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In late May, Jannik Sinner, the world’s top-ranked men’s-tennis player, was closing out a dominant second-round performance against Juan Manuel Cerúndolo, ranked fifty-sixth, at Roland Garros. The French Open, as English speakers tend to call it, takes place during a traditionally splendid time of the year, when Parisians delight in the temperate air. Sinner, a lanky twenty-four-year-old Italian with red hair, sharp elbows, and ground strokes like shotgun blasts, had won thirty consecutive matches and was the overwhelming favorite. After cruising through two sets, he was one game from victory, with a win probability above ninety-nine per cent. Then, as Cerúndolo prepared to serve, Sinner grimaced, took a few gingerly steps, and stretched his apparently cramping legs. They had played for less than two hours, but this May felt like August. Paris lay beneath a heat dome, and the red-clay courts were baking in direct sunlight. Ordinarily, Sinner attacks with explosive energy, crouching on his long legs before springing upward, his feet leaving the ground after each shot. Now he slapped awkwardly at the ball, his shoes seemingly stuck in the clay. He lost the next two games badly, then complained of possible dehydration and called for a medic. After a nearly ten-minute time-out, play resumed. Sinner hobbled on, grimacing, dizzy, stiff, and weak. He lost the set, then the next two, and so the match. “The heat got to him,” an announcer proclaimed, deeming the famously fit Novak Djokovic a new favorite to win the tournament. The next day, Djokovic vomited on the court during the fifth set and lost. Jakub Menšík, a twenty-year-old Czech who reached the semifinals, fared even worse. After winning a four-hour-and-forty-one-minute match, he collapsed onto the clay, where he writhed with cramps for more than four minutes as medics brought ice. “It’s insane to play in this weather,” he said, after being taken to the locker room in a wheelchair. For generations, the French Open has embodied the ideal gateway to the European summer. In 2026, the spectacle of world-class athletes breaking down in sweltering conditions became the latest sign that the Continent’s rapid warming is bringing that ideal to an end. Although climate scientists warned in the nineteen-eighties that extreme heat posed a growing danger, policymakers in Europe didn’t come to grips with it until August, 2003, when a heat wave killed more than seventy thousand people. What those officials still failed to plan for was that Europe would warm at a rate more than twice the global average. A number of regional factors explain why. One is the Continent’s connection to the rapidly warming Arctic. Retreating snow exposes darker ground that absorbs more sunlight; as soils dry, they’re less able to help with cooling the air. Meanwhile, strong winds carry hot air north from Africa, while declining aerosol pollution admits more sunlight, in a process called “brightening.” Together, these effects have set the stage for the fierce summers, destructive wildfires, and suffocating heat domes that now threaten Europe’s health, culture, and economy. Historically, heat has evaded crisis-level attention. Unlike camera-ready disasters—hurricanes, earthquakes, tornadoes, floods, fires—blistering temperatures rarely produce images dramatic enough to go viral. The typical victims are old, poor, and socially isolated residents of disadvantaged areas, making heat waves invisible killers of people largely invisible in public life. In the past five years, however, Europe’s heat has become too lethal to ignore: an estimated seventy thousand heat-related deaths in 2022, forty-eight thousand in 2023, and sixty-three thousand in 2024. This June—Western Europe’s hottest on record—at least ten thousand excess deaths were recorded as Paris topped a hundred and five degrees Fahrenheit, Bordeaux hit 108.5, and Gascony reached 111.7. The Eiffel Tower and the Louvre closed early, the museum explaining that its building is “not sufficiently adapted to climate change.” A nuclear plant in southwestern France shut down a reactor because the river water used to cool it was too warm. Dozens of young people, including a twenty-one-year-old professional footballer, drowned while seeking relief in open water; three French children died in broiling cars. Some Parisians took refuge in air-conditioned hotels. The fact that rooms were available during Europe’s prime vacation season suggests trouble for its nearly three-trillion-dollar tourism industry, a hefty share of the Continent’s G.D.P. More visitors are giving up on summer holidays in cities that feel like saunas and booking “coolcations” in Iceland, Norway, or Denmark. In one survey, more than half of travellers said that they had avoided particular destinations because of extreme weather or natural disasters. A 2023 European Commission study forecast “a clear north-south pattern in tourism demand changes” that “becomes more pronounced for higher warming scenarios,” signalling growing economic consequences for Madrid, Lisbon, Athens, and Milan. “Travelling to European cities is about cultural experience,” Daniel Scott, a scholar of tourism at the University of Waterloo, in Ontario, told me. “Whether it’s Paris, Barcelona, or Rome, you amble around, sightseeing, street shopping, taking a riverboat tour, going to a museum, a restaurant, a café. Most of that time, you’re outside.” Climate change increasingly turns pleasure trips into exercises in escaping pain. “When I’m in Europe, I always notice how many people crowd under trees and shaded areas,” Scott said. “But at some attractions, like the Acropolis, there’s no way to avoid the blazing sun. On especially hot days, they’ve got the Red Cross giving out water, medics pre-positioned because they know many people are going to be in trouble when they come down.” Paris, like the Acropolis, was not built for this weather, and has been slow to adapt. The French are famously wary of air-conditioning, considering it an affront to cultural norms, a threat to architectural beauty, and an accelerant of warming. An Ipsos survey published in June found that seventy-eight per cent of the French regard air-conditioning as “not respectful of the environment.” Only some twenty-five per cent of French homes have air-conditioning, compared with around sixty per cent in Italy and eighty-nine per cent in the United States. The problem is that this summer is only a preview of the treacherous weather that lies ahead. Météo-France projects that France will experience five times as many extreme-heat days by 2050 and ten times as many by 2100. Survivors of the coming climate-changed decades may remember 2026 as mild. Most European governments, unlike the current American Administration, recognize climate change as a profound threat. More mechanical cooling, through conventional air-conditioning or less carbon-intensive technologies like heat pumps and geothermal systems, seems inevitable, even in France. So do electrification and greater investment in renewable energy. Yet nation-states have moved slowly. It’s in cities, where the danger is most urgent, that architects, planners, and officials are racing to lower the temperature of the landscape, inventively redesigning it around three ancient forms of cooling: water, soil, and shade. This July, I arrived in Milan just as Europe’s third heat wave of the summer entered its fiercest stretch, following what meteorologists called an “Omega block”: a high-pressure system had trapped hot air from the African deserts over the Continent, blocking cooler currents. The heat posed more than an urban public-health threat. Across Western Europe, it was creating the conditions for a vast wildfire emergency that in late July displaced more than three hundred thousand people in France and Spain and killed a dozen in Almería province. I had come to meet Stefano Boeri, a Milanese architect and urban planner known for designing buildings, and occasionally whole metropolitan areas, around trees. First, I wanted to see his signature project, Bosco Verticale: two apartment towers, twenty-six and nineteen stories high, whose deep balconies hold enough trees, shrubs, and flowering plants to fill twelve acres of woods, not to mention some twenty species of birds, all of which Boeri considers tenants. I had seen Bosco Verticale in countless photographs and videos, but encountering it at street level was exhilarating and surreal. Squinting up from the muggy sidewalk, I saw the buildings towering like giant redwoods, their leafy, wind-rippled façades startlingly alive. One of Boeri’s assistants arranged for me to visit a compact one-bedroom apartment that Boeri’s firm maintains on the ninth floor of the taller tower. Its windows framed the skyline through tangles of dark, leathery holm-oak leaves and, disorientingly, thick tree trunks rooted in enormous planters and secured against storms by vertical cables. I slid open the heavy balcony door. Light filtered through the foliage, and soft gusts carried scents of olive trees, sage, and rosemary. The air was markedly cooler than on the sidewalks below. Though the morning was already approaching ninety degrees, I wanted only to sit there, listening to birdsong as the building, and I, breathed. That afternoon, Boeri greeted me at his studio, a bright-yellow converted publishing house in a verdant neighborhood near the city center. Casually elegant at sixty-nine, he has silver hair, a warm smile, and a habit of propping his glasses on his forehead. Around him, some sixty-five people hunched over long tables lined with models of new vertical forests. As we walked among them, Boeri told me that he had been fascinated by the symbiosis between people and trees since childhood, when he read Italo Calvino’s novel “The Baron in the Trees.” In it, a twelve-year-old boy, Cosimo, rebels against his aristocratic family, and the Enlightenment’s false divide