Record-breaking heat is now routine. The devastating heatwave that wracked southwestern Europe in 2003 and claimed more than 70,000 lives produced temperatures not experienced in the region since the sixteenth century. Subsequent summers have extended this trend. In 2024, the continent recorded its hottest summer on record.
In urban environments, where most of the world’s population lives, the problem is especially acute. “If you build a city, inevitably it will be hotter,” says Edward Ng, an architect at the Chinese University of Hong Kong. “This is just physics.” A 2021 study1 evaluated more than 13,000 cities around the world, and found that the extent to which city-dwellers were exposed to temperatures above 30 °C nearly tripled between 1983 and 2016. This problem will grow as the climate crisis worsens and cities expand.
Nature Outlook: Cities
Living with ever-intensifying heat could have huge economic and health consequences. “About 9% of the human population is living out of the so-called human niche, which means they are living at temperatures that we never had before,” says Matthaios Santamouris, a physicist at the University of New South Wales in Sydney, Australia. “This may increase up to 25% or 30% by 2050.” He points out that rising temperatures have been linked to increased hospitalization rates and mortality. Sweltering conditions also contribute to mental-health problems, including increased rates of suicide and violent behaviour. As usual, the danger is greatest for the poorest populations, including those in otherwise wealthy cities.
The solutions available range from simple urban-planning strategies, such as providing more shade, to sophisticated coatings for surfaces that reflect the Sun’s heat back into space. But effecting large-scale change in cities is difficult and requires careful analysis of the root causes of the problem. Deploying the wrong solution in a city could not only fail to mitigate the heat, but even exacerbate it, along with its associated health issues. “You have to implement a scientific approach,” says Santamouris. “It’s like in medical science — if you don’t scan the patient, you cannot offer the right medicine.”
Made in the shade
Hunkering down in air-conditioned comfort is not always an option. In many low- and middle-income countries especially, only a minority of the population has access to climate control. Even in countries such as India, where air conditioning is common, frequent blackouts during heatwaves limit its benefits.
In many places, class determines who will bear the brunt of soaring temperatures. David Sailor, an urban-climate researcher at Arizona State University in Tempe, notes that the United States’ history of systemic discrimination has stranded many racial minority communities in greenery-free environments with lots of concrete and asphalt, which typically trap and store the Sun’s heat, rather than reflect it away. For some groups, such as construction workers and children, staying cooped up all summer is problematic. “Certain people have to be outside to work, and many more benefit from outdoor activity,” says Jennifer Vanos, an environmental-health researcher also at Arizona State University.
Researchers have assembled a diverse toolbox to map the temperature landscape and reveal hidden urban hotspots (see ‘Taking the temperature’). Elie Bou-Zeid, a civil and environmental engineer at Princeton University in New Jersey, boils the problem of urban heat down to what he calls the three Fs: form, function and fabric. Form refers to the geometry of buildings, which can reduce airflow and trap radiation, Bou-Zeid explains, and function encompasses what cities do to create heat. These two aspects are difficult to alter in an urban environment that's already been built, so most mitigation efforts focus on the third F, fabric. This relates to the properties of the materials that form the city’s surfaces.
Taking the temperature Mapping the urban temperature landscape is more complicated than it might seem, despite the array of tools available for making such measurements. Satellites that use infrared imaging to survey surface temperatures are a staple of urban climate research. But scientists face a trade-off between the level of detail they can get from satellite imagery and the frequency at which they sample. For example, NASA’s Landsat satellites deliver resolution down to tens of metres, but take 16 days to get around the planet; the speedier Terra satellite can provide daily data, but only with kilometre-scale resolution. Snapshots of this nature are useful, but do not describe the temperatures that people experience. “We don’t care as much about what roof temperatures are for people’s health as what the two-metre-height air temperature is, but that’s much harder to measure on that scale,” says Jennifer Vanos at Arizona State University. Such readings can be delivered by land-based temperature sensors, but these are typically located in places such as airports and golf courses at the edge of metropolitan areas, not in the more-populated centres. To fill in the gaps, some researchers are taking to the streets. “We drive cars around the city with an array of temperature sensors mounted onto the car, use bicycle traverses through parks, and things like that,” says David Sailor, also at Arizona State University. His team’s work in highly urbanized Maricopa County, Arizona, which includes Phoenix, showed that Hispanic residents are more likely than other demographic groups to live in overheated environments8. Even a good on-the-ground perspective offers only a partial view of what city dwellers experience, cautions Elie Bou-Zeid at Princeton University in New Jersey. “We need to map the indoors. There are studies in New York City that show indoors can be way hotter than the outdoors if they’re not air-conditioned,” he says. To capture the full range of indoor and outdoor temperatures that a person might be exposed to throughout the day, Bou-Zeid and others are turning to wearable sensors. These devices could also monitor the physiological effects of high temperatures and provide personalized heat warnings. In Phoenix, which hit a record August temperature of nearly 48 °C this year, Vanos is using wearable sensors in a study involving dozens of participants. Her goal is to identify groups whose health and socioeconomic status, among other factors, leave them ill-equipped to cope with high heat.
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