Ask a Scientist: How Can We Cool Off Urban Heat Islands and Protect the Most Vulnerable?

August 17, 2021
Shutterstock: Logoboom
Elliott Negin
Senior Writer

We recently received a question from a UCS supporter in Denver about how to best address the urban heat island effect. That’s when cities, because of their buildings, pavement and other impervious surfaces that absorb and retain heat, experience significantly higher temperatures than surrounding rural areas.

“We often hear about planting trees to combat the urban heat island effect, but here in the West, we also are concerned about our diminishing water supply, and trees—especially recently planted ones—take a lot of water,” Andy J. wrote. “Has there been any research into the drawbacks of planting thirsty trees here in the West? Are there other, less water-dependent, ways to fight urban heat islands?”

I forwarded this question to Dr. Juan Declet-Barreto, UCS senior social scientist for climate vulnerability, who is quite familiar with the situation Andy J. described. Before moving to the Washington, D.C., metro area, Declet-Barreto lived in Phoenix for 17 years. While he was there, he obtained his bachelor’s and master’s degrees in geography and his doctorate degree in environmental social sciences at Arizona State University. His doctoral work focused on understanding the impact of climate change-related extreme heat on cities, and he recently co-authored a report on how to make neighborhoods at the northern end of Manhattan more resilient to summertime heat.

EN: Before we get to the specifics of Andy J.’s question, it would be helpful if you gave us some background on the urban heat island effect writ large. I imagine that, given the growing climate crisis, it has been getting worse over the last several decades.

JDB: Cities have dense concentrations of asphalt, cement and other surfaces that absorb solar heat during the day and radiate it back into the environment. They make cities hotter than the more rural, less-developed areas outside of them. Scientists call this the urban heat island effect because, when you look at a map of temperatures, cities will appear as hot “islands” surrounded by a cooler “ocean” of lower temperatures in the surrounding areas. But large differences in temperatures don’t just occur between cities and rural areas. There are also dramatic temperature differences within cities due to the variety of their landscape features, which range from buildings and roads to trees and parks. These are called micro-urban heat islands.

EN: Heat kills more people every year on average in the United States than any other weather-related event. Besides being life-threatening, extreme heat can cause a number of illnesses.

JDB: That’s right. Exposure to extreme heat can lead to illnesses that range from a mild heat cramp or rash to more severe—and often fatal—heat stroke that can be triggered once the body has lost the ability to cool itself by sweating. In particular, the very old and the very young; people with chronic medical conditions, physical or mental disabilities; and people who work outdoors are at higher risk for severe heat-related illnesses. With a changing climate, however, even people who are generally healthy can be at high risk of illness or death after a prolonged exposure to high temperatures.

EN: A new study by Climate Central ranked more than 150 U.S. cities with the most intense urban heat islands in the country. The top five were relatively big cities—New Orleans, Newark, New York City, Houston and San Francisco—and surprisingly, there were smaller cities among the top 20, including Bend, Oregon; Erie, Pennsylvania; and Burlington, Vermont. But as you mentioned earlier, there major differences in the heat island effect within individual cities themselves, depending on the neighborhood.

JDB: Absolutely. And these pronounced temperature differences within cities are largely due to longtime environmental injustice. Low-income neighborhoods, especially ones where people of color have been forced to live due to discrimination, generally suffer from higher temperatures than wealthier, whiter neighborhoods.

I coauthored a new report that makes recommendations for five neighborhoods in northern Manhattan trying to cope with extreme heat. For example, surface temperature maps show that one of those neighborhoods—Inwood—has more areas where summertime temperatures exceed 100°F than wealthier Manhattan neighborhoods that have more trees and parks. Most of the residents of Inwood and the four other neighborhoods we looked at—Washington Heights and East, Central and West Harlem—are low-income or people of color or both, and many cannot afford air conditioning.

Another recent study, which reviewed 108 urban areas across the country, found that formerly “redlined” neighborhoods where mostly people of color have historically lived are hotter than non-redlined neighborhoods with mostly White residents, some by nearly 13°F.

Why the disparity? There are both physical and socioeconomic reasons. The main physical mechanisms are lack of tree cover and an abundance of pavement and other impervious, heat-absorbing surfaces. The social and economic mechanisms that created this inequity were institutional racism and urban “renewal” policies that excluded people of color.