between nature and culture, by climbing a holm oak and spending his life in the canopy. There, he hunts, harvests, builds a library, falls in love, and writes a declaration of rights for men, women, children, animals, fish, insects, and all vegetation. Boeri hoped to invent buildings that had this feeling of vitality. He explained, “I wanted to make plausible and possible something that apparently was impossible, to create a new perception—a forest that’s a façade, a building, a hundred and twenty metres tall, that lives and breathes and constantly changes as the seasons change and the trees grow.” But his reasons for the vertical forests weren’t merely romantic. “Green buildings have a lot of ecological benefits,” he said. “Façades made of minerals absorb the high temperatures, which means the interior gets hotter, and the streets do, too, because the building gives off heat at night. A vertical canopy keeps the interior several degrees cooler, which means it requires less energy. It takes pollution out of the air, because trees eat carbon and dust.” He added, “Of course, it’s also a habitat. It sustains life.” Though Bosco Verticale is astonishing from the street, the more enchanting view I saw inside is accessible only to those rich enough to reside there. Boeri, who has thirty vertical forests around the world that are completed or under construction, acknowledges the affordability issue. In his studio, he showed me recent projects designed to spread the benefits of vertical forests more widely, including social housing in Eindhoven, a rehabilitation center in Shenzhen, and an arboreal retrofit in Brussels. Internationally, Boeri is best known for building upward. But an even more ambitious project of his extends across the ground. Around 2007, he proposed Metrobosco, a seventy-four-thousand-acre greenbelt meant to encircle Milan with hundreds of thousands of trees. “The tree canopy here covers only fifteen per cent of the city,” he said. In Barcelona, the figure is roughly twenty-five per cent; in New York, twenty-three. “We need more trees to clean the air, but we also need more to cool it,” he said. In most modern cities, “shade is one of our most precious resources, and planting trees—lots and lots of trees—is the best way to get more of it.” A project this ambitious requires substantial government support. Boeri, the son of the acclaimed architect Cini Boeri and a member of an influential Milanese family, had the social capital to launch it. But, after some three hundred thousand trees were planted, the project foundered owing to a shift in political tides. Bosco Verticale’s success gave Boeri another opening. In 2019, he founded Forestami, a civic organization that is aiming to plant three million trees, one for every resident of metropolitan Milan, by 2030. “We might not get that many,” Boeri conceded. “But we now understand that it’s not the number of trees that matters. It’s the number of leaves and the size of the canopy, the amount of surface shade we can produce.” The next afternoon, with “real feel” temperatures in the nineties, I travelled to Parco Nord Milano, a twenty-three-hundred-acre urban forest, where Maria Chiara Pastore, Forestami’s scientific director, and Riccardo Gini, its technical director (and the director of Parco Nord), had agreed to show me some fresh canopy. After a long residential-and-commercial stretch, the park, which was established in the nineteen-seventies on a former industrial site, suddenly came into view. There was a vast green recreation zone, where bare-chested cyclists and runners (Gini remarked on the “Parco Nord uniform”) passed by small fountains and ponds. Pastore, in a patterned white-and-red sundress and sandals, mercifully led us from the hot stone parking lot into a small air-conditioned office, where she shared digital maps showing Milan’s uneven distribution of trees. In the city center, options for adding shade by planting trees are limited. Beyond a few parks and private estates, neighborhoods typically have narrow sidewalks that leave little space for planting. “The reality is, there is not much room left,” Pastore told me. Creating it—by de-paving the streets—inevitably brings the political heat that comes from asking people to cede parking spaces and change driving habits. Still, in four years, Milan has completed more than seventy de-paving projects, mostly small, on blocks where residents pushed for more shade and green space. “People love it,” Pastore said. “It just takes a lot of money, a very long time, and a lot of work to get it done.” Gini, an agronomist who has overseen the planting of some two hundred thousand trees in Parco Nord in the past quarter century, has the tan skin, strong hands, and hearty disposition that are requisite for the job. After we left the office, Gini drove us through a lush expanse of mature trees in grassy meadows to a patchwork of saplings and shrubs. He crouched to inspect young maples and oaks, some only a few feet high, their