New research suggests cities could help combat the dangerous urban heat island effect with something we all encounter every day: trees.
The phenomenon occurs when asphalt, concrete, traffic, and the close proximity of people and buildings in cities combine to absorb the warmth of the sun and exacerbate its heating effects. It can result in urban daytime temperatures averaging 1-7 degrees Fahrenheit higher than in outlying rural areas.
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Heat, however, is not the only issue affecting the greater health and well-being of people in urban areas. Higher temps often go hand in hand with other exposures like air pollution, leading to poorer air quality as well as increased risk of respiratory issues and other health problems.
But Northeastern University researchers say trees’ leafy canopies can help cities find relief from these environmental hazards, meaning civil engineers and other planners can help improve the situation.
Daniel O’Brien, Ph.D., professor of public policy and urban affairs and criminology and criminal justice at Northeastern, said research into heat and air pollution in cities is often siloed – yet most urban areas are affected by both. To understand the scope of the issue, as well as identify any potential solutions, he and his colleagues examined multihazard exposure in the census block groups of 55 cities across the U.S.
“I really started to think about these effects in terms of the people and communities who experience it every day,” said O’Brien. “What does it mean to experience heat and air pollution at the same time? Where does that happen? Who is suffering from both those exposures at once? And what might differentiate cities where both exposures are high and places where they aren’t?”
O’Brien and colleagues combined temperature and air pollution data from different cities with information about community demographics, street density, and other area features. They discovered that heat and air pollution generally occur together – but the level of multihazard exposure varied significantly among urban centers.
In fact, the cities with the worst combination of heat and air pollution were those with high street density and a lack of tree canopy. O’Brien and his colleagues found that the neighborhoods with the highest rates of multihazard exposure also tended to be communities of color and lower-income people.
Nathan Guzman via Unsplash“It matters to be exposed to more than one hazard at a time,” O’Brien said. “And, certainly, a lot of work suggests that things like heat and air pollution come together, but as we learned in this study, that is often true but isn’t always.
“There are cities that have both high heat and air pollution, and then there are other cities that don’t have as much of either. And what seems to set these places apart is having tree canopy.”
The benefits of tree canopy
Timothy Jiang, Ph.D., who formerly studied the effects of trees on urban heat mitigation at Canada’s University of Guelph, said there is substantial research showing that trees provide great benefits to cities.
“They reduce heat, they provide shade, they help ventilate pollutants, they reduce energy use in the summer because they shade buildings with their leaves, then can help heat buildings in winter because as they shed their leaves, they let sunlight through,” Jiang explained. “They are very adaptive and are a promising solution for a variety of problems that you see in urban areas.”
Yet the data shows that trees are less common in lower-income neighborhoods. O’Brien said it’s unclear why – it could be due to historical housing policies, budget disparities, real estate development practices, or a combination of these factors. No matter the cause, O’Brien’s data makes clear that the lack of tree canopy in these poorer urban areas helps drive disquieting inequities in environmental exposures.
“These are things that can disproportionately affect health and greater well-being,” he said. “And they haven’t been and aren’t really being addressed in the way they should be.”
Creative solutions to a complex problem
Given O’Brien’s findings, it would seem there is a straightforward solution: plant more trees. Place the bulk of them in urban areas, particularly in underserved neighborhoods. But it’s not that simple, said Jiang.
“If you go out to some neighborhoods and talk with the residents, they don’t necessarily want more trees,” he said. “They’ll tell you that the city will plant them and then not take care of them. So, the burden of maintenance falls to the people who live there.”
That can be time-consuming and expensive for city residents. Jiang added that trees can affect infrastructure in negative ways, particularly during inclement weather.
“Branches can fall and affect homes and power lines,” he said. “During autumn, they can cover streets and sidewalks with leaves. There can be lots of pollen in the spring. We need to think about how we can best mitigate those effects that aren’t seen as so beneficial to the people who live in these places.”
Then there’s the matter of where to place the trees. One of the most alarming data points highlighted by O’Brien’s research is that the neighborhoods that could most benefit from tree canopy often lack adequate space to add them. Too much of these areas is already developed – often making it harder to justify the cost of adding new green spaces.
“There’s more than just the inequity between people of different socioeconomic backgrounds and disenfranchised minorities living with fewer trees,” he said. “They have fewer places to put trees because so much of the neighborhood is paved over. The inequity sort of doubles up on itself.”
Jiang agreed – and said that it is difficult and expensive to reclaim green space. But, he said, many urban planners are looking at innovative ways to keep cities cool.
“Some cities now mandate green roofs as a way of increasing greenery, and these do have benefits, although they are only marginally effective at cooling cities at ground level, where we walk,” Jiang said. “To better prioritize greenery when few resources are available, the greatest effect of trees on urban climate is their ability to provide shade. So, cities could consider preferentially placing trees in areas that aren’t shaded, such as the north side of east-west streets.”
The findings suggest that civil engineers, developers, and urban planners should think creatively about where and how additional tree canopy can be incorporated into urban landscapes.
O’Brien said adding pocket parks, planters, and redesigned sidewalk spaces can help. But he cautioned that where trees are planted matters. Careful, data-driven planning should help community stakeholders determine where the addition of new trees can offer the most benefit. And new development may provide the greatest opportunity to really move the needle and help the neighborhoods that have historically lacked the tree canopy needed to better protect residents from multihazard environmental exposures.
“Communities and those that serve them need to be really proactive in thinking about how to incorporate great infrastructure into new development efforts,” he said. “We need more trees. We need more grass. We need to make sure that our spaces aren’t just bigger and bigger seas of pavement.
“We can disrupt current infrastructure with these new projects – and there is a real opportunity there for community and municipal leaders, as well as private developers, to negotiate and demand that any new buildings are held to a higher standard. They can help provide the green space and the tree canopy that can make these neighborhoods cooler and healthier.”
Lance Cheung, U.S. Department of Agriculture