American Scientist

Jeremy Hoffman was examining temperature maps of the city of Richmond, Virginia, when he noticed a disturbing pattern. Hoffman, a climate scientist at the Science Museum of Virginia, had been working with local nonprofits to measure the intensity of the summertime heat island effect, in which urban areas are hotter than the surrounding areas. The effect was pronounced in Richmond, but the city’s excess heat was not at all evenly distributed. “The hottest spots in the city tended to be areas with lower resources and minoritized communities,” Hoffman says.

The heat island effect is caused in large part by brick, asphalt, and other urban building materials that absorb solar energy during the day and release it as heat in the afternoon and evening. Hoffman compares the heat island effect to walking across a parking lot on a hot day. You can feel the sweltering heat radiating up from the asphalt, and the rush of warm air as you pass by a running car. Tall structures block wind that could cool the parking lot, leaving you sweating. On the scale of a city, these factors—the hard surfaces, the waste heat from vehicles and air conditioners, the absence of tree canopies and breezes—combine to make U.S. cities up to 4 degrees Celsius hotter than surrounding areas during the day.

“Extreme heat is the leading cause of weather-related fatalities in this country,” Hoffman says. “It’s this silent killer.” Heat increases physical stress and exertion; exacerbates preexisting respiratory, kidney, and liver problems; and has been shown to cause premature birth. Heat also increases economic burdens: A few degrees may be the difference between leaving your air conditioning off and turning it on, Hoffman notes.

Within a city, trees and parkland can reduce the heat island effect, but such amenities are most often found in wealthier, predominantly white neighborhoods. Lack of green space could account for the hot spots Hoffman found in lower-income communities of color. The problem of urban heat and the disparities it exacerbates is only growing, as climate change increases the frequency and intensity of summer heat waves and urbanization makes cities even warmer. When “there are particular places in our cities that then turn up the heat even further,” Hoffman adds, “that has a disproportionate impact on people’s health.” Hoffman’s data tell a story of climate injustice: systemic, institutionalized racism that places the burden of climate change—including heat waves—disproportionately on poor Black and Brown communities.

To better understand this phenomenon, Hoffman and his colleagues Vivek Shandas and Nicholas Pendleton decided to investigate whether historically redlined neighborhoods are hotter than those that weren’t redlined. Redlining began in the 1930s with the federally funded Home Owners’ Loan Corporation (HOLC), which offered low-interest mortgage loans to help people keep their homes. However, these loans were not offered to everybody. To determine which areas were desirable for lending, the corporation drew up maps of about 240 U.S. cities, classifying neighborhoods in four categories from A (“best” investment) to D (“hazardous” investment). On these maps, “hazardous” areas were outlined in red, hence the term redlining; they consisted primarily of working-class Black and immigrant neighborhoods.

Redlining was made illegal with the passage of the Fair Housing Act in 1968, but the damage was done. Historically redlined areas have suffered an ongoing legacy of disinvestment, resulting in lower home ownership rates, lower property values, and reduced credit access today. People living in historically redlined areas are exposed to more pollution than those in neighborhoods that were greenlined, and are at greater risk for a number of health conditions including cardiovascular disease and diabetes. However, no previous studies had explored the relationship between historically redlined areas and present-day temperature.

Hoffman and his team set out to do just that by examining summer temperature data for 108 urban areas. Within each city, they calculated the surface temperature anomaly, or how the summertime surface temperatures of each HOLC tier from A to D compared with the city’s summertime surface temperature overall. The researchers also calculated the average percentage of impervious land surface cover (heat-absorbing materials, such as asphalt) and the percentage of tree cover for each HOLC category, because those factors have been shown to influence local temperature.

The results were clear: In 94 percent of the cities studied, summer temperature anomalies followed a stepwise function, with each successive HOLC category from A to D significantly warmer than the category above. Historically D-rated areas were an average of 2.6 degrees warmer than A-rated areas, but the difference was as high as 7 degrees in some cities. Cities with large temperature anomalies demonstrated corresponding disparities in land use: The warmer neighborhoods had more impervious land surface cover and less tree cover than their cooler counterparts. Hoffman’s results line up with previous studies, which show that historically redlined areas have higher rates of health complications, including asthma, kidney disease, and preterm birth, all of which are associated with extreme heat.

HOLC maps were typically accompanied by notes that influenced which neighborhoods were flagged in A-rated green and which ones in D-rated red. Greenlined neighborhoods were described as shady and leafy, whereas redlined areas were considered built-up and smelly—and were already recognized as hotter. “Think of the trajectory of those two different categories, where wealth and power is concentrated in one and completely deprived in another,” Hoffman says. Accessing the amenities that keep a neighborhood cool, such as parks and tree-lined streets, and pushing back against freeways, strip malls, and factories that produce heat, takes economic and political power. According to Hoffman’s hypothesis, the practice of redlining codified this long-standing power imbalance—already visible in the temperature differences between neighborhoods—and paved the way for decades of urban renewal and infrastructure projects that further reinforced it.

Although overt forms of redlining are now illegal, Hoffman’s research highlights the degree to which redlining continues to affect people’s health and lives. The most important corrective step, he says, is boosting public investment in historically redlined communities. Multiple cities, such as Boston and Baltimore, are already using redlining maps to identify areas for tree planting; in the future, prioritizing historically redlined neighborhoods for urban greening and park building projects could help mitigate the disproportionate effects of heat waves.

According to Hoffman, the inequalities we see today are the direct result of communities of color being left out of decision-making processes. If historically redlined communities are empowered to develop and implement their own responses to extreme heat waves, Hoffman expects to see better, more just results in the future. There are also steps that anyone can take to help, such as pushing local governments to build more bus shelters so that people who rely on public transportation have shade from the heat.

“Climate change can feel very far away in time, in space, and in power,” Hoffman says, “but there are things that we can do today in our own backyards that will keep people safe and healthy.”