trunks a finger’s width. “The big trees come from a nursery in Tuscany where the soil is similar to what we have here,” he explained, pointing out trees that were flourishing and smaller ones that, despite ample irrigation, might not survive. “I used to know which species to get, but climate change is making it difficult. Maybe now we need trees from the south.” Pastore, who’s also a professor at the Politecnico di Milano, has colleagues testing which species will grow best in the new, hotter environment. “There are so many variables,” she said. “More heat. More rain. More drought. Insects. New pathogens. The science is hard.” The saplings, their bases encased in white plastic shelters, were not yet tall enough to form a canopy, and, after a few minutes in the direct afternoon sun, we were sweating through our clothes. “Let’s go into the air-conditioning,” Gini said, guiding us into a more mature forest with twenty-year-old hornbeams, whose serrated, deeply corrugated leaves looked almost pleated. Immediately, we felt relief. V. Kelly Turner, an environmental planner at U.C.L.A., told me that “a person standing in the shade can feel twenty to forty degrees Celsius cooler than someone who’s standing in the sun just a few feet away.” (Yes, Celsius.) Here, with the thermometer approaching a hundred, the grove of hornbeams turned a Las Vegas summer day into a San Francisco spring. Planting trees in an established park is the easier part of Forestami’s work. “Now we’re going to show you something more challenging,” Pastore said. We drove seventeen miles west to Vittuone, where Forestami had partnered with a local coöperative and the municipal government to turn an empty industrial lot surrounded by factories, warehouses, and motorbike shops into a wooded area. I asked Pastore why they’d chosen this site and whether they expected visitors. “Maybe someday,” she said. But the trees brought another benefit. “The land won’t become another factory, with more carbon emissions and more heat,” she said. There was not an inch of shade in the first vegetation zone we inspected, and the grass, brittle as hay, crunched beneath our feet. “I think the irrigation system is broken,” Gini said glumly, pointing to three rows of desiccated wild-cherry saplings. He made a phone call, then marched toward a stand of thriving field maples and poplars, their fluttering leaves glossy in the sun, and beckoned us. The canopy was smaller and thinner than Parco Nord’s, but the shade worked its magic, lowering the temperature and delivering a gentle, refreshing breeze. On my last day in Milan, I grabbed a reflective umbrella I’d packed and walked through the neighborhoods around Bosco Verticale. Like Manhattan’s High Line, another influential green-design project, it’s a breathtaking work of charismatic megaflora that has seeded both a new crop of urban parks and a luxury ecosystem of designer shops and corporate skyscrapers. But, in the shadows of Milan’s runaway development, new forests are growing. Nearby, I found another stretch of green: the Biblioteca degli Alberi—the Library of Trees—a postindustrial sprawl that the city helped turn into a twenty-five-acre public park that opened in 2018, complete with twenty-two circular forests and a hundred and thirty-five thousand plants. A few blocks away, on Via Toce, the city had recently melded two small parks by pedestrianizing and de-paving the busy street between them. A nursery school on the green’s edge now has a safe—and, soon, shaded—area for children to play. “We have to get the asphalt and the cars off the streets and return them to plants and people,” Boeri said. “It’s not just about making Milan a better place to live. It’s about making Milan cooler.” For Boeri, such projects bring his life’s work full circle. In Calvino’s novel, young Cosimo remakes his relationship with society by moving into the trees; in Boeri’s vision, society remakes its relationship with nature by moving the trees into the city. From Milan, I chased the heat to Barcelona, where the dry-bulb temperature was slightly lower but the humidity so persistently high that a friend I’d hoped to meet, and two climate experts I’d planned to interview, had fled. “I can’t stop sweating,” my friend messaged me, sheepishly. “People have died on the streets. . . . Be careful.” Another contact noted that the city had just set a heat record, of a hundred and five degrees, but suggested an upside: perhaps I’d avoid Barcelona’s notorious crowds. Of all the problems that heat poses for Barcelona, losing some of the hundred and sixty thousand tourists who typically visit each day ranks low on the priority lists of most officials and residents. Some locals have staged protests, chanting “Tourists go home” while targeting visitors with water guns. But tourism employs more than a hundred and sixty thousand people in Barcelona and accounts for around thirteen per cent of its economic output, which explains why, despite concerns that the city gets