A version of this story appeared in Science, Vol 381, Issue 6665. View it here.
MUMBAI, INDIA—In Lal Matti, an informal settlement of tin-roofed homes in this coastal city, summer can be unbearable. Before the monsoon rains arrive, temperatures can top 37°C, with humidity at a sweltering 95%. The hot, muggy afternoons are particularly oppressive, says Tamrunissa, a young woman who lives with her extended family in a large room above a shop, accessed by a ladder. Despite running three small ceiling fans all day—the family can’t afford an air conditioner—she often gets headaches from the heat, and her children fall sick. Her elderly father, who has diabetes and high blood pressure, retreats to an alcove below the apartment to nap. “He can’t handle the heat up here,” she says.
Sometimes, Mumbai’s heat becomes deadly. In April, on a day the temperature reached 36°C, 11 people sitting through an hourslong outdoor ceremony died from heat stroke. At least 20 others were hospitalized.
Indians are increasingly at risk from extreme heat and heat waves that are more frequent and last longer. The country saw a 55% rise in heat-related deaths between 2000–04 and 2017–21, according to research published last year in The Lancet. Global warming will continue to elevate the risks, particularly in India’s fast-growing urban areas, where heat-absorbing tar and concrete boost temperatures. By 2025, cities are expected to house more than half of India’s population, now at 1.7 billion.
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“It is a very clear story of increasing hazard,” says social scientist Chandni Singh, a climate researcher with the Indian Institute for Human Settlements. “The combination of climate change–driven heat and urban heat, generated by buildings and roads, means that many more cities are going to suffer from extreme heat.” The most affected, she adds, will be the millions of urban poor who can’t afford air conditioning or labor outdoors.
Historically, India’s disaster planning efforts focused on addressing cyclones and floods. It wasn’t until 2015 that officials designated heat waves as a natural disaster at the national level. But now, India is emerging as a laboratory for a world coping with climate change, as researchers try to better understand the risks posed by heat and find ways to prevent the worst outcomes.
In some cities, researchers have been working with authorities to improve emergency response plans that guide decisions on when to issue heat alerts and open cooling centers. Some are also experimenting with inexpensive ways to retrofit dwellings to make them cooler. Others are studying how land use and architecture influence temperatures in different neighborhoods, producing insights that could reduce the impact of future urban expansion.
It’s urgent to turn such research into action, says Rajashree Kotharkar, an architect and urban planner at the Visvesvaraya National Institute of Technology who leads one of the country’s longest running heat studies. Science must “evolve, keep pace,” Kotharkar says. “We aren’t mathematicians doing something abstract that will find application a hundred years down the line. Whatever research we do should also provide some solutions for this particular time.”
KOTHARKAR'S URBAN HEAT research project is based in one of India’s hottest cities: Nagpur. The city of 3 million sits in the semiarid plains of central India, where summer temperatures can reach nearly 49°C.
One morning in May, on a narrow lane lined with apartment buildings in Nagpur’s Mahal neighborhood, researcher Parikshit Dongarsane clambered up a wall to retrieve a sensor attached to a slender pole. A colleague then plugged the sensor, which had been recording temperatures and humidity levels for a month, into a laptop and downloaded its data.
It was just the first stop on a long, blazingly hot day in the field. Equipped with hats, water bottles, and even packages of oral rehydration solution, the young researchers retrieved data from 14 sensors across the city. Kotharkar set up the network as part of a decadelong effort to map how temperatures vary across the city and gain insight into how the built environment shapes microclimates.
Some of the results have been surprising. One might assume, for instance, that daytime temperatures would be highest in relatively dense and treeless neighborhoods like Mahal, which sits near the city’s center. But the sensors often record the highest daytime readings in the less built-up and slightly greener areas on Nagpur’s outskirts. That’s likely because more open land is directly exposed to the Sun there, Kotharkar says. Those fringe areas are usually cooler at night, she adds. Downtown, denser buildings block trapped heat from radiating into the sky, and the nights remain hotter.
That daily pattern has implications for human health, Kotharkar notes. Research has shown that unusually warm nights can contribute to health problems, especially when they follow hot days, because the body gets no respite from the heat. And consecutive hot days and hot nights are becoming more frequent in India, studies have found.
