The Bhalswa landfill, on the outskirts of Delhi, is an apocalyptic place. A gray mountain of dense, decaying trash rises seventeen stories, stretching over some fifty acres. Broken glass and plastic containers stand in for grass and stones, and plastic bags dangle from spindly trees that grow in the filth. Fifteen miles from the seat of the Indian government, cows rummage for fruit peels and pigs wallow in stagnant water. Thousands of people who live in slums near the mountain’s base work as waste pickers, collecting, sorting, and selling the garbage created by around half of Delhi’s residents.

This March was the hottest on record in India. The same was true for April. On the afternoon of April 26th, Bhalswa caught fire. Dark, toxic fumes spewed into the air, and people living nearby struggled to breathe. By the time firefighters arrived, flames had engulfed much of the landfill. In the past, similar fires had been extinguished within hours or days, but Bhalswa burned for weeks. “The weather poses a big challenge for us,” Atul Garg, the chief of the Delhi Fire Service, said, nine days after the fire began. “Firefighters find it difficult to wear masks and protective gear because of the heat.” A nearby school, blanketed by hazardous smoke, was forced to close. In the end, it took two weeks to extinguish the blaze. The charred bodies of cows and dogs were found in the debris.

I have family in Delhi, and have visited regularly over the decades. Each year has always felt hotter than the last. But this spring’s heat wave, which continued into the summer, has been unprecedented in its severity, duration, and geographic expanse. Across much of northern India, where more than a billion people live, temperatures have regularly soared past a hundred and ten degrees, and slightly lower temperatures have often combined with very high humidity—a dangerous combination. “The heat is rising rapidly and much earlier than usual,” Prime Minister Narendra Modi said, in April. “Fire has broken out in many forests, historical monuments, and hospitals.” Indians who work outside—about half the population—have sometimes had to stop in the afternoons, relinquishing their wages; schools and businesses have had to adjust their hours or shut entirely; and farmers have seen their crop yields drop by a third or more. On a particularly hot day in May, the high in Delhi hit a hundred and twenty-one, and overheated birds fell from the sky.

According to the official count, the heat wave has killed around a hundred people. But the true toll is certainly higher: in the summer of 2003, a less severe event killed seventy thousand across Europe. Only eight per cent of Indians have air-conditioning, and many lack reliable electricity, a situation that limits their use of fans and other cooling devices. In 2010, during a heat wave in Ahmedabad, the financial center of the state of Gujarat, officials counted seventy-six heatstroke deaths during the hottest week—but a later analysis of death certificates revealed that there had been at least eight hundred more deaths than usual during that time, some two hundred of them on a single day. Research has shown that, each day the temperature rises above ninety-five degrees in India, the annual mortality rate increases by three-quarters of a per cent. (In the United States, the rate increases by only .03 per cent.)

The dusty road leading to Bhalswa is lined with ramshackle shops and gutters full of stagnant water. When I visited, in May, some sections of the landfill were still releasing angry coils of smoke. In the car, I consulted my phone, which told me that it was a hundred and three degrees outside, with thirty-two-per-cent humidity. Still, when I opened the door, I was stunned by the three-dimensionality of the heat. The sun fried my skin but also somehow roasted me from within. I felt as if I’d swallowed a space heater.

A dirt path wound between tents and shacks. Tattered sheets, hung from wires attached to wooden poles, provided only a little shade. Fat plastic bags full of trash for resale leaned against crumbling brick walls; alongside them were broken chairs, metal buckets, plastic bottles, cracked pots, torn trousers, errant shoes, and a dirty diaper. Two women cooked over an open fire while an elderly man pushed a wooden cart, a young child lugging a sack behind him.

In a small brick hut, a man sat cross-legged amid a thick knot of flies, braiding human hair that he’d collected from the landfill. Half a dozen women in brightly colored clothing, their heads covered in scarves, sat on the floor.

“It didn’t use to be this hot,” Saira, the woman in charge of the group, said. “Before, it felt like it was possible for humans to work the landfill.” Now, because of the heat, they tried to stay out of the sun. “If you see five hundred people working there right now, you’ll see at least two thousand people up there at night,” she said.