too many visitors, more than seventy per cent of residents consider tourism beneficial. At midday, when I arrived on La Rambla, the city’s most heavily visited and photographed corridor, the street had little shade and surprisingly few people. At La Boqueria market, normally jam-packed at lunchtime, I found my way to a famously busy bar counter and took one of several open seats. Later, the emptiness became the subject of conversation. “Everybody’s making way less money,” Diego Penzo Vivas, a veteran tour guide who often works with Americans, told me at a hotel bar near the Plaça de Catalunya. “We’re doing fewer tours, and we have fewer people on the tours. I’m on a WhatsApp chat with a bunch of guides, and the stories are crazy. People are getting heat stress, vomiting, asking to go back to the hotel—not just here, all over Europe. The Vatican, Paris, Madrid.” Penzo added, “We’ve had to start doing night tours. Right now, that’s the main business I’m getting.” After sundown, I met Andreina Seijas at a tapas bar in Eixample. An urban-planning consultant, she helps cities improve life at night. Seijas, who is forty-one, got her doctorate at Harvard and has lived in seven cities. “What I’m starting to see in Barcelona is people trying to organize their schedules the way they do in Seville or Málaga,” she told me. “You stay home with your A.C., close your shutters, take a siesta, and then, after 8 or 9 P.M., you emerge.” For visitors, many of whom can afford late-night bars and restaurants, this merely means avoiding La Rambla in the sunshine. For locals, it can be harder. “Barcelona has great social infrastructure, but many parks, libraries, schools, and senior centers are closed late in the evening, and public transit is limited after midnight,” Seijas said. “Maybe we need to adopt Dubai’s approach toward keeping beaches open twenty-four hours.” “We’re definitely changing how we do things,” the architect Clara Sola-Morales told me, after we switched cafés because the air-conditioning wasn’t working in the first one. “You know the toldos, those textile canopies they hang over the streets in Seville? There’s one planned here now, near the Gothic Quarter, as part of the citywide shadows project.” Sola-Morales has long appreciated Seville’s cooling strategy, which the city, among the hottest in Europe, adopted generations ago. Transplanted to Barcelona, it signals the city’s cultural and ecological transformation. “The heat has changed my practice, too,” she said. “In my entire career, I’d never installed air-conditioning until this year. It was always cross-ventilation, fans, outdoor space. Now I have no option. The first one I put in is in my office.” On another muggy day in Barcelona, the air-conditioning worked perfectly in the city-council offices—a far cry from the steamy Metro stations and streets I’d passed through on my way across the Gothic Quarter, where a toldo would have been welcome. At the entrance, a security guard handed me a roll of paper towels with a sympathetic look. In meetings with the city’s first deputy mayor, Laia Bonet, and its chief architect, Maria Buhigas, I learned that adapting to extreme heat, wildfires, and drought had become a citywide planning effort. Barcelona was a leader in green urbanism long before the term existed. In 1859, the engineer Ildefons Cerdà submitted an expansion plan that broke with convention by making even residential streets twenty metres wide—five for each sidewalk and ten for carriages. Committed, as the formula goes, to “ruralizing what’s urban and urbanizing what’s rural,” Cerdà called for vegetation throughout the streetscape. Over the next century, residents and city agencies planted thousands of trees, including several forests’ worth of London planes, whose enormous branches and broad crowns often meet above the street. Thousands remain, forming green corridors alive with cicadas and birdcalls, reducing traffic noise to a hum. Yet the city’s tree cover falls short of what Cerdà envisioned. In the nineteen-nineties, amid runaway development and increasing traffic and pollution after the 1992 Olympics, there was demand for a return to Cerdà’s model: more planting and, radically, fewer cars. In 1993, Barcelona opened its first “superblock,” a protected area in El Born where automobile access was highly restricted, giving space to people and plants. Two more followed, in Gràcia, in 2005. Between 2016 and 2023, the mayor Ada Colau expanded the initiative and added four linear parks, known as “green axes,” including one almost two miles long. In the renovated areas, newer trees stand beside the heritage London planes, and children kick soccer balls across leafy streets. Most of Barcelona, however, remains ungraced by superblocks or green axes. A quarter of the city has tree canopy; the rest bakes under the midday sun. Extending the superblocks and parks proved difficult. Fire trucks and ambulances need access to every block, and vegetated surfaces cost far more than asphalt