Editor’s note: This year What If?, our annual collection of scenarios, considers the future of health. Each of these stories is fiction, but grounded in historical fact, current speculation and real science. They do not present a unified narrative but are set in different possible futures

IN NEW DELHI, India’s capital, the roads have begun to melt. Temperatures in the city reached 49.3°C (120.7°F) as the deadliest heat wave in the country’s history entered its third week. It was even hotter in the south, where temperatures rose above 50°C, peaking at a record-breaking 52.1°C in the town of Markapur, Andhra Pradesh, on June 23rd. But the centre of the crisis is the city of Chennai, where hospitals are buckling in the face of heat-related illnesses. The worst scenes were outside Jawaharlal Nehru Hospital, where 11 people died from heat exposure while queuing.

The real killer in Chennai is the humidity. The combined measure of heat and humidity in air is the “wet-bulb temperature”—the lowest temperature to which something can be cooled through evaporation from its surface. In dry air, even at temperatures well above 37°C—human body temperature—people can sweat to cool down. But at wet-bulb temperatures of 32°C and higher, “it becomes unsafe to perform most physical labour,” says Moetasim Ashfaq, an atmospheric physicist at the Oak Ridge National Laboratory in Tennessee. Few people can survive a wet-bulb temperature above 35°C. In the past decade, wet-bulb temperatures in Chennai have regularly risen above 32°C. But for much of the past week, wet-bulb temperatures have repeatedly crossed 36°C—a fatal level.

Initially, the deaths were concentrated among those who could not escape the heat, especially the city’s homeless population. But the high energy demand of air conditioning soon stretched the city’s power grid to its breaking point, resulting in city-wide blackouts lasting hours. That exposed anybody without a generator to the deadly heat. According to official statistics, 17,642 people have died from heat-related causes in Chennai since the heat wave began—more than a third of the 52,348 deaths reported nationally. Those are shocking statistics, but all the more so when juxtaposed with the experience of nearby Hyderabad.

The two cities have similar temperatures in their immediate surroundings and similar populations of around 10m people. But Hyderabad has registered just 26 deaths, fewer than any other big city in southern India. The comparison has not been lost on officials. “Whatever Hyderabad is doing, it’s working,” said Ramanatha Srinivasan, the mayor of Chennai, this week. Hyderabad is in fact one of the leading cities in heat-wave mitigation—not just in India, but globally. So what exactly is it doing differently?

Heat of the moment

India’s current heat wave is the result of an unlucky confluence of factors. Unusually strong north-westerly winds blowing in from Pakistan kept moist air from the Bay of Bengal from drifting inland onto the subcontinent. As a result, the region’s customary pre-monsoon rain showers failed to materialise this year, leaving much of southern India drier and more arid than usual. The unprecedented length of the current dry season—the monsoon usually starts at the beginning of June—has compounded matters. A strong El Niño effect has turned the heat up even further.

It was a similar, though much less deadly, heat wave 26 years ago that jump-started Hyderabad’s efforts to reduce city heat. In 2015 at least 585 people died as blistering temperatures enveloped the city and the surrounding state of Telangana. It was a turning point. Hyderabad has since become a crucible of experimentation in urban heat-reduction techniques.

In most heat waves, the highest temperatures are experienced in cities, towns and other urban areas. One of the simplest ways to reduce heat is to boost the reflectiveness, or “albedo” of city surfaces—especially the roofs of buildings. The more solar radiation is reflected away from a city, the less is absorbed to be re-radiated as heat. To that end, the city government of Hyderabad tested a “cool roofs” programme in low-income neighbourhoods in the city in 2017.

The results were striking. Indoor air temperatures in homes fitted with a cheap, white polyethylene roof coating were, on average, 2°C cooler than similar homes without them. As a result, in 2019, Telangana committed to a statewide cool-roof programme, making cool roofing mandatory for commercial and government buildings, and in low-cost housing provided by the government. By 2027 more than 8,000 buildings in Hyderabad had been fitted with cool roofs.

These efforts received a further boost in 2030, when India’s National Rural Employment Guarantee Act (NREGA)—a rural jobs guarantee—was extended to poor urban neighbourhoods. Under the programme, the city of Hyderabad put unemployed residents to work painting shacks, shanties and other makeshift structures with a lime-based whitewash. More than 250,000 homes have been made heat-resilient in this way. The city also borrowed an idea from South Africa, planting 2.5m trees, which reduce surface and air temperatures by providing shade and through the evaporation of moisture from their leaves.

An analysis by the University of Hyderabad found that all these initiatives have collectively reduced the average outdoor temperature in the city by 0.9°C since the early 2020s. That may not sound much, but this small change can make an enormous difference, as events in Chennai are now demonstrating. Until the 2030s, Chennai remained largely unscathed by deadly heat waves because of its proximity to the ocean. But in recent years heat waves have increased in frequency and intensity as a result of climate change.

This heat wave is likely to be a harbinger of things to come. Though the world is on track to reach net-zero carbon emissions around 2062, the effects of past emissions will continue to manifest themselves for decades. Deadly heat waves are expected to increase in frequency well into the next century. Scientists have known this for some time. Since the early 2020s experts have warned that deadly temperatures could be commonplace in the Middle East, South Asia and parts of China by 2100. Yet the danger posed by heat waves continues to surprise many policymakers.

Few cities have made interventions of the type undertaken in Hyderabad, and the consequences are now becoming clear. Fortunately, it is never too late to start. The best time to begin adapting may have been 20 years ago, but the second-best time is now.

Full contents of this What If?
Freedom to tinker, October 2029: What if biohackers injected themselves with mRNA?
The other epidemic, June 2025: What if America tackled its opioid crisis?
A tale of two cities, June 2041: What if a deadly heat wave hit India?*
You are what you eat, January 2035: What if everyone’s nutrition was personalised?
iHealthy, September 2028: What if smartphones become personal health assistants?
Mrs Chippy’s benediction, February 2055: What if marmosets lived on the Moon?
Novel treatments, August 2050: What if dementia was preventable and treatable?
Rage against the machine, December 2036: What if an AI wins the Nobel prize for medicine?
Germ of an idea: What if germ theory had caught on sooner?

This article appeared in the What If? section of the print edition under the headline “A tale of two cities”