Redlining is a textbook combination of both. Redlining was a racist policy because the only criteria used to assess a mortgage lender’s financial risk of lending to people in redlined neighborhoods was the color of their skin. And that had two broad, pernicious effects that persist to this day. First, the systematic denial of transgenerational wealth accumulation to people of color by not allowing them to purchase a home, which is how most US households became middle-class and were able to build financially stable lives for their families. In other words, redlining held many generations of people of color captive in poverty.

Second, redlined neighborhoods were also deprived of the public investment that makes neighborhoods desirable: good schools, well-built houses, safe streets, and—of course—trees and parks. Instead, zoning laws and other state and local policies made those neighborhoods a dumping ground for factories, freeways and other urban development that threaten public health. And all of that infrastructure drives up temperatures.

Nearly every one of the redlined neighborhoods in the 108 urban areas cited in the study I just mentioned are hotter than non-redlined areas!

EN: Let’s take a look at Denver, Andy J.’s hometown. Climate Central found that Denver suffers from summer temperatures that are as much as 23°F higher than nearby rural areas, that summers there are on average 4.9°F hotter than the rural areas, and that the city experiences 26 more days per year with temperatures above 90°F than the rural areas. That makes Denver the No. 3 city in the country with the biggest gap between urban and rural temperatures. But what about the differences among Denver neighborhoods?

JDB: Denver is just a microcosm of what we find in most, if not all, major US cities. The lower the income and the higher the proportion of people of color in a neighborhood, the more intense the heat island effect. In Denver, those disadvantaged areas are in the northern and western sections of the city. Likewise, as in many other US cities, residents of poorer neighborhoods in Denver have less formal education and are less likely to speak English, less likely to have air conditioning, and less likely to own a car, so they have to rely on public transportation.

EN: As Andy J. pointed out in his question, Denver and other Western cities suffer from routine drought, and one of the major solutions federal and state authorities recommend to reduce the heat island effect is to plant more trees, which require a lot of water. Besides planting trees, what other approaches could Western cities take to address this problem?

JDB: We have to remember that arid cities have been implementing xeriscaping—a landscape style favoring trees, grasses and shrubs that require minimal water—for years to conserve water. But research has shown that xeriscaping has limited benefits to mitigate the urban heat island effect, and in some instances it may increase urban warming.

Global and regional climate change are creating multiple pressures, and as we can see with water conservation and heat-island mitigation, some solutions seem to be at odds with each other. So cities suffering from drought need to evaluate the relative effectiveness of the main accepted strategies to combat the urban heat island effect. They include cool roofs, which are designed to better reflect sunlight; green roofs, where a layer of vegetation reduces a roof’s temperature; cool pavements, which reflect more solar energy than traditional paving; and trees and vegetation, which lower surface and air temperatures and provide shade.

Now, to get to Andy J.’s question. To answer it directly, we would need more locally based research to understand what mix of urban heat mitigation techniques would be best for Denver, because the interplay between them and local climate conditions is complex and can have important implications for long-term sustainability. There are no one-size-fits-all solutions. That said, maybe we can get a clue from Phoenix, a city with similar regional drought challenges. It suffers from a substantially dangerous urban heat island effect that I have studied for some time now.

The city of Phoenix has an ambitious climate action plan with multiple heat-mitigation goals. For example, residents of impacted communities, scientists and policymakers are working together to develop “cool corridors” of shaded, walkable areas that protect residents from extreme heat when they walk to and from school, work, the grocery store, and other establishments. The plan also calls on developers to use more reflective materials in new infrastructure to increase what’s called “albedo”—the amount of radiation a surface reflects—and thus reduce the amount of solar heat that buildings absorb. Finally— and for me this is the sine qua non of equitable urban heat island mitigation—Phoenix plans to achieve a 25 percent tree and shade canopy in pedestrian areas in neighborhoods with the most vulnerable residents.

So while we can’t provide a blueprint for Denver, we can say that Denver—as well as every other city across the country—would do well to follow the Phoenix example by working together with local residents—especially the most vulnerable—and their advocates to come up with socially equitable solutions to protect those at highest risk of heat-related illness and death.

About the author

More from Elliott

Elliott writes about UCS-related topics for a range of news organizations. Prior to joining UCS, Elliott was the Washington communications director for the Natural Resources Defense Council, a foreign news editor at National Public Radio, the managing editor of American Journalism Review, and the editor of Nuclear Times and Public Citizen magazines.