Last month, the Washington Post conducted its own study of the city's tree canopy, with some findings that may not surprise anyone who lives in the capital: Lower-income neighborhoods were substantially less likely to have trees, with the city's densest greenery clustered west of the 16th Street Northwest fault line that divides some of Washington's wealthiest neighborhoods from the rest of town. Tree density in Washington, in short, provides a kind of proxy for wealth (and if you've spent time in Washington, you also know that wealth is a proxy for race).
Lest other cities scoff at Washington's arbor-inequality, research just published online in the journal Environmental Health Perspectives confirms that a very similar pattern appears all across the country. Researchers from the University of California at Berkeley looked at 63,436 census block groups from across the country covering 304 metropolitan areas and more than 81 million people. And they identified those blocks most at risk in extreme heat waves thanks to the lack of tree cover or the presence of too much asphalt (or impervious surfaces). Both of these factors have been shown to exacerbate the urban heat island effect, raising surface temperatures, suggesting that people who live in these neighborhoods may be at the highest heat risk as temperatures warm with climate change.
According to the findings, blacks were 52 percent more likely than whites to live in such neighborhoods, Asians 32 percent more likely, and Hispanics 21 percent more likely (controlling for factors that explain variation in tree growth, like climate and rainfall).
"It’s in the same range of elevated risk that we see for a number of environmental concerns," says Bill Jesdale, one of the authors, referring to similar findings in the environmental justice literature that show minorities living in communities with greater exposure to traffic, pollution and other environmental hazards. "Often, unfortunately, you see relative risks that are quite a bit higher than that."
The study yielded one other curious finding: In metropolitan regions with greater levels of racial segregation, everyone – whites included – is more likely to live in such heat-prone areas (we can add this to the evidence that segregation is bad for everyone). This is another way of saying that integrated metropolitan areas may do a better job of mitigating heat for everyone. Why might that be?
"When you have an area that’s more racially stratified, where race really dominates how people live with each other, there’s less shared investment," Jesdale says. "People are not as concerned with the common good. And that shows up in trees. "
Segregated places may be less likely to make collective investments, and collective investments are precisely what's needed to support environmental improvements for everyone like planting more trees in the public way (or paving roads and sidewalks with pervious material).
Jesdale notes one interesting exception in the tree-canopy literature to these findings: In Baltimore, blacks have been found to live in neighborhoods with more trees than whites. But the city has undergone significant demographic change with time, and 50 years ago, the city's tree canopy corresponded with where whites used to live.
The problem may ultimately be compounded by the fact that the heat-prone neighborhoods may also be home to households with the least capacity to withstand hot weather.
"If you’re living in one of these heat-prone areas in a city like Phoenix, and you have air conditioning, well you’re going to be able to weather it OK as long as you can afford the electric bill," Jesdale says. "But when you start getting into a situation where you’re in a city where people don’t tend to have air conditioning, or you’re in a situation where being able to afford keeping the air conditioning running is a real problem, that’s where you’re going to see significant heat problems."