“We eat up there, sleep up there sometimes,” another worker added.

Hema, a thin woman in a purple sari, sat on the stairs. “When the sun hits, it feels like your body is on fire,” she said. “I drape a shirt over my head—that makes it feel even hotter. When we come back home, our heads feel like they will explode. We take water with us, but it’s boiling by the time we can drink it.”

The women described headaches, exhaustion, dizziness, rashes, fever. The stench of the landfill—an acrid mixture of excrement and rotting trash—was sickening, they said, but the heat made it hard to tolerate masks. Outside the hut, children kicked a ragged soccer ball. A scrawny dog panted on a mound of refuse. Flies swarmed a heap of dung.

“We are living,” Saira said. “But we are also dying.”

The human body is an exquisitely effective temperature-regulation machine. As your core temperature rises, neurons in your brain’s hypothalamus tell your peripheral blood vessels to dilate; this increases blood flow near the skin, where heat can dissipate through sweat. But the system struggles to keep up when temperature and humidity become extreme. Initially, heat increases the body’s metabolic rate: cells consume more oxygen, your heart rate rises, and your breathing grows rapid. As internal heat mounts, enzymes cease to function and proteins become misshapen. An overheated person might experience dizziness, confusion, inflammation, nausea, seizures, or coma. In the worst cases, the body desperately shunts blood to the extremities in an effort to release heat, in the process starving internal organs of oxygen and causing damage to the gut, liver, nerves, and blood vessels. This is heatstroke; up to two-thirds of cases are fatal.

Even short of causing heatstroke, excessive heat is bad for you. The Paharganj Mohalla Clinic, in central Delhi, is one of more than a hundred public clinics in the city, built by the local government to offer medical care to the poor; it serves the residents of Paharganj, a bustling area filled with budget hotels and roadside restaurants. (“Mohalla” means “neighborhood” in Hindi.) At 10 a.m., when I visited, it was already uncomfortably hot. The sun seemed angry at the dusty earth outside the clinic, a two-room prefabricated cabin. Inside, men, women, and children sat on metal chairs. Some coughed behind their masks, while others slumped over, looking vaguely distressed. A nurse near the reception desk, surrounded by a small crowd, checked people in on a tablet. Nearby, a pharmacist at a table strewn with medications wrote on a clipboard as she handed them out.

Deepika Sharma, the clinic’s doctor, stood in the second room, behind a desk on which a large plastic shield had been installed. In her white mask and purple shalwar kameez, she radiated the easy alertness of a friendly teacher.

A man made a namaste from the other side of the shield. “Thank you, Doctor,” he said.

“Take care,” she replied, as he limped away. “See you soon.” The pharmacist appeared at the door to confirm a prescription for another patient; Sharma checked her notes and nodded.

The clinic was busy, Sharma told me, but not unusually so. She typically sees a hundred patients a day, with each visit lasting about three minutes. As a physician myself—I practice internal medicine at a large academic hospital in New York—I found this pace dizzying to contemplate. On a normal day, I might see fifteen or twenty patients. But Sharma’s task was made easier, she said, by the fact that so many patients had the same problems. Nearly half the people she saw had respiratory issues—such as asthma, bronchitis, and emphysema—which she attributed to the city’s punishing air pollution. (India has among the world’s highest rates of chronic respiratory disease; on the worst days, breathing the Delhi air is equivalent to smoking two packs of cigarettes.) During the heat wave, she said, around a quarter of her patients suffered heat-related rashes, and a fifth presented with signs of dehydration. Inhalers, calamine lotion, oral rehydration salts—these three medications made up the bulk of her prescriptions.

Outside, the temperature seemed to have ticked up a few degrees, and the sun felt more intense than before. A line had formed to get into the clinic. On the sidewalk, a slight man leaned against a bright-green auto rickshaw. I caught his eye as I walked past.

“How’s the heat been?” I asked.

“It’s been very hard,” he said. “We’re in really tough shape.” His wife, eight months pregnant, had been coming to the clinic for prenatal care; they lived on the fifth floor of a building, in a structure made of wood and tarpaulin, and relied on a small fan for relief. I moved with guilty steps to my air-conditioned car. Arun Kumar, my thirtysomething driver, had put a cooler in the back seat, filled with bottles of water and iced coffee. As we pulled away, I reached for some water and looked out the window. A woman was sitting on the back of the green auto rickshaw, looking miserable—the driver’s wife, baking in the sun.

