A man sits in the shade as people wait at a bus stop in the Bronx borough of New York City. Some cities are installing more shelters to provide better refuge from extreme heat. David 'Dee' Delgado/Bloomberg

The practice of redlining in the 1930s helps explain why poorer U.S. neighborhoods experience more extreme heat.

American cities face increasingly unbearable summers—but the heat isn’t distributed equally. Low-income and minority neighborhoods can get significantly warmer than their surrounding areas due to the urban heat island effect. These areas typically lack trees and other cooling infrastructure that provide shade during the day, and stay uncomfortably warm at night as the heat absorbed by impervious surfaces escapes back into the air.

A new study finds that these disparities correlate closely with neighborhoods that were subject to decades of housing discrimination through federal redlining policies that prevented African Americans from buying homes in certain neighborhoods during the New Deal era. In 94 percent of 108 cities that the researchers looked at, they found that historically redlined neighborhoods were nearly 5 degrees Fahrenheit warmer on average than non-redlined neighborhoods, though some cities showed starker differences—as much as 12.8 degrees.

“It’s been something that we [researchers] have recognized on a case-by-case level for many years, but we never ask, ‘how did it get to be this way?’” says Jeremy Hoffman, chief scientist at the Science Museum of Virginia, who led the study. “Our paper is the first to really establish that this is systematically related between cities, and partially explained by the historical policies of redlining.”

The finding adds to the body of research on how redlining has imposed decades of economic and health disparities on black and brown communities—in ways that are now being exacerbated by climate change.

Hoffman and his colleagues at the Virginia Commonwealth University and Portland State University compared satellite images of neighborhood-level surface temperatures with the digitized color-coded maps of the 1930s. That’s when the federal agency Home Owners’ Loan Corporation, in trying to revive the housing market, graded how risky neighborhoods were for real estate investment. Neighborhoods made up of largely low-income and immigrant or African American residents were deemed “hazardous,” or “definitely declining.” Whiter and wealthier communities were considered “best” or “still desirable.”

A map of redlining in Portland. Neighborhoods made up of largely low-income and immigrant or African American residents were outlined in red deemed “hazardous,” or colored yellow and labeled “definitely declining.” Whiter and wealthier communities were considered “best” or “still desirable,” and colored green and blue, respectively. (Mapping Inequality Project)

The researchers’ analysis shows that the gaps in temperature among neighborhoods were pronounced in southeastern, northeastern, and western cities. The largest disparities were in Portland, Oregon, and Denver, where redlined neighborhoods were roughly 12 degrees hotter than the “best” neighborhoods. Midwestern cities, meanwhile, had the least disparities between neighborhoods on average, which the researchers say is likely due to both the variation in how neighborhoods were ranked across the country and how land was used. Still, the largest metropolitan areas followed the national trend. In Minneapolis, the temperature difference varied, on average, by 11 degrees.

“We don’t claim that [redlining] caused this environmental disparity but they likely codified it, and turned the neighborhood disparities into law,” Hoffman says. Redlining prevented minority families from owning homes, fueling today’s racial wealth gap. They also led to decades of disinvestment in already-struggling neighborhoods, which helped shape the current built environment, exacerbating the disparities of the urban heat island effect.

Historically, wealthier neighborhoods were greener, with more parks and greater tree density to provide relief from the heat. That remains true today. Developers meanwhile, had more incentives to construct buildings and roadways through the lowest-income communities. In the mid-1900s, the federal government subsidized the construction of large building complexes in redlined areas, due in part to the inexpensive land. Those buildings, many of which still stand, were often made with materials that retain heat, like cinderblock and brick.

Then there was the Interstate Highway System, which was widely employed as a revitalization tool for struggling city centers during the 1950s and ‘60s; often, urban highways bisected poor and less-educated neighborhoods that couldn’t fight back against the massive project.

To advocates like Calvin Gladney of Smart Growth America, a nonprofit pushing for equitable and sustainable revitalization of communities, the finding isn’t surprising. But it can be “eye-opening” to local policymakers. “Redlining was banned in the ‘60s, and oftentimes people think there are no current-day ramifications,” he says. “But this study crystallizes that just because you take away a rule doing damage at the time, that damage doesn’t go away.”  

But this isn’t merely a design problem. Curbing the urban heat island effect isn’t just about looking at the data and building more shaded bus shelters or planting more trees in hotter communities—decisions often made separately by various departments. Rather, Gladney is calling for better coordination at the municipality level to understand the bigger picture about what communities of color need and how to include their voices at the table.

Climate change, he says, will only exacerbate those historic disparities unless equity becomes the “default” solution.

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