Updated at 11:40 a.m. ET on August 17, 20222.
When the heat index—a figure that takes into account both temperature and humidity—reaches 80 degrees, the National Weather Service advises Americans to take caution. When it reaches 90, that advisory gets bumped to possibly dangerous; at 100, it’s likely so. At a heat index of 125 or above, the National Weather Service warns of “extreme danger” and describes its effect on the body concisely: “heat stroke highly likely.”
Until now, that kind of extreme heat has been limited to relatively small parts of the country. But that might not always be the case. According to a new heat model released yesterday by the First Street Foundation, a nonprofit organization that assesses future climate risk, more than 100 million Americans live in counties that are expected to experience at least one day with a heat index of 125 degrees or above in the next 30 years. That’s 13 times more than the 8 million people who are forecasted to experience such world-melting temperatures this year, the group notes. And it helps to illustrate just how much of the country will need to start preparing today for more regular periods of intense heat.
The nonprofit, which has previously modeled wildfire and flood risk, predicts that an “Extreme Heat Belt” will form along the Mississippi River, enveloping most of Louisiana, Mississippi, Arkansas, Missouri, Illinois, Iowa, and Indiana—as well as portions of Oklahoma, Texas, Kentucky, and Tennessee.
To create the model, the team used historical weather data and satellite imagery to calculate the seven hottest days at every property in the United States. (The satellite pictures helped them understand land-surface temperatures, which can vary based on what covers the ground—concrete holds more heat than grass.) It then used the United Nations Intergovernmental Panel on Climate Change’s global climate models to analyze 30 years into the future and see how many days an area will hit its current maximum three decades from now. Miami, for example, spends its hottest seven days a year sweltering under a 100-degree heat index. By 2053, the First Street Foundation predicts that the city will hit that temp 34 days of the year—more than a full month’s worth.
How can communities nationwide prepare for more regular extreme heat, as well as more really-really-really-hot-but-maybe-not-125-degree-heat-index days?
For starters, it’s important to understand the nature of the threat. Experts told me that one-off hot days aren’t necessarily the most dangerous, nor are the hottest periods. When temperatures hit triple digits in the Southeast, people seem to adapt their behavior, and the region sees fewer emergency-department visits for heat-related illness, Maggie Sugg, an associate professor in the geography and planning department at Appalachian State University, told me. In Arizona, more people die from heat-related causes outside of heat waves, Ladd Keith, an assistant professor of planning and sustainable built environments at the University of Arizona, told me. This is in part because heat waves represent a smaller portion of the year—but it also helps underscore the danger of less attention-grabbing, more regular heat. And the hottest places aren’t necessarily the biggest problem. Last summer’s heat dome in the Pacific Northwest, which is estimated to have killed more than 1,000 people, underscored how communities in areas that don’t traditionally get blasted with heat can be at particular risk because they’re not equipped to handle it.
Heat’s effects can also be cumulative, becoming worse over long stretches. Our bodies have to work to keep us cool, by making us sweat. The stress of all that hard work can build up, and our bodies need rest—or else our performance will lag. “When we look at workers performing day-long work in the heat, we see a gradual, progressive deterioration in their ability to lose heat,” Glen Kenny, a professor and the research chair in heat strain monitoring and management at the University of Ottawa, told me. “So Monday, they may have a given capacity to lose heat. So while they’re doing the same work on Tuesday, you see a gradual reduction.”
As temperatures rise, people who have flexibility may start to adapt their routine. Sara Meerow grew up in South Florida but now lives in Arizona, where she is an associate professor in the School of Geographical Sciences and Urban Planning at Arizona State University, and where the heat is drier and can hit higher temperatures. “One of the things I noticed when I first moved to Phoenix was that, in the summer, you would hardly see anyone walking around in the middle of the day,” she told me. “But you go outside at 10:30 at night, and there’s tons of people out jogging, walking their dogs. It was a very strange thing to see that at first.” Sugg likewise said that some occupational workers in the South, such as groundskeepers, have changed their work hours to start at 6 a.m.
But not everyone can shift their schedule, and not everyone can afford air-conditioning. Heat tends to prey on society’s most vulnerable. My colleague Vann R. Newkirk II has argued that, globally, heat will be the defining human-rights issue of the 21st century. And older people are particularly at risk: Our body’s ability to lose heat declines by about 5 percent every decade, Kenny explained.
Preparing for more extreme heat will require rethinking how cities are planned. Researchers have known for more than 200 years that cities tend to be hotter than their rural counterparts because of the urban-heat-island effect. Essentially, the materials used in cities often trap more heat from the sun than a natural landscape. Additionally, people collectively use more energy in these places while doing things like driving and running AC, which release waste heat. And within cities themselves, all of this extra heat isn’t equally distributed, in part because neither is tree coverage: “We know from research that the hotter areas of the cities tend to be lower-income, marginalized, historically redlined communities. And so there’s a lot of equity implications with urban heat, where those exposed to the hottest temperatures are often those least able to address it through their own kind of personal resources,” Keith explained.
To counter, cities can construct more parks, plant vegetation, build with more reflective or lighter-colored materials, and push to be more sustainable overall energy-wise, Keith told me. Meerow pointed out that making cities more walkable or encouraging public transportation can have a bonus heat-mitigation effect because fewer people release energy while driving. Developers can also design with heat in mind, she said, by considering a building’s orientation or adding mechanical shade structures, while city officials can make sure their power grid is resilient in case of an extreme heat event.
And when heat inevitably arrives, cities have options for managing it. They can provide cooling centers (with backup sources of power, just in case) and build out their heat-alert communication systems, with a focus on reaching the most vulnerable. They can also educate the public on the dangers ahead of time. At the state level, officials can consider labor laws that protect those who work outside.
In the short term, Kenny stresses the importance of using air-conditioning to save lives, despite its environmental impact: “It’s like a seat belt, right? If I’m going to protect [someone] from a crash, I need to have the best protection possible: air-conditioning for the most vulnerable—without any question.” That doesn’t, he said, mean blasting it all summer; he argues that people should save it for heat events. Kenny also believes that experts can start informing people of other home-based heat-mitigation strategies, such as putting temporary shading on their windows, only using rooms that have less exposure to the sun, or creating living spaces in cooler parts of the house, like a basement.
Recently, Miami, Phoenix, and Los Angeles each appointed a chief heat officer. And if a heat belt is looming, difficult summers won’t be limited to the Southern half of the U.S. With large swaths of the country—including cities such as Indianapolis, Chicago, and Kansas City—under threat of extreme heat in the next few decades, more places might want to consider doing the same.
This article originally mischaracterized how the heat index is calculated.