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The humble tree has long protected humans from sickness and even death—and in the modern city, it’s still doing so. As global temperatures rise, so too does the “urban heat island effect”—the tendency for cities to absorb and hold on to the sun’s energy, which is a growing public-health crisis worldwide. On a small scale, the shade under a single tree is an invaluable refuge on a blisteringly hot day. Scaling that effect up, neighborhoods with more tree cover are measurably cooler.
Now research is showing just what an impact this can have on people’s health. A new paper finds that in Los Angeles, planting more trees and deploying more reflective surfaces—something as simple as painting roofs white—could lower temperatures so dramatically, it’d cut the number of heat-related ER visits by up to 66 percent. That research follows a previous study by the same scientists finding that one in four lives lost during heat waves could be avoided with the same techniques.
As urban populations are rising around the world, so are temperatures, putting ever more people in ever-hotter environments. “We’re primarily urban dwellers at this point,” says UCLA environmental researcher Edith de Guzman, coauthor of both studies and the cofounder and director of the Los Angeles Urban Cooling Collaborative. “We know that that’s problematic, because there’s a magnification of heat that occurs in those spaces, because of the preponderance of heat-retaining surfaces that then release that heat at night, when the body seeks to cool off.”
If a heat wave stretches day after day, the physiological stress builds and builds on residents. Elderly and young people are especially at risk, since their bodies can’t cool themselves as efficiently. Extreme heat also leads to the formation of ozone, which exacerbates asthma.
For the new study, published in the International Journal of Biometeorology, de Guzman’s team looked at four different LA heat waves between 2006 and 2010—the same ones were also referred to in the previous study on mortality. These heat waves differed in their length, high temperatures, and humidity. (The higher the humidity, the more difficult it is for the human body to cool off by sweating.) The team then married this data with land-use data showing where in LA County there’s good tree cover, which reflects heat, and where there are impermeable surfaces like pavement, which absorb it. And lastly, they got data on heat-related ER visits in those areas.
They plugged all that data into an algorithm that modeled scenarios in which modifying the urban environment—with more reflective surfaces and more trees—could lower temperatures and avoid those heat-related ER visits. With 25 percent more tree cover, visits might decline by 7 to 45 percent. Bumping that up to 50 percent more tree cover, the heat-related ER visits decline by 19 to 58 percent. And then if LA were to max out its allowable tree cover—40 percent of its total area—those visits would drop by 24 to 66 percent.
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“We are able to quantify how outcomes would have been different in real events,” says de Guzman. “I know that projections are for a higher number of extreme heat days in each of these communities.”
In other words: The time to act is now. Tall trees provide shade, for one, but plants in general release water vapor as they photosynthesize, essentially “sweating.” Accordingly, a high-income neighborhood with lots of parks and landscaping might be 15 degrees Fahrenheit cooler than a low-income, more industrialized area—like so many health threats, heat disproportionately affects those who are the most deprived. The urban heat island effect varies not only from neighborhood to neighborhood, but also from block to block and even house to house. Different building materials, like wood and brick, absorb and retain the sun’s energy in different ways.
So the utility of this kind of research is to first of all find whole neighborhoods to prioritize for greening, but then to target specific places to put single trees. “We don’t necessarily in urban areas plant entire forests—we plant an individual tree at a time,” says Portland State University climate adaptation scientist Vivek Shandas, who studies the heat island effect but wasn’t involved in the new research. “If you have a limited amount of funds, and you know where there’s going to be the highest impact from the deployment of this, it’s a no-brainer to be able to identify those locations.”
But it’s not as easy as just sticking a bunch of trees in the ground and calling it a day. LA has embarked on a multiyear Urban Forest Management Plan to increase its tree canopy, especially in its most vulnerable neighborhoods. It has to carefully identify locations to add the plants, but also collaborate with the residents there—for instance, with community feedback meetings. Some people might not want trees outside their home: Maybe a branch once fell on their car, or their sidewalk cracked from underlying roots, reducing the mobility of a grandparent in a wheelchair.
So getting more trees in the urban landscape is also about collaboration and education, getting folks to understand the significant cooling benefits that could well save their lives—and, at the very least, reduce their air-conditioning bills. “If you have a negative tree story, we have to do some education on how we’re going to do better in the future,” says Rachel Malarich, the City Forest Officer of Los Angeles. “How we’ve learned as an industry is to be more selective about what we put where, and make it appropriate for the site, or to change the site so that we can plant something larger.” For example, if a given patch of dirt is too small to fit a tree species big enough to provide good shade, perhaps the city could rework the sidewalks to accommodate that, simultaneously improving mobility and reducing local temperatures.
In LA and elsewhere, scientists are rushing to find tree species that’ll be able to withstand temperatures 10, 15, 20 years from now—you wouldn’t want to plant a tree only to discover it can’t survive in the new climate. Climate change may also help tree pests and diseases spread, adding yet more precarity to urban greenery: A given community might be keen on planting more of their neighborhood’s trademark species of tree—magnolias or what have you—but the future may be increasingly hostile to it.
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A tree also needs more maintenance and water in its vulnerable early years, a resource that may be in increasingly short supply as Southern California warms. So in addition to increasing its tree cover, LA is trying to make itself spongier: Lots more green spaces in general that allow rainwater to soak into the underlying aquifer or cisterns for later extraction. Back in February, this spongy infrastructure helped the city capture 8.6 billion gallons of stormwater over just three days.
Being an urban environment with pedestrians, cars, and buildings under the trees, the city has to ensure that the plants aren’t dropping branches. That means inspections and boots on the ground—unlike a proper forest, the urban forest needs constant attention. “We have to manage a little bit higher-touch in an urban environment, because we have these risk factors,” says Malarich. “We want to do planting, maintenance, and then preservation. So keeping existing mature trees in place is actually really critical to canopy expansion.”
And a neighborhood doesn’t have to stop at adding extra trees to mitigate the urban heat island effect. Scientists are researching ways to grow crops on rooftops, shaded by solar panels, which produces both food and free electricity as well as cooling the top floor of a building. And extra points if it’s a “blue-green roof” capable of storing rainwater for residents to use to water plants and flush toilets.
Scientists are also experimenting with “cool pavements,” which bounce more sunlight back into space instead of absorbing it. Reflective coatings do the same for the sides and rooftops of buildings. Painting more surfaces white helps cool an area, but urban designers have to take care not to unintentionally bounce that reflected solar radiation toward people. “I’m a big fan of white paints up high,” says Shandas. “That’s what clouds do naturally: On a cloudy day, a lot of that sunlight is already reflected back up into space. And so if we can get every roof in some way a lighter color, we can start moving in that direction.”
It’s not a matter of whether LA or any other city will be greener or more reflective in the years to come, but how much greener and more reflective it can manage to be. And accordingly, how many lives it can save as the world warms. “While it’s generally a feel-good kind of investment for cities, we need to tie those investments to public health outcomes,” says de Guzman, “because the improvements that can be made are really meaningful.”
