“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. ♦