to maintain. Then the warming climate intervened. “From 2021 to 2025, we had a terrible drought here,” Bonet said. “At times, we couldn’t plant. We couldn’t irrigate, across all Catalonia. Just when Barcelona was trying to increase the green space, we lost around seventy-five hundred big trees.” Heat and drought were destroying the measures meant to temper them. Trees were hardly the only casualties. Barcelona has suffered thousands of heat-related deaths and illnesses in recent years, concentrated in poor and immigrant neighborhoods, where air-conditioning and green corridors are scarce. “We know that not everyone in the city experiences heat waves the same way,” Bonet said. “So heat is not just an environmental issue. It’s a social-justice issue, because it affects older people, children, and low-income households the most.” For that reason, she said, “we’re prioritizing the areas where vulnerable people are using public space. We’re installing two hundred new shade structures around the city, many in playgrounds. We’re acclimatizing schools with air-conditioning and paying for it with the tourist tax, so that tourism can help the city adapt.” Since Jaume Collboni became the mayor, in 2023, Barcelona has shifted its heat interventions from superblocks and green corridors toward direct protection for those most at risk. In 2025, it expanded its network of Refugios Climáticos, or Climate Shelters, a program hailed by the World Health Organization which has inspired similar measures from Paris to Rome. More than five hundred libraries, churches, stores, and other public and private spaces now welcome anyone who needs air-conditioning, water, or rest. Some offer free Wi-Fi and rest rooms; all appear on a city app with directions. Another program, Radars, connects isolated elderly residents with neighbors who provide companionship and can check in on them during extreme weather. For children, Barcelona is opening small water parks in neighborhood plazas, with showers and water guns. One sultry Saturday morning, I visited one and found some twenty children drenched and delighted as their parents sat contentedly in the shade. Buhigas found particular satisfaction in another initiative: turning certain in-between areas—“leftovers,” as she calls them—into absorbent green spaces. “There’s a variety of them,” she explained in a small conference room. “Some are rooftops. Some are abandoned spaces. Some are the spaces in between buildings.” The city aims to transform a fifth of Barcelona’s leftover land. A colleague projected an image of the Jardins d’Oriol Martorell. “The zoning says that there is a station underground and a green space on top,” Buhigas said. “Do you see the green space?” The screen showed a postindustrial hellscape: a vast expanse of rough concrete bordered by a graffiti-covered wall. “I don’t. You cannot use this space. It’s dead. And not only does it separate two different neighborhoods—it makes both of them hotter by attracting the sun and then releasing all that energy at night.” She went on, “Now look what we’re doing.” The next slide showed the site transformed into a park, with trees, flowers, permeable pavement, and comfortable seating. “This is going to help connect neighbors, and it’s going to help them cool down,” Buhigas said. “The other important thing is that it’s not very expensive.” The plan was to make over fifty acres of land in this way during the next year. “It won’t get the attention of a giant park,” she said, “but it’s going to make a difference.” On my last night in Barcelona, Spain won the World Cup. When the final whistle blew, near midnight, all 1.7 million residents, along with every tourist, seemed to be dancing, sweaty and exhilarated, in the streets. The next morning, sleepless but still buzzing, I met a team from Buhigas’s office in the giant park getting that attention. Parc de les Glòries, which opened in 2025, contains, within its twenty-one acres, around a thousand trees, more than fifty thousand plants and shrubs, rain gardens, playgrounds, sports facilities, a water park, and a broad meadow. Its vegetation is irrigated with groundwater unfit to drink but suitable for plants, a luxury in a drought-plagued city. By nine-thirty, people were stretched out on the soft grass, napping, reading, and revelling in the previous night’s victory. I rode an outdoor escalator up into the park through a cactus garden beside the Glòries Metro station. The morning was already steamy, and much of the park remained exposed to the bright Catalonian sun. The designers and I took refuge beneath an immense pergola, nearly ten metres high and seventy metres long. Its white-painted metal columns resembled the legs of a giant caterpillar, and, under the shaded bands of its densely lined shell, the air was suddenly tolerable. Skateboarders shot past. Day campers raced toward the meadow. The site was a former highway interchange, in what had once been a large industrial area, and as we stood there it was