“It’s happening all across the state,” said coach Rene Hernandez, with teams switching to predawn hours to avoid afternoons that are markedly hotter than several decades ago. Hernandez rescheduled his preseason workouts when he became Chapin’s head coach in 2007, and he’s likely to do the same for the full season next year.

Traditional after-school practices are just becoming too risky, he fears, and coaches are getting smarter about protecting players. When he played in the 1970s, Hernandez remembers, “there weren’t water breaks. . . . Water was weakness.”

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Across the Southwest, people have long made accommodations to the heat, but climate change and urban development are forcing far more considerations. An increasing number of cities face extreme heat for much of the summer, with highs surpassing 100 and even 110 degrees for weeks at a time. Even in the final days of August, Phoenix was sweltering at 107 and San Antonio at 104.

Such relentless, triple-digit temperatures — the equivalent danger of rising seas in many coastal communities — are straining power grids, buckling roads, grounding planes and endangering lives. The Phoenix area reached a dubious record last year: at least 155 heat-related deaths.

“Extreme heat is not just an inconvenience,” said Kim Knowlton, deputy director of the Natural Resources Defense Council’s science center. “It is killing people, and it’s making people sick to a higher and higher degree.”

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Although few people fear that communities throughout the region will become unlivable by 2100, as various projections suggest for parts of the Middle East and Africa, researchers and urban planners say that local governments can’t ignore the threat.

The challenge is what to do. According to David Hondula, senior sustainability scientist at Arizona State University and one of the nation’s leading experts on how to adapt to or mitigate extreme urban heating, many efforts to date “have been disconnected from one another or operated in an ad hoc manner, [so] that it’s really hard to get a sense of the big picture and really hard to understand which are most helpful and which are at least helpful or redundant or maybe even have undesirable trade-offs.”

Los Angeles, where the number of 95-plus-degree days is expected to triple by 2050, is among the municipal pioneers. In 2013, it became the first major U.S. city to require new and remodeled homes to install what is called cool roofing, made from materials of lighter shades such as white, pale gray or tan.

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The mandate took effect in 2016, and officials say at least 18,000 homes across the L.A. basin are now topped with cool roofs.

“L.A. is way out in front,” said George Ban-Weiss, an environmental engineer at the University of Southern California, who calculates that temperatures in the basin would drop by up to 2 degrees Fahrenheit if all buildings and households adopted cool roofs.

The city also has experimented with pavement treatments. Crews covered about a dozen streets in a milky coating, CoolSeal, as part of a pilot last summer to test the substance’s ability to reduce extreme heat. The project found that the streets’ surface temperature dropped by up to 10 degrees, but Ban-Weiss cautions that more research is needed.

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“What people sort of experience isn’t really surface temperature, unless you’re walking barefoot,” he said. “Air temperature plays a more important role in determining a person’s thermal comfort.”

The issues are particularly pronounced in the Southwest’s largest metropolitan areas given the “heat island” effect caused by pavement and construction, which reflect heat instead of allowing it to be absorbed into the ground. As a result, temperatures are often several degrees warmer than those outside the city — and sometimes more than 20 degrees warmer at night.

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“Urbanization has been the dominant driver of regional warming in many of the heat belt cities,” Hondula said. In cities with some of the greatest growth, overnight temperatures have risen as much as 10 degrees over the past several decades. “That’s a much larger [change] than what we estimate the effect of global warming to be over that time period.”

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Phoenix is already one of the hottest cities in the country, as well as one that is warming the fastest. Six years ago, it received a grant from the nonprofit organization Cities of Service to tackle rooftops on city buildings. Volunteers helped paint white reflective coating on the targeted sites, and the results showed that it reduced air-conditioning costs, energy use and carbon emissions.

Today the coating is standard for any new city project. “When a new roof is constructed on a building, a cool roof goes in,” said Michael Hammett, Phoenix’s chief service officer.

And for the past six months, this time backed by a Mayors Challenge grant from Bloomberg Philanthropies, city officials have gathered data for a first-of-its-kind program to make Phoenix “HeatReady” through education, public communication, infrastructure, housing and emergency services.

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They now have a tree-shade master plan that has helped to plant 500 desert-friendly trees in neighborhoods with little shade — and temperature monitors at some sites to determine if the temperature impact can be measured. As an experiment several weeks ago, the city installed misting sprayers at a public bus shelter to see if they would effectively cool people waiting. Increased ridership would be an added bonus.

“We need to move on this. We need to show that we’re moving on this,” Deputy City Manager Karen Peters said. She acknowledges that the climatic trajectory could put the city’s economic future at risk. “We need to be able to communicate to our residents, our businesses, our visitors, ‘You can navigate this comfortably and safely.’ ”

Hondula, who works with local governments in the Phoenix metro area, said cities throughout the heat belt face numerous barriers as they try to mitigate or adapt to their new normal. Bureaucracies are slow to innovate. Cost-sensitive developers are reluctant to take steps that could add to the price of new construction. But the biggest barrier, he believes, is “community inertia.”

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He blames the chronic nature of heat and the fact that it is an everyday experience. “To some extent [people] accept that they’re going to have a decreased quality of life in some ways because of the heat,” he said. Weather forecasters deal with this constantly. “They issue a public heat warning and the response from many community members is, ‘Thanks. It’s hot. I know.’ And if that is the reaction when we are proposing new interventions or new programs . . . that’s not a particularly good starting point.”

The greatest threats are faced by low-income people who struggle to afford air conditioning and often work outdoors.

“That’s something that a lot of us don’t appreciate: that a lot of U.S. households face energy challenges in paying their bills or having the wherewithal to have adequate heating and cooling,” the NRDC’s Knowlton said.

In the Chihuahuan Desert, El Paso remains somewhat protected by its 3,800-foot elevation. But even here, the climate has shifted dramatically. A century ago, the city averaged about six days a year when temperatures soared past 100. Since 2010, it has averaged almost 26 such days annually — and 44 days reached that mark this year.

Numbers like those are a central focus of Nicole Ferrini’s job as El Paso’s chief resilience officer, someone who is helping the city reshape its streetscapes and change how it operates to blunt the heat island effect.

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Yet local government officials, like many of their counterparts elsewhere, have been reluctant to mandate changes.

“With the private sector, what we can do is start to incentivize them and say, you know, if you’re going to come in and do this type of a project in this footprint, we want to encourage you to build in this way,” Ferrini said.

Adapting takes many forms. Maria Kennedy, athletic director for the El Paso Independent School District, has a granddaughter who is a cross-country runner at one of the city’s high schools. “Throughout the summer, they were running at, like, 5 o’clock in the morning,” she said.

And football coaches like Hernandez concentrate on much more than drawing up and calling plays as they get their athletes ready.

“I emphasize hydration every single day. I write it on my notes every day when I talk to them,” he said. “I talk to them about the color of their urine: ‘If it’s dark, you’re already dehydrated.’ ”