鈥淢y office felt like a steamer on Monday morning,鈥 wrote Chinese influencer Bi Dao in a social media post in August. He fetched a drink from a supposed cold-water dispenser 鈥 it was 40.8掳C (105掳F). Bi, who lives in Hangzhou, a regional capital on China鈥檚 east coast, decided to roam the city with a temperature gun, pointing it at things to find out exactly how hot they had got. 鈥淭he ground was 72.6掳C, the seat of a sharing bike was 56.5掳C, the handrail in the metro station was 45掳C, even the tree bark was 38.7掳C,鈥 he wrote. He ended his post by thanking Willis Carrier for inventing the air conditioner.
Hangzhou is known for its beautiful lake, large pagoda and rolling green tea farms 鈥 not for heat. But what Bi witnessed was only one of the 60 鈥渉igh-temperature days鈥 鈥 ones that topped 35掳C (95掳F) 鈥 that grilled the city and its 12.5 million inhabitants this year. Hangzhou isn鈥檛 alone. Many cities worldwide are feeling the heat. Things are getting so bad that growing numbers of people face temperatures that are beyond human endurance.
Already, such conditions kill around half a million each year. That will inevitably rise as climate change increases the number and intensity of heatwaves around the globe. Cities are on the front line of this unfolding crisis. And China鈥檚 vast, densely packed megacities are leading the way. As well as providing a glimpse of what we are in for, they offer lessons that could help urbanites everywhere adapt, from their 鈥渧ertical forest cities鈥 to cleverly designed parks 鈥 and new, cool building materials.
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Many temperature records have been broken this year, with astonishing readings reported worldwide, from to . This isn鈥檛 a flash in the pan. The probability of heat extremes has nearly . And things are getting worse. 鈥淚n the past decade or so, there has been a tremendous increase in intensity and frequency of heat extremes,鈥 says , who co-chairs a working group at the Intergovernmental Panel on Climate Change. The past northern summer was the hottest on record, with average temperatures between June and August 0.69掳C higher than the 1991 to 2020 average for those months. , set just last year, prompting UN Secretary-General Ant脫nio Guterres to comment that .
Unbearable heatwaves
, if current climate policies continue, a third of the world鈥檚 population will live outside of a safe temperature range by the end of this century. The study looked at the 鈥渉uman climate niche鈥 鈥 the temperature range most suitable for our survival (see Deadly heat, below). 鈥淲e project that every 0.1掳C of further global warming will expose around 100 million people to extreme heat risks. That is a major concern,鈥 says one of the researchers, at the University of Exeter, UK.
Urbanites will bear the brunt because cities suffer what is known as the urban heat island effect. This is a result of having less vegetation 鈥 which can counter heat 鈥 than rural areas and many buildings, pavements and roads that absorb sunlight and radiate heat. 鈥淭he experience of heat in cities is very different from heat in the countryside,鈥 says at the World Resources Institute鈥檚 cities programme. 鈥淭hink about the way it feels to stand on a hot, unshaded pavement with no breeze and the sun beating down on you.鈥
Cities are already home to over half of us and . This exodus from the countryside is especially pronounced in China, where the shift to city dwelling has exposed over the past two decades. By mid-century, the population experiencing heatwaves there is projected to jump by .
To put things into context, the Yangtze River Delta region in the east of China, which includes the enormous cities of Hangzhou, Shanghai and Nanjing, is now one of the world鈥檚 largest urban clusters. One-and-a-half times the size of the UK and home to 220 million people, this region baked in record-breaking heat for more than two weeks straight from the end of July this year, earning it the nickname . Summers also happen to be humid in many of the nation鈥檚 economic hubs 鈥 including Shanghai, Chongqing and Hong Kong 鈥 due to the monsoon climate. Humidity compounds extreme temperatures because it becomes harder for people to sweat.
