When the heat index — the temperature multiplied by humidity — hits 80 degrees, the National Weather Service advises Americans to exercise caution. When it reaches 90, this hint is encountered on possibly dangerous; at 100 it probably is. From a heat index of 125, the National Weather Service warns of “extreme danger” and describes the effect on the body concisely: “Heat stroke very likely”.
Until now, this type of extreme heat has been limited to relatively small parts of the country. But that may not always be the case. More than 100 million Americans live in counties expected to experience at least one day with a heat index of 125 degrees, according to a new heat model released yesterday by the First Street Foundation, a nonprofit organization that assesses future climate risks or more will experience the next 30 years. That’s 13 times more than the 8 million people expected to experience world-melting temperatures this year, the group notes. And it helps illustrate how much of the country today must start preparing for more regular bouts of intense heat.
The nonprofit, which has previously modeled wildfire and flood risks, predicts an “Extreme Heat Belt” will form along the Mississippi River, covering most of Louisiana, Mississippi, Arkansas, Missouri, Illinois, Iowa and Indiana — as well Portions of Louisiana – Surrounds Oklahoma, Texas, Kentucky and Tennessee.
To create the model, the team used historical weather data and satellite imagery to calculate the seven hottest days for each property in the United States. (The satellite imagery helped them understand land surface temperatures, which can vary depending on soil cover — concrete retains more heat than grass.) The United Nations Intergovernmental Panel on Climate Change’s global climate models were then used to analyze 30 years into the future and see how many days in three decades an area will reach its current maximum. Miami, for example, spends its hottest seven days of the year below a 100-degree heat index. By 2053, the First Street Foundation projects the city will reach that temperature 34 days a year — more than a full month.
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?
First of all, it is important to understand the nature of the threat. Experts have told me that one-off hot days aren’t necessarily the most dangerous, nor are the hottest periods. When Southeast temperatures hit triple digits, people seem to adjust their behavior, and the region sees fewer emergency room visits for heat-related illnesses, Maggie Sugg, an associate professor in the department of geography and planning at Appalachian State University, told me. In Arizona, more people are dying from heat-related causes outside of heat waves,” Ladd Keith, assistant professor of planning and sustainably built environments at the University of Arizona, told me. This is partly because heatwaves make up a smaller part of the year – but it also helps underscore the danger of a less attention-grabbing, more regular heatwave. 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 not traditionally prone to heat can be particularly vulnerable because they are not prepared for it.
The effects of heat can also be cumulative and worsen over long distances. Our body has 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 – otherwise our performance will lag behind. “If we look at workers who work all day in the heat, we see a gradual, progressive deterioration in their ability to lose heat,” said Glen Kenny, professor and research chair in heat stress monitoring and management at the University of Ottawa me. “So on Monday they may have a certain capacity to lose heat. So as they do the same work on Tuesday, you see a gradual reduction.”
As temperatures rise, people who are flexible can begin to adjust their routine. Sara Meerow grew up in South Florida but now resides in Arizona, where she is an adjunct professor in Arizona State University’s School of Geographical Sciences and Urban Planning and where the heat is drier and temperatures can reach higher. “One of the things that struck me when I moved to Phoenix was that in the middle of the day in the summer, you hardly ever see anyone walking around,” she told me. “But you go outside at 10:30 p.m. and there are a lot of people jogging and walking their dogs. It was very strange to see at first.” Sugg also said some Southern workers, such as groundsmen, had changed their hours to 6am
But not everyone can shift their schedule, and not everyone can afford air conditioning. Heat tends to prey upon society’s most vulnerable. My colleague Vann R. Newkirk II has argued that heat will be the defining human rights issue of the 21st century worldwide. And older people are particularly at risk: our body’s ability to shed heat decreases by about 5 percent every decade, Kenny explained.
Preparing for more extreme heat requires a rethink of how cities plan. Researchers have known for more than 200 years that cities tend to be hotter than rural areas due to the Urban Heat Island effect. Essentially, the materials used in cities often retain more heat from the sun than a natural landscape. Additionally, people in these places collectively use more energy while doing things like driving and running air conditioners that release waste heat. And within the cities themselves, all that extra heat isn’t evenly distributed, in part because tree cover isn’t either: “We know from research that the hotter areas of cities tend to be lower-income, marginalized, historically weakened communities.” . And so there’s a lot of impact in urban heat, where those exposed to the hottest temperatures are often those least able to manage it through their own personal resources,” explained Keith.
To counteract this, cities can build more parks, plant vegetation, build with more reflective or brighter materials, and push to be more energy-efficient overall, Keith told me. Meerow pointed out that making cities more walkable or promoting public transport can have an additional heat-reduction effect, as fewer people release energy when 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 ensure their power grid is resilient in the event of an extreme heat event.
And when heat inevitably arrives, cities have ways to manage it. They can deploy cooling centers (with back-up power sources just in case) and scale up their communication systems for heat alerts, with a focus on reaching the most vulnerable. You can also educate the public about the dangers in advance. At the state level, officials may consider labor laws that protect those who work outside.
In the short term, Kenny emphasizes the importance of using air conditioning to save lives despite its impact on the environment: “It’s like a seat belt, right? If I will protect [someone] before an accident I need the best possible protection: air conditioning for the weakest – without question.” That doesn’t mean, he said, that you blow up the whole summer; He argues that people should save it for heat events. Kenny also believes experts can begin to educate people about other home heat control strategies, such as: like a basement.
Recently, Miami, Phoenix and Los Angeles each appointed a chief heat officer. And if a heat belt emerges, difficult summers won’t be limited to the southern half of the United States. As large parts of the country — including cities like Indianapolis, Chicago, and Kansas City — face extreme heat threats in the next few decades, more places should consider doing the same.