Hazy, hot and… shady? How street trees counteract air pollution and heat in American cities
Researchers measured the “multihazard exposure” of extreme heat and air pollution across 55 American cities.
It’s a catchy tune, but The Lovin Spoonful’s “Summer in the City” doesn’t paint the picture of an appealing environment with its description of a sidewalk as “hotter than a match head.”
New research from Northeastern University may offer relief from those conditions, however, finding that extreme heat and air pollution don’t necessarily go hand in hand when the concrete jungle is made a little more green.
“If neighborhoods with lots of streets also had lots of trees, then you [have] very little correlation between heat and air pollution,” said Daniel O’Brien, professor of public policy and urban affairs and criminology and criminal justice at Northeastern. Trees, his research found, “disrupted the things that make heat and air pollution come together.”
O’Brien said that scientists typically examine heat and air pollution as individual hazards. But when talking about urban demographics, heat and air pollution are often implicitly combined – in other words, if it’s hot, it’s also polluted – O’Brien said. Like with other forms of environmental hazards, the understanding has also been that the association between heat and air pollution tends to have more of an impact on lower income and communities of color in the United States.
O’Brien wanted to see if there was any evidence to support these assumptions.
To find out, researchers measured what’s known as “multihazard exposure,” where two or more hazards are present at the same time in the same community. O’Brien’s team specifically focused on extreme heat and air pollution across 55 American cities.
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The results, published this month in the journal Environment & Planning B: Urban Analytics & City Science, found that there was a connection between presence of heat and air pollution within cities, but not always.
“It’s just stark,” O’Brien said. “There is just this line, and it’s clear who has heat and air pollution and who has less of both.”
In the San Francisco Bay Area’s neighboring cities of Oakland and Berkeley, for instance, heat and air pollution tended to be present at the same time.
Detroit, on the other hand, was “all over the spectrum,” according to O’Brien’s research. In that city, “There were neighborhoods that were hot and had very little air pollution and neighborhoods with air pollution that were not hot at all,” O’Brien said.
What seemed to make the difference was the presence of tree-lined streets. Areas with many tree-lined streets tended to not have both air pollution and extreme heat at the same time. An absence of such streets often meant those neighborhoods were dealing with both environmental hazards.
In Detroit, the research found that neighborhoods with high street density often had higher levels of tree canopy. And there was a negative correlation in other cities. Some neighborhoods were hot, and other neighborhoods had high air pollution, but very few were subject to both hazards – in Roanoke, Va. and Newark, N.J., according to the research.
We often think of street networks as “baking in the sun in the summer,” O’Brien said. “But if you put a bunch of trees out there, it’s not that bad.”
As for demographics, residents of neighborhoods where heat and air pollution coincided were predominantly low income and people of color, although O’Brien said it’s unclear how that might have happened.
“Did people of those demographic backgrounds already live in those communities, [which were] then designed to have lots of streets and fewer trees?” O’Brien asked. “Or did these uncomfortable conditions exist and market processes drove those with less income to be the ones who were more likely to live in those communities?”
Regardless of the origin story, the research suggests a self-perpetuating cycle keeps such communities hot and polluted. If people with political power don’t live in a neighborhood with both heat and air pollution, then they are less likely to protest when infrastructure such as more roads or polluting industries expand into that neighborhood.
Still, it will take more than saplings and a shovel, he said.
O’Brien referenced New York City’s recent effort to increase equity in green space and tree canopy, an effort that he said “was able to move the needle a little bit,” in increasing tree canopy in low-income neighborhoods, “but not much.”
“They couldn’t [easily] find places to plant the trees in low-income neighborhoods, and it’s because it [had] already all been paved,” O’Brien said.
Instead, city planners, neighbors and researchers must consider alternate means to incorporate trees, perhaps by using planters, eliminating sidewalk space, making pocket parks and more.
“Every tree is an intervention and knowing precisely how to…use your resources as efficiently and effectively as possible takes some data and it takes some really good planning,” O’Brien said.