Public clinics refer their sickest patients to public hospitals. One such institution, Civil Hospital, is situated just off a highway in Palwal, a city of a hundred and thirty thousand people outside of Delhi. Brahmdeep Sindhu, its chief medical officer, met me in a large, wood-panelled office on the first floor. With silver hair, a navy tie, and a long white coat, he reminded me of the senior physicians in my own hospital. He welcomed me with a broad smile and directed me toward a couch near his desk.

“The heat has created a rise in physical, psychological, and social symptoms,” he said. “Heatstroke, dizziness, low blood pressure, dehydration, exhaustion—we are seeing these almost daily.” The hospital was on a main road, and often admitted travellers. “The A.C.s in cars can’t function at such high temperatures,” he said. “The cars become extremely hot. The other day, three passengers came in—one person vomiting, one person with fever, one person so weak he couldn’t move.”

A psychiatrist by training, Sindhu was especially troubled by a surge in psychological distress caused by the heat. “Patients with bipolar disorder, with schizophrenia—they are really struggling,” he said. The Intergovernmental Panel on Climate Change, in its 2022 report, highlighted for the first time the dire mental-health effects of a warming planet: anxiety, grief, stress, post-traumatic stress disorder. In recent years, heat has been linked to a rise in suicides among Indian farmers, whose livelihoods have been imperilled.

“Those are the clinical aspects of behavioral problems,” he said. “But we should talk about the non-clinical aspects as well. People can’t concentrate at work. When they’re stopped at red lights, they’re ready to fight with each other. The high temperatures are causing irritation and aggression. I’ve never seen anything like it before.”

He took a sip of tea and offered me a cup.

“We are moving to a hotter, more dangerous world,” he said. “We have damaged the environment so much. Now the environment is damaging us.”

Since 1980, the number of heat waves—defined by the World Meteorological Association as periods of at least three consecutive days during which temperatures significantly exceed the historical average—has increased by a factor of fifty around the world. Climate models suggest that India’s current heat wave was made thirty times more likely by global warming, and they predict that another degree of warming could lead to thirty-two times as many extreme heat waves as there were at the end of the twentieth century, each lasting five times as long. India, which is home to one in every six people on earth, has emitted just over three per cent of the planet’s greenhouse gases—and yet it will be among the nations most ravaged by the climate.

It’s tempting for Americans and Europeans to conceive of extreme heat as a scourge for poor, faraway people without reliable electricity or air-conditioning. But climate change has inflicted lethal heat in Western countries, too. This summer, Europe has experienced record heat. Last week, temperatures reached a hundred and eight degrees in Spain and a hundred and seventeen in Portugal; more than seventeen hundred people died of heat-related causes. In the U.K., where the temperature reached a hundred and four for the first time ever, the government issued a “red warning” for extreme heat, urging people to stay indoors so as not to risk “serious illness or danger to life.” In France, triple-digit temperatures contributed to raging wildfires. Meanwhile, heat waves already kill more Americans on average than any other extreme weather event. Last summer, in the Pacific Northwest, temperatures soared thirty degrees above normal, reaching a hundred and eight in Seattle, a hundred and sixteen in Portland, and a hundred and twenty-one in British Columbia. Streetcar cables melted, roads buckled, crops burned, and schools were closed. In just three weeks, the heat wave caused nine billion dollars’ worth of damage, and more than fourteen hundred people died.

Rewari, a rural town about fifty miles north of Delhi, is known for its ornamental brass work, but its economy is powered by agriculture, mostly mustard and wheat. By early afternoon on the day I visited, the temperature had climbed to a hundred and two, and the humidity made it feel ten degrees hotter. A community center—an imposing concrete structure—stood by a dirt road near the highway, surrounded by bicycles and scooters; local farmers had gathered inside to get out of the sun during the hottest part of the day.