Kotharkar’s team is also examining how other meteorological factors—such as humidity and wind speed—affect heat stress. And it is looking at how infrastructure and socioeconomic factors can shape vulnerability to that stress. Poorer people may not be able to afford air conditioners or other cooling devices, for example, and the elderly may have ailments that make them more susceptible to heat. (Most of those who died in Mumbai’s April heat tragedy were older.) Some neighborhoods lack the water supplies or health clinics that can help tackle heat-related illness. Nagpur’s outlying areas might experience cooler nights, for example, but they also have less access to health services.
How people perceive heat can also be important, Kotharkar’s team has found. During a heat wave last year, researchers set up mini–weather stations on sidewalks and interviewed passing pedestrians, to see how objective data such as temperatures lined up with the subjective experience. Even at temperatures of 45°C, many of those interviewed expressed little concern. “People would say: ‘It’s always been like this,’” Kotharkar says.
The nonchalance is worrying, she says, because it suggests people are not aware of the dangers posed by heat—and might not pay heed to official heat wave warnings. One take-home message, Kotharkar says, is “people who live in hot regions are not always able to see heat as a risk.”
SUCH FINDINGS have important practical applications, particularly in helping cities develop or improve heat wave response plans.
These heat action plans, or HAPs, have been proliferating in India in the past few years. In general, an HAP spells out when and how officials should issue heat warnings and alert hospitals and other institutions. Nagpur’s plan, for instance, calls for hospitals to set aside “cold wards” in the summer for treating heatstroke patients, and advises builders to give construction laborers a break from work on very hot days.
The first HAP was introduced in 2013 in the arid western Indian city of Ahmedabad, with the help of local scientific institutions as well as the nonprofit Natural Resources Defense Council (NRDC). That plan’s success—a study published in 2018 estimated it had saved at least 1190 lives—spurred the creation of more. India’s disaster management agency is now working with 23 of India’s 28 states to develop HAPs.
But implementation of existing HAPs has been uneven, according to a March report from the Centre for Policy Research (CPR), a prominent Indian think tank. It reviewed 37 plans adopted by cities, states, or administrative districts.
Many lack adequate funding, it found. And their triggering thresholds often are not customized to the local climate, says Dileep Mavalankar, director of the Indian Institute of Public Health, who has been closely involved in Ahmedabad’s HAP. In some areas, high daytime temperatures alone might serve as an adequate trigger for alerts. Ahmedabad, for example, set its threshold for initial alerts at 41°C based on data showing that heat-related deaths began to climb at that point. But in other places, nighttime temperatures or humidity might be as important a gauge of risk as daytime highs.
Mumbai’s April heat stroke deaths highlighted the need for more nuanced and localized warnings, researchers say. That day’s high temperature of roughly 36°C was 1°C shy of the heat wave alert threshold for coastal cities set by national meteorological authorities. But the effects of the heat were amplified by humidity—an often neglected factor in heat alert systems—and the lack of shade at the late-morning outdoor ceremony. Ironically, the state of Maharashtra, which includes Mumbai, had adopted its own HAP just 2 months before the tragedy. It advised shifting outdoor events to early mornings on hot days.
To help improve HAPs, Kotharkar’s team is working on a model plan that outlines best practices and could be adapted to local conditions. Among other things, she says, all cities should create a vulnerability map to help focus responses on the populations most at risk. (The CPR study found that only two of the 37 HAPs it examined identified the most vulnerable populations.)
Such mapping doesn’t need to be complex, Kotharkar says. “A useful map can be created by looking at even a few key parameters.” For example, neighborhoods with a large elderly population or informal dwellings that cope poorly with heat could get special warnings or be bolstered with cooling centers. The Nagpur project has already created a risk and vulnerability map, which enabled Kotharkar to tell officials which neighborhoods to focus on in the event of a heat wave this summer.
HAPs shouldn’t just include short-term emergency responses, researchers say, but also recommend medium- to long-term measures that could make communities cooler. In Nagpur, for example, Kotharkar’s team has been able to advise city officials about where to plant trees to provide shade. HAPs could also guide efforts to retrofit homes or tweak building regulations. “Reducing deaths [in an emergency] is good target to have, but it’s the lowest [target],” Singh says.
IN THE LONG RUN, cooling India’s cities will mean changing the way they are built. One possibility is to look to the past, when structures were designed to insulate people from their local climates.