possible, briefly, to imagine Barcelona finding a way to cool itself. Climate adaptation offers no easy victories. Beyond the pergola, we entered a vast playground with soft synthetic terrain and a play “mountain” fitted with long metal slides. Nobody was using it. The designers, after getting so much right, had neglected to shade it. The slides were already hot enough to burn. Spain is hardly alone in struggling with the details. While I was in Barcelona, Tour de France organizers tried to keep riders from overheating by shortening a difficult hill stage and spraying white paint on the road to lower surface temperatures. But the paint allegedly became slippery in the sun, and three riders, including one known as a master of descents, crashed. The weather had cooled by the time I reached Paris, where the city’s new mayor, Emmanuel Grégoire, was focussed on the summer’s three record-setting heat waves, and on those to follow. Moments before our meeting, his press secretary messaged: “Can we do it by bike?” Soon, in the Hôtel de Ville courtyard, Grégoire greeted me in a smart blue suit, brown sunglasses, and jet-black oxfords. Nearby, someone adjusted a bicycle whose seat turned out to be broken. “Take mine,” Grégoire said, offering me his green Voltaire e-bike. “It’s the best bike in Paris.” Our first stop was the plaza in front of city hall. The previous year, Paris replaced part of its heat-radiating stone esplanade with a dense urban forest, including more than fifty large trees and twenty thousand shrubs and plants. Beneath a honey locust, Grégoire removed his sunglasses and acknowledged that, although climate adaptation had been central to his campaign, he had not expected so many heat waves so soon. “We spent the last twenty years trying to transform the city, saying we need to adapt it to climate change,” he said. “Now even people who were in doubt about our plans say, ‘Go faster, faster, faster!’ ” Grégoire, a forty-eight-year-old Socialist who took office in March, is lean and energetic, with short silver hair, a neatly trimmed beard, and a tendency to race ahead, both on his bicycle and in conversation. He inherited a city undergoing one of Europe’s most ambitious ecological transformations. Anne Hidalgo, his predecessor, spent twenty-five years in city government adding bicycle lanes, reducing car traffic, and expanding pedestrian space. Over the past two decades, Paris eliminated tens of thousands of parking spaces, planted more than a hundred and fifty thousand trees, and added thick vegetation to residential and commercial streets. Car traffic fell by more than fifty per cent, bicycle use tripled, and air pollution plummeted. Paris is cleaner, greener, and healthier than it was at the turn of the century—except when it broils. We cycled to the pedestrianized promenade along the Seine’s Right Bank, now a four-mile park abuzz with cyclists, joggers, and strolling tourists. Past a climbing wall, a playground, and a beach-volleyball court, we reached an elevated misting station. A shirtless old man luxuriated in the spray while children ran through it, giggling. “Let’s give it a try,” Grégoire said. I followed him in. “There was real social tension during the heat wave,” he told me, after our clothes had dried with unsettling speed. “People were nervous, anxious, and angry. Dealing with the heat requires improving the social situation, not only health. That’s why we’re making parks like this, and opening new swimming areas.” The most famous new Parisian swimming spot is, improbably, the Seine, which opened to Olympic competitors in 2024 and to the public the next year, after France spent $1.5 billion upgrading the region’s sewer, stormwater, and wastewater systems. But Grégoire was referring to the Canal Saint-Martin, which runs through crowded residential neighborhoods. Long polluted, it is now clean enough for swimming. During the heat waves, young people who normally gathered beside it began jumping in. “I saw videos of policemen running after boys and girls to say it’s forbidden to swim,” he said. “But people needed a way to cool down, and I needed police doing other things. Now, when the water is safe, we have three new swimming areas in the Seine. They’re important for heat waves, but they also stimulate joy and happiness. They improve the quality of daily life.” When I asked what Paris could do for those most endangered by the heat, Grégoire’s tone grew sombre. He described programs similar to Barcelona’s for reaching socially isolated residents, and acknowledged a need for more mechanical cooling for the elderly. Around two-thirds of excess deaths in France’s heat wave this June were of people seventy-five or older. Still, he said, another immediate challenge was protecting children. Nearly all Parisian schools lack air-conditioning or adequate ventilation. “Until this year, our major heat waves happened during summer holidays,” he said. “Now we realize that we will have heat waves in May, June, September, and