Indoors, the farmers milled around in the heat. Thin, rugged, and stoic, they wore white kurtas or solid-color long-sleeved shirts with dark cotton pants. After a while, they climbed the stairs to the second floor and sat in rows of plastic chairs. Fans whirred overhead; a young woman offered water and fruit juice.

“It’s good to be indoors at this hour,” an elderly man rasped to a younger one sitting next to him.

“You had better stay indoors at all hours,” the other replied, joking.

These past months, they said, had seemed like the culmination of a crescendo that had been building for years. Because of the high temperatures, their children no longer worked the farms; they themselves now stayed inside when the sun was at its most punishing, usually between eleven and three. They estimated that, as a result of heat and drought, their crop yields had fallen by a quarter to a third; for some crops, there was no point in harvesting them. Heat stress had also reduced the fertility of their cows, further compounding their financial problems.

I asked the farmers how they planned to contend with a hotter future. What if this year represented not the end of a crescendo but the start of one?

The question seemed unwelcome. “We don’t know what we’ll do,” one said.

Across India, thousands of government-run Industrial Training Institutes offer vocational instruction to poor students. Just down the road from the community center was an I.T.I. that occupied a pale-yellow building near a small lake and a Hanuman temple. Each morning, scores of girls, most of them seventeen or eighteen years old, travelled from surrounding towns and villages to attend classes in dressmaking, basic information technology, and the servicing of electronics. This institute had partnered with the American India Foundation, a nonprofit focussed on social and economic mobility; some students were training to become electric-vehicle technicians.

Inside, sunlight streamed into a wide atrium that opened onto hallways to either side. In one room, girls in gray-and-navy uniforms sat in rows, hunched over small cream-colored sewing machines, talking happily. Across the hall, a bespectacled man stood at a whiteboard and lectured about computers. About forty girls sat behind bulky black monitors, backpacks and textbooks scattered at their feet. The instructor asked a question. A girl stood to attention and rattled off the answer with ease.

“Very good,” the teacher said.

“Sir, thank you, sir,” she said, and sat down.

I walked to the front of the room, and the class rose in unison. I apologized for interrupting and said that I wanted to learn about their experiences during the heat wave.

“I fainted, sir,” a girl in the first row said. “My blood pressure has been low. I had to take a week off school. People in my village don’t have enough water to drink.”

A student in the back stood up. “My dog used to be happy,” she said. “Now he has rashes everywhere—he barely moves, barely eats.”

“Has it been hard to study?” I asked.

A murmur of assent rippled through the class. “I used to study for two, two and a half hours a night,” another girl said. “Now I can barely concentrate for half an hour. I don’t have the strength. I’m exhausted just sitting here in class.”

I asked how many had experienced symptoms of heat stress—light-headedness, fatigue, nausea, fainting. Nearly every hand shot into the air.

Outside, I felt a leaden tiredness. Sweating, uncomfortable, I saw with relief that Kumar’s car was gliding toward me. Inside was a tenuous oasis, available to Kumar only when I joined him; he turned off the A.C. while he waited, because gas was too expensive. “The car quickly becomes an oven,” he told me.

On the way back to the city, we stopped at the Karol Bagh market, which is among the busiest in Delhi. Its roads are lined with stores selling auto parts, clothes, shoes, sweets, spices, bangles, and electronics; street venders push carts full of lassi, pani-puri, mangoes, and samosas. A little after 4 p.m., it was a hundred and six degrees, with thirty-two per cent humidity. The driver of a white Maruti Suzuki sedan haggled with a roadside parking attendant, who oversaw a thrumming array of double-parked vehicles, while the asphalt radiated heat back into the air. A few venders had gathered on a corner. I approached a short man with a sharp nose and neatly parted black hair.

“Is it hard working in this heat?” I asked.

“You can’t imagine,” he said. He rolled up his pant legs and showed me an angry rash on his shins. Some days, he said, he worked in temperatures as high as a hundred and sixteen degrees; he estimated that, because of the heat wave, business had dropped by ninety per cent.

“Who wants to come out in this heat?” a woman in a red sari demanded. “We can barely stand it ourselves.” She sold rice and vegetables; because she didn’t have a refrigerator, any food that she didn’t sell rotted. A few weeks ago, she’d felt dizzy and lost consciousness, twisting her ankle as she fell. Now she walked with a limp.