In Nagpur, Kotharkar likes to show her students—and visiting reporters—a one-story, 300-year-old mansion in Mahal. It is built of brick, stone, and wood, not the concrete and plastics that make nearby modern apartment blocks absorb and radiate heat. The walls are thick, delaying heat gain. A double-layered roof provides extra shade and ventilation, and a courtyard allows hot air to float into a natural sink—the sky. Smaller windows on the external walls, along with the courtyard, help create differences in air pressure that keep air moving and cool temperatures.
On a May morning, the outdoor temperature is 39°C, but the ground-floor rooms feel comfortable and the interior courtyard, with its fountain flowing, is almost cool. “When the temperature outside is highest, at say 2 p.m., the inside isn’t so hot,” Kotharkar says. “And by the time the inside temperature peaks, the outside has started cooling down.”
In recent years, India’s housing ministry has produced detailed guidelines for constructing climate-resilient buildings, which include some principles drawn from traditional architecture. But the recommendations remain largely on paper, with builders and local government slow—or reluctant—to change their practices. To make cool building easier, Kotharkar’s dream is to create a predictive model, perhaps even an app, that local officials can use to forecast the impact of a new building or development policy on local temperatures—and plan for mitigation if needed.
Given the rapid pace of development in India’s large cities, Singh warns that “there’s a narrowing space” to erect cooler architecture. Still, Kotharkar sees hope in smaller towns where growth is just beginning to accelerate. “We’re so focused on the big cities,” she says, “we forget the smaller ones are still manageable.”
FOR MANY LOW-INCOME Indians living in cities, the possibility of moving to a newly constructed, climate-resilient residence is remote. That is why some groups are retrofitting existing homes.
The most popular retrofit involves covering roofs with white reflective paint. Ahmedabad was the first city to experiment with this technique in 2017, as part of its HAP. The project, which was supported by NRDC and the Mahila Housing Trust, a local women’s group, has had encouraging results. One assessment found that homes with painted tin roofs were at least 1°C cooler than nonpainted ones, and more expensive roofing technologies produced cooling of up to 4.5°C.
In 2020, Ahmedabad expanded its cool roof program to cover 15,000 low-income homes. This year, the desert city of Jodhpur launched a similar initiative, and in April the state of Telangana pledged to create 300 square kilometers of cool roofs by 2028.
In Mumbai and other cities, meanwhile, a private firm called cBalance is testing other low-cost passive cooling interventions for poorer households. Among those selected for the experiment this year: Tamrunissa’s family in Lal Matti.
To cool her house, cBalance workers lined her ceiling with aluminum foil, providing insulation against the heat from the tin roof. But the bigger intervention was the installation of a second roof, made of panels of recycled plastic waste, a little above the original. From her kitchen window, Tamrunissa can reach out to pull a lever to close the panels in the morning, creating a shield against the Sun, and then open them again at night, allowing heat to radiate.
“It’s like wearing a hat,” says environmental engineer Vivek Gilani, the firm’s founder. “The [extra] barrier doesn’t allow heat to accumulate in the roof, and what does accumulate gets released into the sky at night.”
The new roof has made a real difference for Tamrunissa and her family. This summer has been more comfortable than usual, she says. “We can sleep better.”
A philanthropic foundation provided the funding for those improvements. But finding money to pay for cooling solutions remains a challenge, says Abhiyant Tiwari, NRDC’s lead climate resilience and health consultant in India. The special paint, for example, can cost several thousand rupees, a large outlay for poorer families. To save money, some have tried painting their roofs with regular white paint. But it has a different formulation and doesn’t work, says Savitaben who worked on a cool roof project in New Delhi with the Mahila Housing Trust and goes by one name.
In Mumbai’s slums, Gilani and cBalance have encountered obstacles beyond funding. Shading panels can’t be installed on some roofs because they are crisscrossed by power lines or being used for other purposes. They’ve also had to ensure that roof modifications don’t interfere with the waterproofing measures that protect homes against the intense rains of the annual monsoon.
Despite these challenges, researchers and engineers remain motivated to find cooling solutions for India’s cities, especially for their most vulnerable inhabitants. “Cooling is not a matter of luxury,” Tiwari says. “It’s a matter of justice.”