Exposure to extreme urban heat has tripled worldwide since the 1980s, study finds

Increases in dangerously high temperature and humidity were responsible for roughly a third of the global boost in exposure, while increased population accounted for the rest.

The study adds vivid context to the threats posed by a human-warmed planet, and to the challenges facing delegates at the United Nations climate summit taking place in Glasgow, Scotland.

Led by Cascade Tuholske, a postdoctoral researcher at Columbia University's Earth Institute, the study used new fine-grained data sets to analyze the geographic overlap between urban growth and dangerous combinations of temperature and humidity.

"Many of the fastest-warming cities are in the humid tropics," said Tuholske in an email.

The study analyzed 13,115 urban areas over the period from 1983 to 2016. Collective heat exposure was assessed in terms of person-days, or the number of days above a particular threshold in each city multiplied by the number of people affected.

The extreme heat was assessed by using day-to-day peaks in wet bulb globe temperature, or WBGT (max), a metric that takes into account humidity, sunlight and wind, in addition to temperature. The measure is considered particularly dangerous when it exceeds 86 degrees.

Using the 86-degree WBGT (max) threshold, the authors found that collective exposure to extreme heat and humidity across the cities studied soared from around 40 billion person-days in 1983 to 119 billion in 2016. Close to half of the cities studied showed increases in exposure that were statistically significant.

Higher WBGT (max) readings were the main culprit behind the exposure increase in many areas, including much of India. In a few other parts of the world, including East Africa, exposure climbed mainly due to population.

- - -

The new study by Tuholske and colleagues joins a rapidly growing body of work on the impact of extreme heat and how climate change will exacerbate the problem. The World Weather Attribution project concluded that July's deadly, record-smashing heat wave over the U.S. Pacific Northwest and southwest Canada would have been "virtually impossible without human-caused climate change."

There is high confidence that heat waves over land have become more intense and frequent across most of the world, according to the latest Working Group I assessment, released in August by the Intergovernmental Panel on Climate Change (IPCC).

Cities are at particular risk. "Compared to present day, large implications are expected from the combination of future urban development and more frequent occurrence of extreme climate events, such as heat waves, with more hot days and warm nights adding to heat stress in cities," said the IPCC.

By late century, billions of people could be experiencing annual average temperatures now found only in the world's hottest cities, such as Bangkok, and heat on some days could reach virtually unsurvivable levels in a growing number of locations.

A recent study led by Cassandra Rogers at Washington State University found that some of the world's largest increases over the past four decades in "humid heat" - extreme heat coupled with relatively high humidity - have been in South and Southeast Asia and in the southeastern United States. "These increases are concentrated over densely populated regions in the tropics and subtropics, where humid-heat levels are already high," said Rogers and colleagues.

Across the planet's cities, extreme heat is increasing because of global-scale warming from greenhouse gases together with the urban heat island effect, the tendency of built-up areas to absorb and retain heat. Tuholske and colleagues did not attempt to separate out the two effects in their analysis, but they acknowledged that both are involved.

"The global approach of this paper is very powerful to highlight the scope of the problem and the intensity with which it has been increasing," said Koen Tieskens, a Boston University research scientist in environmental health. "In terms of health consequences, it is even more worrying, as within cities those most vulnerable to extreme heat tend to live in the hottest parts with the least protection."

- - -

It's clear that people are continuing to migrate toward some of the world's most heat-vulnerable areas. In the United States, the 2020 census showed that, among the 10 largest cities, the three fastest-growing were Phoenix, Houston and Dallas - which also happen to be three of the nation's hottest big cities.

Meanwhile, in the developing world, economic imperatives are pushing people toward cities, many of them increasingly heat-vulnerable.

"I don't think [extreme heat] will stem the flow of rural migrants to urban areas in rapidly urbanizing low- and middle-income countries," said Tuholske. He stressed the need to learn more about migration patterns and dynamics.

Kristie Ebi, a professor at the University of Washington's Center for Health and the Global Environment, said that the analysis in Tuholske's study can help guide efforts to introduce interventions in the most vulnerable areas.

"Understanding the relative contributions can better target heat action plans," she wrote in an email. "Having a globally accurate map of the intersection of urban populations and extreme heat adds to understanding of regions at higher risk during heat waves, particularly regions with limited data."

There's a need to drill even deeper on variations within cities themselves, said Patricia Fabian, an associate professor at Boston University who welcomed the new study.

"In our community heat studies, we've recorded differences of seven degrees Fahrenheit within a couple city blocks when comparing temperatures at a park versus the downtown area," said Fabian in an email.

Fabian stressed the importance of better understanding geographic variations in how urban residents respond to heat physiologically, as well as other factors that influence a person's heat vulnerability, including their mobility, occupation, housing quality, access to air-conditioning and ability to pay utility bills.

Since the 1990s, many cities across the world, especially in richer nations, have implemented safety measures such as cooling centers and programs to check on vulnerable residents. These have likely helped to keep the toll from heat exposure from growing as fast as it otherwise might have.

Such innovations won't always work in poorer cities unless they are planned and carried out thoughtfully, according to Tuholske.

"Building cooling centers is a great tool, yet if people cannot access them because they have to work on hot days to feed their families, then they are useless," said Tuholske. "We also need policies - like occupational heat health standards - that address the structural problems that lead to exposure."

Knowing what we know, Tuholske added, "no one should die from extreme heat exposure."