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Study highlights US cities at risk of rising heat exposure

20 August 2020

by Sarah Wray

Citizens’ exposure to extreme heat could increase substantially in US cities by the end of the century, according to a new study. Cities are urged to better integrate climate modelling into their resilience plans.

Sunbelt cities such as Austin, Orlando and Atlanta could be among the worst affected, as well as major metropolises like New York and Los Angeles.

Researchers from Arizona State University modelled how climate change, urban development and population growth in individual cities could jointly affect human exposure to extreme temperatures. They found that in the worst-case scenario, where no adaptation or mitigation measures are taken, heat exposure could increase thirty-fold in some cities by the end of the century, compared to the beginning.

Ashley Broadbent, an assistant research professor at Arizona State University’s School of Geographical Sciences and Urban Planning, told Cities Today: “The results of this paper inform cities and municipalities how serious the problem of heat exposure could become without any climate change mitigation and heat adaptation. This is the first estimate of heat exposure that includes population dynamics (migration and growth), greenhouse climate change and urban-induced climatic warming.”

Heat waves are already the leading cause of weather-related deaths in the United States and the findings of the Motley drivers of heat and cold exposure in 21st century U.S. cities paper, published online in the Proceedings of the National Academy of Sciences (PNAS) journal, are worrying as they come during intense hot spells. Several states experienced their hottest July ever last month and extreme heat has led to rolling blackouts in California over the last week. Recent reports say that what could be the highest temperature ever reliably recorded on Earth – 130F (54.4C) – may have been reached in Death Valley National Park, California.

Heat’s human impact

The researchers used a metric they call “person-hours” to describe humans’ exposure to extreme heat and cold.

“It’s an intuitive metric,” said Broadbent. “For example, when one person is exposed to one hour of an extreme temperature, that exposure equals one person-hour of exposure. Likewise, if 10 people are exposed to 10 hours of an extreme temperature that exposure equals 100 person-hours.”

“I think this definition is more representative of what people experience, which is what this study is about versus a study that simply communicates temperature changes without any human element attached to it,” he added.

Overall, the researchers found that the average annual heat exposure at the start of this century in the United States was about 5.2 billion person-hours. They calculated that assuming a worst-case scenario of peak global warming, population growth and urban development, the annual heat exposure would rise to 150 billion person-hours by the end of the century, a nearly 30-fold increase.


However, heat exposure is not projected to increase uniformly in all cities across the US. “There will be hotspots where heat exposure grows sharply,” Broadbent said.

The researchers defined heat thresholds based on local city definitions, which they say is also something previous studies have not done.

“People die at different temperatures in different cities because what is extreme in one city may be normal in another,” said Broadbent.

The study concludes that human exposure will increase the most in areas where climate change and population increase in tandem. It finds that urban development has a smaller, but “non-negligible” effect.

The largest absolute changes in population heat exposure are projected to occur in major US metropolitan regions such as New York, Los Angeles and Atlanta. The biggest increases relative to the start of the century are forecast in rapidly growing cities located in the sunbelt, including Austin, TX; Orlando; and Atlanta.

Beating heat

Reducing greenhouse gas emissions globally is key to mitigating rising heat but local actions will make a difference too, the researchers say. Infrastructure adaptation could include planting trees, providing shade and cooling areas and constructing buildings using materials that absorb less heat.

Many cities are taking action to address heat but the researchers urge them to do more.

“The successful steps taken will require holistic thinking that embraces contributions from urban planners, engineers, social scientists and climate scientists with a long-range vision of how we want our cities to be,” said Matei Georgescu, an associate professor in ASU’s School of Geographical Sciences and Urban Planning. “We therefore call on cities to start asking some very foundational questions regarding the projected exposure of their constituents to future environmental change. Is the work of the urban climate modelling community being integrated into their environmental adaptation plans? If so, how, and if not, why not?”

A follow-up paper which is in progress, will provide city-specific estimates of the avoided heat exposure derived from urban adaptation measures and greenhouse mitigation.

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