Why rising night-time temperatures pose a growing risk to cities

Relentless night-time heat puts billions at risk in growing megacities

By David Fickling

 

Most people in reasonable health could probably complete a 100-meter run without serious ill effects. Get them to jog non-stop for 24 hours, and they might be in serious danger.

 

That’s a useful way to think about how urbanization on our rapidly warming planet is risking the well-being of billions of people.

 

We’re used to measuring dangerous heatwaves in terms of day-time highs, such as the scorching 48.2 degrees Celsius (119 Fahrenheit) in a town in India’s Uttar Pradesh province last month. We should probably be more worried about the 32.5 Celsius night-time low in Delhi the following week, though. The sprint of one sweltering afternoon is survivable for most people. The marathon of days on end with no night-time respite can be fatal. 

 

 

Scientists are only just starting to understand this phenomenon, and data on the subject is still sparse. But it’s all pointing in one direction. A 2023 study of 25 million deaths in Japan between 1973 and 2015 found that mortality was as much as 10% higher on hot nights, even after controlling for day-time heat. Another found such conditions raised the risk of death in parts of Switzerland by as much as a third. The same effects have since been observed in the US and 28 East Asian cities. When day-time and night-time heatwaves coincide, leaving no opportunity to cool down, the effects could be even worse. 

 

The comparison with endurance athletes isn’t a flippant one. The toll on the body of heat and exercise are quite similar: Sweating increases, the skin flushes, blood vessels dilate, the heart beats faster, and dehydration accelerates as our systems work to dump excess internal warmth into the atmosphere.

 

Like an endurance athlete deprived of recovery time, a person exposed to constant high temperatures without the opportunity to cool off during sleep finds their reserves run down, day after day. As the cardiovascular system weakens, the chances of stroke or heart attack rise markedly.

 

The risk is greatest in the burgeoning megacities of South Asia, Southeast Asia and Africa, where urban populations are set to grow by about a billion people over the coming decades.  

 

In the tropics and hot subtropics, overnight temperatures are already high and set to get higher. Worse still, cities are growing fastest where incomes are lowest. New housing isn’t going to be built to the best standards of well-ventilated, sun-shaded passive cooling, let alone come with air conditioners installed.

 

Take India’s Ahmedabad, where one notorious 2010 heatwave killed at least 1,300 people. The city has pioneered painting roofs white, a cheap technique which can lower indoor temperatures several degrees Celsius by reflecting the sun’s rays away. But even that simple expedient only appears to have been implemented on a minority of roofs.

 

Worse still is the way that urbanization itself is exacerbating the problem. Cities in the developing world typically expand by absorbing neighboring farmland: felling trees, filling in wetlands and paving soil, and then progressively building over even the small pockets of nature that remain. Bengaluru was once famed for its lakes and water-tanks, essential infrastructure for millennia in Indian cities that can go through months without rain. Since the 1970s, about 80% have disappeared, to be replaced by stadiums and bus terminals. Nowadays, as much as a quarter of the population depends on water tankers to meet their daily needs.

 

That process eliminates areas that can act as temperature regulators and replaces them with concrete and asphalt. These hard surfaces suck up solar radiation during the day and then act as immense radiators overnight, flooding homes and streets with heat just when people need a reprieve.

 

Air conditioners fix the problem for those who can afford them, but dump further excess warmth on those who can’t. In India, the unfortunate latter group comprises about 90% of the population.

 

It’s not too late to mitigate the problem. The greening of global cities has been going on since the 19th century, when London, Paris and New York made building city parks and tree-lined boulevards fundamental elements of urban development. More recently, Curitiba in Brazil and Shenzhen in China made parklands central to their development plans, while Medellin in Colombia, Seoul and Singapore have managed to claw back green space from already built-up urban fabric.

 

Doing the same to the next wave of megacities is going to require levels of civic ambition that are all-too rare in emerging Asia and Africa, however.

 

Delhi, the largest city in the most populous country, can’t even protect its 30 million people from choking pollution caused directly by human activity in and around the city itself. What hope is there that the same politicians will tackle the more insidious threat from a whole planet dumping carbon dioxide into the atmosphere? If you think of the future of night in the city, don’t think of peace. Think instead of sweats, sleeplessness, and a racing heartrate.   

Disclaimer: This is a Bloomberg Opinion piece, and these are the personal opinions of the writer. They do not reflect the views of www.business-standard.com or the Business Standard newspaper