces with no shade can feel: Researchers at Arizona State University measured a mean radiant temperature of 169 degrees Fahrenheit at one such site.) The amount of sun that hits a person’s body is by far the determining factor in how hot they actually feel, V. Kelly Turner, an urban-heat expert at UCLA, told me. But, because measuring a city’s average air temperature is easier than measuring mean radiant temperature for every person in a city, the role of mean radiant temperature and the power of shade can be missed.
If you’re walking through a shadeless city on a hot day, you probably understand all of this instinctively. On maps of the urban-heat-island effect, a long suburban sidewalk next to a treeless lawn will show up as “cool” because the lawn is vegetated. But try trekking to a grocery store in a city like Tallahassee, Florida, or Austin, Texas (to give examples from my personal experience), and you’ll find that those shadeless walks are miserably hot. Strolling around the dense core of a downtown would actually feel cooler, because even though hardly any plants grow there, the tall buildings throw lots of shade. Heat is most dangerous when the body doesn’t have a chance to cool down, so the more relief from the sun people can have as they move around a city, the better.
“If the goal is to cool the city, trees are more helpful than a built structure,” Hondula said. “If the goal is to cool people, which is the way we are trying to talk about these efforts in Phoenix … I think that’s a different toolbox.”
And that toolbox includes lots of built shade (which has an added advantage in a desert city: no water required). According to Hondula, Phoenix has installed shade structures on 3,054 out of the city’s 4,000 or so bus stops, and plans to add shade to “all bus stops where it’s feasible to do so” in the next 10 years. Los Angeles recently debuted the first of many mint-green modernist bus shelters that the city plans on installing across the metropolis over the next decade. Cities around the country are adding shade sails—large swaths of suspended fabric, often in swoopy triangular shapes—to playgrounds. And in some cities around the world, shade sails have been installed above plazas and pedestrian shopping streets.
Mostly, though, cities are still thinking about planting trees. With a group of colleagues, Turner surveyed 175 municipal plans produced by the 50 most populous cities in the United States to see how they are planning for heat. The team found that few cities are trying to systematically increase shade, and of those that are, 75 percent mentioned trees and just 10 percent mentioned shade structures in their plans. Trees, Turner pointed out, have a “mature institutional infrastructure” that has been speaking for them, Loraxlike, for many decades, in part because greenery beautifies cities, improves real-estate values, controls erosion, and boosts biodiversity. Shade structures don’t really have any organized lobbying groups. “There’s so many people who are already tree advocates,” Turner said. “Heat now is a new entry point to advocate for trees. And that’s not necessarily a bad thing. But it’s different than coming at it from an angle of: It’s hot. We need to produce shade.”
Jonathan Beaver, a landscape architect at Knot Studio in Portland, led the design of Cully Park. Those metal shade structures cost tens of thousands of dollars each, he told me, but they should last a long time—at least 50 years. Fabric shade sails might last a decade, until they’re worn out from being bombarded with heat and UV radiation so people don’t have to be. Tree longevity depends on species, but oaks can live 1,000 years, and some shade upwards of 1,500 square feet. But trees that grow that old and big don’t work everywhere. For Cully Park, Beaver included the awnings in the design partly because large trees can’t be planted on the site. (Their roots might break the waterproof membrane that covers the buried garbage from the old landfill.)
“This question about shade is coming up more and more on our projects,” Beaver said. He’s working on a playground now, and the local parents have asked for shade sails over the play equipment. “It takes a long time for a tree to grow up to the size that it can really provide shade in these spaces. I certainly wouldn’t think shelters are a replacement for trees, but I think both have their place.” A group of researchers examining strategies to combat heat in a public square in Seville, Spain, likened shade structures to “temporary urban prostheses” that help people enjoy city spaces while waiting 20 or 30 years for newly planted trees to mature. In other cases, shade structures might be the best long-term solution, especially for spots where trees cannot be planted or maintained, or where space is too tight.
Shade structures can have their charm, but they aren’t as romantic or beautiful or complex as trees. They aren’t (typically) homes for insects and birds. They don’t store carbon dioxide. They don’t release chemical compounds that smell good and calm our minds and boost our moods. They don’t grow fruit or nuts. But they can stop someone from passing out at a bus stop on the very same day they are installed.
“Shade is transformational,” Hondula said. “That difference of 30, 40, 50 degrees Fahrenheit in radiant temperature—it’s like moving months. If we’re in the shade in July, it might be like we’re in the sun in April or May.” Shade is so powerful that he imagines that even if the overall temperature is higher, cities of the future, designed right, could feel cooler than cities do today.