Across China, this past summer was a taster of what is to come. Relentless waves of extreme weather scorched many places for weeks on end, leading to a barrage of heat warnings and broken records. July was the . But as early as mid-June, temperatures in the capital, Beijing, were already pushing 40掳C (104掳F). Crossing the city鈥檚 notoriously wide roads in the sun became a physically demanding mission, even for the young and healthy. Iron-willed tourists on Tiananmen Square equipped themselves with sunglasses, parasols and anti-UV face masks as they admired the country鈥檚 political heart in the heat of the day. Increasingly, people wear portable air conditioners around their necks.
Record-breaking temperatures
鈥淔requent extreme heatwaves have arrived earlier than we had expected, and their intensity is getting stronger and stronger every year,鈥 says at Nanjing University. Summers are also lasting longer. On 22 August this year 鈥 a day that should have been chu shu, meaning 鈥渢he end of heat鈥, according to China鈥檚 traditional season calendar 鈥 another regional capital, Chengdu, issued a red alert for a heatwave. During the week to follow, the city of 21 million in the south-west due to surging electricity demand driven by the use of air conditioners.
In China and beyond, exposure to excessive urban heat is, in many ways, an issue of social justice. Vulnerable groups ranging from low-income communities to the elderly are expected to be disproportionately affected. For example, those who toil outdoors will probably as extreme heat becomes more and more frequent. 鈥淭he same heatwave is not the same for all people, such as those with pre-existing conditions or living in certain types of buildings or maybe buildings without air conditioning,鈥 says at the Hong Kong Polytechnic University. And, globally, the countries most affected by extreme heat are often those with lower and middle incomes: research indicates that the five with the are India, Nigeria, Indonesia, the Philippines and Pakistan.
Yet, we have been slow to react to the growing threat, perhaps because heat is an invisible killer. The first thing cities should do, according to Vautard, is 鈥減repare for temperatures that are much higher than current extreme ones鈥. For at the University of Nebraska-Lincoln, a crucial aspect of this is to improve the outdoor environment. 鈥淭here鈥檚 a big difference between a city being survivable and a city being livable,鈥 he says. 鈥淭he question is how do we make sure that, in the face of the rising heat, we still maintain the same quality of living where people, especially children, can go out there and enjoy their time in the outdoors.鈥
Exposure to urban heat is, in many ways, an issue of social justice
One answer is trees. Research shows that they can cool the surface temperature of cities by up to 12掳C (22掳F). And creating urban forests within 1 kilometre of residential areas is the most effective way to reduce heat-related deaths, by at Xi鈥檃n Jiaotong-Liverpool University in Suzhou, China, and her colleagues. She hopes this finding will guide planners in compact cities to identify suitable spots for such planting.
Urban green space
Trees are already being used to remarkable effect in Nanchang, an industrial city and the capital of the landlocked Jiangxi province in south-east China. Its is a 51-hectare wetland featuring dozens of wooded islands on a lake. Completed in 2021, it was built by a team led by Yu Kongjian, the brains behind that can mitigate excess water, as part of the national flood-control programme. It has since become a go-to place for the city鈥檚 6.2 million residents seeking refuge in its notoriously humid summers.
There is good reason for the park鈥檚 popularity. Average temperatures on its forested islands are between . 鈥淚t鈥檚 a unique piece of urban ecological park,鈥 says Wang, who made these measurements with his colleagues in summer 2023. Because it was built as a sponge city park, its elevation is lower than the surrounding urban areas, a feature that doesn鈥檛 just let water drain away, but also allows cooler air to circulate. 鈥淵ou get lower temperature and increased wind,鈥 says Wang.
Other Chinese cities have adopted a variety of initiatives to boost their greenery. Shenzhen, a city of more than 17 million in the south, for example, is . These can by around 1掳C (2掳F) and slash the electricity demand associated with cooling by 8 per cent. And in the province of Guangxi in southern China, a is being built in Liuzhou, featuring high-rise blocks planted with thousands of trees and shrubs.
Increasing urban green spaces is a 鈥渉ighly effective way鈥 for cities to adapt to extreme heat, says Lenton. It has the added benefits of offsetting carbon emissions and improving well-being. And it is relatively cheap. 鈥淭here needs to be targeted investment in increasing green space in the global south and especially the poorer parts of cities,鈥 he says.