Another man, in khaki pants and a white shirt, stepped forward. Originally from Rajasthan, he had gray hair and few teeth. “I feel sick,” he said. “On really hot days, I just keep vomiting. I think, O.K., I should at least put up an umbrella over my cart. But the stores behind us won’t let us. They say it blocks the view of their storefronts.”

They all agreed that it was the worst heat they could remember.

Midway through my trip, I met up with my cousin, a computer scientist, and his wife, a schoolteacher. They live in a middle-class neighborhood in Gurgaon, an I.T. hub on the border of Delhi and Haryana. We went to a kebab restaurant, which was pristine, spacious, and powerfully air-conditioned—I almost wished I had a sweater. The host led us up a regal staircase, and we sat down and ordered beers amid scents of cardamom, fenugreek, and garam masala. Over the years, I’d suffered my share of Delhi Belly, but I couldn’t help myself: we ordered butter chicken, tandoori prawns, dal makhani, garlic naan, and a mixed grill. We ate and reminisced about past visits. Once, when I was a kid, my cousin had taken me to get a haircut, and I’d told the barber that I wanted my hair styled like my favorite Bollywood star’s; he’d misheard which one, and I ended up nearly bald.

After dinner, I walked the streets, passing families in tattered tents under overpasses or sleeping out in the open. It was still hot, in the eighties and muggy. Half-clothed people struggled to sleep on roasting pavement. Extreme maximum temperatures grab the headlines, but high minimum temperatures are perilous, too. Normally, the body cools off during sleep; hot nights disrupt that return to equilibrium, and heat deaths spike when nighttime temperatures fail to drop below eighty-five degrees—a regular occurrence for much of this spring in northern India.

The world will become even less hospitable to poor people in the decades ahead; the degree of danger they face depends, to a great extent, on the behavior of wealthier people who are, for now, shielded from the worst effects of climate change. Activists talk about “climate justice,” a view that takes into account the fact that the countries that have contributed the least to global warming will suffer its effects first, and more profoundly. The first step in adopting such a view may be “climate recognition”—an acknowledgment of the pain we inflict through the burning of fossil fuels.

India is doing what it can to adapt to a painful new reality. After the deadly 2010 heat wave in Ahmedabad, the municipal government there developed a heat-action plan. It launched a public-awareness campaign, implemented early-warning procedures, bolstered health-system capacity, trained medical professionals to recognize heat stress, and increased supplies of potable water in temples, parks, and other public places. Similar plans are now active in cities around the country, including in Delhi, and are thought to avert twelve hundred deaths a year. Cities have begun pushing for the installation of “cool roofs,” made of light-colored, reflective surfaces, especially in slums. On some station platforms, Indian Railways has added misting systems; the tiny water droplets absorb heat, reducing ambient temperatures by as much as thirteen degrees. (Misting is less effective in humid conditions.)

Poor air quality combines dangerously with heat. In 2020, nine of the world’s ten most polluted cities were in India, and Delhi remains the world’s most polluted capital. Radio spots now encourage people to plant trees, which improve air quality, lower air and surface temperatures, and provide shade. The government of Delhi has also introduced a slew of incentives to speed a transition to electric vehicles: in 2019, only one per cent of India’s new-vehicle purchases were electric; in March, 2022, more than twelve per cent were. During my visit, a hundred and fifty electric buses were put into circulation. In the last decade, Delhi has also closed its two remaining coal plants—although eleven more sit just outside the city limits.

On my final day in India, the temperature was a hundred and eight. The U.V. index—a measure of how damaging the sun’s radiation is to human skin and eyes—sat at eleven-plus, its maximum value. Earlier, I’d talked with a cardiologist named Rajat Arora, the managing director of the Yashoda Hospital and Research Centre, a busy three-hundred-bed private facility just east of Delhi. “It’s never been this bad,” he told me, of the heat. Patients were complaining: “They say, ‘The A.C.s are failing, do something, we’re so uncomfortable.’ But what can I do? It’s so hot that even the A.C.s can’t handle it.” The heat had disrupted the construction of new facilities at the hospital. “When you personally cannot stand such heat for five minutes, how can you expect workers to be out there for eight, twelve hours a day?” Arora asked. “I told them, ‘Just hold off, this is not a safe time.’ ” Every other day, a parent brought in a newborn baby with hyperthermia, a condition with symptoms that include fever, lethargy, and difficulty feeding. Arora’s own mother-in-law had been admitted to a hospital in Kanpur, another of India’s hottest cities, suffering from fatigue and dehydration.