probably, one day, October. Our schools were not built for this weather, so we need a solution, now.” During Hidalgo’s second term, the city began transforming its six hundred or so public preschools and elementary schools. It pedestrianized streets outside entrances, removed courtyard asphalt, and added trees and shade structures. These changes improved outdoor spaces but offered little indoor protection during the May and June heat waves. Classrooms baked. Students took consequential exams in dangerous conditions, and young children returned home dehydrated or ill. Officials had to choose between keeping schools open and sending children to equally stifling apartments. Parents demanded answers. Since June, Grégoire has launched another round of renovations. “First, we have to keep the heat out of the buildings,” he explained, “so we’re installing shutters to block the sun. Second, we have to improve air circulation. We’re putting in ceiling fans and also creating a specific kind of door”—it’s heavily perforated—“that helps natural ventilation at night. Third, we are doing more to shade the courtyards, taking out the asphalt, which gets very, very hot, and making them cooler places to play.” Confessing to an American bias, I asked whether it would be easier and safer to install air-conditioning, perhaps using heat pumps. Grégoire said he supports mechanical cooling in some settings but doubts that its immediate classroom benefits would justify the additional energy use. After this summer, many parents may disagree. Marine Le Pen, the far-right politician running for President next year, has proposed a “grand plan pour la climatisation,” presenting air-conditioning as essential to the health of children and older people. Several progressives and Green Party supporters I met in Paris feared that the issue would boost her campaign. In 2017, a team of climate scientists warned that, by the second half of the century, France could experience a “mega-heatwave,” and that, by century’s end, it could see temperatures in excess of a hundred and twenty-two degrees. Alexandre Florentin, a forty-year-old climate-policy engineer who grew up outside Paris, encountered the figure in a science paper and couldn’t shake it. “I love what Mayor Hidalgo did for Paris,” he told me as we sat beneath beech and plane trees in the Luxembourg Gardens, one of the city’s coolest outdoor spaces. “When I was a kid, the air was terrible. I had respiratory problems, asthma, and attempted treatments. Now we have trees and plants on the sidewalks, fewer cars. It’s clean and comfortable, and I bike everywhere. I see bees and butterflies on my block. But in Paris we’ve been acting as if cutting emissions meant we wouldn’t have to adapt. Clearly, that was a mistake.” In 2020, Florentin ran for city council as a Green and joined Hidalgo’s coalition. “I suggested to the Greens that we advance an agenda called Paris at 50°C,” he recounted. “We needed to change the conversation so it was not just about mitigation targets, like carbon zero by 2050, because that gave people the impression that they have twenty or thirty years to adapt. We knew about the dangerous conditions in schools before this summer, but we didn’t do enough.” He paused to watch a duck cross behind our bench. “We need to understand that the future is now.” Florentin and a group of colleagues released a report three years ago. “We had two main policy conclusions,” he told me. “First, we cannot adapt everything. We cannot take our whole society and put it in a different climate. We cannot entirely change our infrastructure, our buildings, our ecosystems. We don’t have enough money or time. That means we have to prioritize, to pinpoint the things we can do better. Second, we cannot adapt to anything. There is a hard limit. It could get too hot. I don’t want to move to Norway and be a climate refugee, but some people are already talking about leaving the city. There’s a real risk that we will lose our habitat. That’s what this is about.” Afterward, Florentin unfolded his bicycle and walked with me toward the Seine, pointing out planters, pedestrianized streets, and the historic zinc rooftops that Paris may eventually have to surrender. The day was clear and bright, but we were both aware that, as we spoke, ferocious wildfires, strong enough to generate an unprecedented pyrocumulonimbus, or “fire cloud,” was tearing through France, forcing vast numbers of people from their homes. He asked what promising interventions I had seen elsewhere. I mentioned urban forests, green axes, water parks, shelters, de-paved streets. He took it in, looked at me intently, and asked, “Have you seen any plans for developing the underground?” I told him I hadn’t. “Well, we should think about it,” he said, “because before long we’re going to have three days in a row when the temperature is more than forty-five degrees, or a hundred and thirteen degrees Fahrenheit. When that happens, we’re all going to need a place to hide.” ♦