Cool roofs
When it comes to cooling indoor spaces, there is another cheap, low-tech solution: 鈥 ones painted white or covered with certain materials, such as light-coloured gravel, to reflect sunlight. 鈥淭here鈥檚 no real cost to it because the materials used for traditional cool roofs are no more expensive than a typical roof,鈥 says at the University of California, Los Angeles. But the benefits are substantial. A cool roof can lower temperatures inside a home lacking air conditioning . In one with air conditioning, it can reduce the amount of cooling needed to keep the space at a comfortable temperature by between 11 and 27 per cent. This is a double positive because air conditioners belch waste heat into the outside air, making cities around during heatwaves, and they are also responsible for around worldwide. Demand for air conditioning is expected to more than triple by 2050, a big worry given that burning fossil fuels is still the primary means to generate power.
We have been slow to react, perhaps because heat is an invisible killer
鈥淯nless all the energy driving air conditioning is clean, you鈥檙e going to have this feedback loop. Hotter temperatures mean more air conditioner use, which means more carbon emissions,鈥 says Raman. He and his colleagues are attempting to curb this vicious circle 鈥 and provide ways to make indoor spaces more comfortable for people who can鈥檛 afford air conditioning 鈥 by developing materials that can cool buildings without using electricity. It began a decade ago, with the invention of a even in direct sunlight because it radiates heat at a wavelength that can escape straight into space without being captured by the atmosphere. Since then, Raman has co-founded a company called to develop a series of other cooling materials. These include a film that can be applied to roofs and outdoor structures and a panel that improves the efficiency of refrigerators.
Raman believes a lot can be done to buildings and outdoor spaces to combat heatwaves 鈥 for example, creating cold surfaces near people waiting for a bus. 鈥淲e are thinking about how this could be used in lower-income countries that don鈥檛 have good insulation in a lot of buildings,鈥 he says. He has even found that a layer of metallised polypropylene would help windows and walls reflect more heat 鈥 it is the stuff used to make crisp packets. 鈥淵ou could imagine retrofitting facades with recycled potato chip bags,鈥 he says.
Other experts say what is needed most 鈥 at least in the short term 鈥 are administrative measures to deal with excessive heat. 鈥淭his can involve immediate actions during a heatwave like rescheduling school and work hours or setting up areas where the public can access drinking water, escape the heat and, if necessary, get medical attention,鈥 says .
Authorities in Hangzhou are clearly aware of this. In August, as the heat became unbearable, the city鈥檚 government opened cool, underground air-raid shelters, capable of holding thousands of people, providing free cold tea, Wi-Fi and ping-pong tables. Apparently, the measure proved to be a hit.
Deadly heat
The combined effect of heat and relative humidity can be : one wrapped in a damp cloth that allows you to get a reading known as a wet bulb temperature (WBT). This is more realistic in terms of how our bodies cope with heat because high humidity reduces or even stops the ability to sweat and stay cool naturally. Using this method, research published in 2010 concluded that the limit of human survival is a WBT of 35掳C (95掳F) 鈥 equivalent to an ambient temperature of 35掳C and 100 per cent humidity or 40掳C (104掳F) and 75 per cent humidity. Above this, even a healthy person in the shade won't live longer than 6 hours, with overheating resulting in heat stress, a .
In fact, our tolerance to humid heat may be even lower. In 2022, a study looked at how people reacted to heat in real life and suggested the survival limit at 100 per cent humidity is . Weng Qihao at the Hong Kong Polytechnic University says that, in such highly humid conditions, even 28掳C (82掳F) WBT can lead to severe strain and heat stroke. , Weng and his colleagues concluded that most existing measures to define heatwaves fail to capture the danger to humans because they aren't applicable to all regions and climate conditions. He is calling on scientists to develop a global framework for better assessing these aspects of heatwaves.
You Xiaoying is a freelance climate writer based in London