I arrived at Yashoda in the early afternoon. Outside the hospital entrance was a tangle of honking cars and scooters. People who had to stand in the sun did so with umbrellas or cloths over their heads. Arora, who is six feet three in a country where the average man is around five feet eight, cut an imposing figure in the hospital lobby; with his black-rimmed glasses, well-fitting khakis, shiny brown loafers, and crisp blue shirt open at the collar, he could have been a Bollywood actor playing a doctor.

Arora gave me a tour of the hospital. It was state of the art, with MRI machines, pet scanners, and cardiac-catheterization labs. Everywhere we went, the waiting rooms were full. The temperature was mostly comfortable, but, in certain corridors, stairwells, and rooms, the A.C. wasn’t working effectively, and an overpowering heat stole in.

At some point during my visit to India, I’d started making a list of groups that are especially vulnerable to severe heat. It grew longer each day. Young children, older adults, and the poor; people with disabilities and chronic conditions; farmers and those who depend on their crops; students who take tests in sweltering schools or play soccer on scorching fields; construction workers in California and Kuwait, Mississippi and Mali; a middle-class couple in Delhi, London, or Seattle whose electricity fades during a brownout; a well-off Texan who overheats when the power grid fails. The occasional hot spots in Arora’s hospital were an unsettling reminder that even those with the means to run the relay race of heat avoidance—air-conditioned home to air-conditioned car to air-conditioned office—will eventually have to drop the baton. You can’t shut out climate change the way a gated community shuts out crime, litter, or traffic. It’s a delusion to think that we can harm the whole planet without suffering too much ourselves.

We stopped to rest on a small couch near an intensive-care unit. Arora offered me a bottle of water and introduced me to Brijesh Prajapat, the head of the pulmonology department. Prajapat had a keen, youthful face but an old-school demeanor—he seemed like the kind of person who prefers memorizing facts to looking them up on the Internet. He wrapped a stethoscope around his neck and told me that many Indians were now being diagnosed as having emphysema in their early forties. The extreme heat, he explained, had worsened their conditions. “Humans increase their respiratory rate to maintain an appropriate body temperature,” he said, and that can be challenging for people with poor lung function. For reasons that aren’t entirely clear, higher temperatures also seem to cause more coughing, breathlessness, and sputum production among such patients. During the heat wave, the number of people admitted to Yashoda with chronic obstructive pulmonary disease, or C.O.P.D., had more than doubled.

For many doctors I spoke with, the heat had become the boiling water in which they swam. It wasn’t entirely different from how covid-19 had recalibrated my expectations back home: as a doctor, I’d grown used to a higher level of death and disease. If a patient in an Indian hospital arrived with heat rashes and a fever of a hundred and four, it was obvious that heat was the culprit. But extreme heat also compromised health in subtle, pervasive ways—dehydration, kidney injury, infectious disease, cardiovascular and respiratory problems—that might have knock-on effects down the road.

I walked through the doors of the pediatric I.C.U. Alarms were pinging loudly; a child screamed behind a curtain and a nurse rushed past. Two pediatricians were completing their rounds, reviewing X-rays, speaking with one family and then another. The sun blazed through a window at the far end of the room.

Behind me, a toddler rested after suffering a febrile seizure—a frightening, uncontrollable shaking, driven by heat and infection. Up ahead, a woman tended to a teen-age boy, his head wrapped in a bloodied bandage. In a nearby bed, a young girl lay sleeping. I traced the I.V. tubing from her arm up along the pole next to her. A bag of fluid hung at the top, dripping its contents one hydrating drop at a time. I thought about how a warmer planet would affect her ability to study, work, and live, and about how little time we have to change course. The dripping of the I.V. felt less like a remedy than a countdown.