It’s just 8 a.m., and the sun is already punishing. Shahbaz Ali, a 32-year-old ride-hailing motorcycle driver, is drenched in sweat before his day has even begun.

Sleep deprived from regular power outages in the hot weather, he heads off into the choking traffic of Karachi, Pakistan’s main port city, for nearly 12 hours of work. By midday, temperatures can exceed 104 degrees Fahrenheit (40 degrees Celsius), with high humidity along the Arabian Sea pushing the heat index past 115.

“It feels like living in a furnace,” Mr. Ali said one day last month, steering his motorcycle over uneven roads as I rode on the back. “But what choice do I have? If I stop working, my family won’t eat.”

I spent a day with Mr. Ali to see how he copes in the extreme heat.

Karachi, Pakistan’s largest metropolis, is a striking example of how rising temperatures can turn cities into pressure cookers. It was recently ranked among the five least livable cities in the world, with its 17 million residents enduring overlapping crises of toxic air, frequent flooding and poor waste disposal.

Life is especially hard for a majority of the population, including Mr. Ali, who live in the city’s informal, low-income settlements, where residents are packed together in poorly ventilated homes without insulation, greenery or essential services.

The two-room house he shares with his wife and two children is a concrete structure that absorbs heat throughout the day and radiates it back into the night. It lacks any cooling system, making it a suffocating oven.

Chronic shortages of water and electricity compound the hardship. Sometimes, the power is suspended for six to 12 hours a day as collective punishment for electricity theft or unpaid bills in the community. Residents frequently take to the streets under the blazing sun to protest such deprivations.

“It’s suffocating inside the house,” Mr. Ali said. “I think of taking the kids to the beach just to get out, but the humidity there makes it just as stifling.”

Once he heads out on his motorcycle for work, an already physically demanding job becomes a test of endurance in the heat.

On the day I was with him, temperatures reached 99 degrees Fahrenheit, with a heat index of 106. Mr. Ali rode for several hours, completing four trips that covered more than 20 miles, before managing even a brief break. He lay on his motorcycle in a rare patch of shade beneath a pedestrian bridge.

Whenever he got thirsty, he would stop by one of the stalls around the city where residents hand out free cups of water and the sweet red syrup drink known as Rooh Afza.

“There’s little chance to rest. But even if there was, where would I go?” he asked. Much of the city lacks trees, and shade is a luxury found only in affluent neighborhoods or beneath overpasses, where vendors compete fiercely for a sliver of relief from the sun.

Mr. Ali is among the 70 percent of Pakistani workers who are in the informal sector, which includes construction laborers, vendors, security guards and delivery riders.

These outdoor workers face the greatest risks from prolonged exposure to extreme heat and are the most vulnerable to financial insecurity.

On a good day, Mr. Ali earns around $8, a fragile income strained by fuel, rent, electricity and water expenses. If he falls ill from the heat and cannot work, as happened to him one day in April, his family’s monthly budget collapses.

That risk has grown as climate patterns have shifted. The hot season now starts sooner — as early as March — and lasts longer. Heat waves have grown more unpredictable and more intense.

“Every summer feels worse than the last,” Mr. Ali said, wiping sweat from his brow. “It feels like there is no other season left anymore in Karachi.”

The unrelenting heat puts the city on edge, he said.

“People fight over the smallest things these days, such as accidents, delays,” Mr. Ali said. “It feels like everyone’s angry.”

The problem is not just that winter has shrunk. The relief that nighttime once brought has also receded. Temperatures remain elevated overnight, depriving the body of the chance to cool down, and raising the risk of illness.

Since 1960, Karachi’s mean nighttime temperatures have risen by about 4.3 degrees Fahrenheit (2.4 degrees Celsius), while daytime highs have risen by about 2.9 degrees Fahrenheit, studies show. Globally, temperatures have risen by about 1.9 degrees Fahrenheit since 1900.

Highway and infrastructure projects showcased by the city and provincial governments have intensified what is known as the urban heat island effect, which is one cause of high overnight temperatures. Heat-absorbing materials like asphalt and concrete make cities several degrees warmer than nearby rural areas.

Amnesty International recently warned that Karachi faces severe risks from heat waves induced by climate change. In 2015, more than 1,300 people died in the city as temperatures reached 113 degrees Fahrenheit. A year ago, deaths surged as temperatures hit 104.

To address the worsening heat, Mayor Murtaza Wahab recently introduced a climate action plan, featuring cleaner transportation, greener industries and expanded renewable energy. However, experts say that climate policy remains a low priority amid Pakistan’s economic, political and security crises.

Residents are left to fend for themselves.

After a long day laboring under the scorching sun, Mr. Ali returned home exhausted around 8 p.m., only to find that the power was out yet again.

For the next two hours, he sat in the stifling heat, unable to sleep without a fan, like millions across the city.

“We are enduring, not living anymore,” Mr. Ali said as he helped his two children with their schoolwork under the dim glow of a rechargeable emergency light.

“This city is cursed by incompetent governance,” he added. “Someone must fix it before it becomes unlivable.”