Human exposure to extreme heat in US may increase by 30 times by 2100 end, sunbelt cities to be most hit: Study

Human exposure to extreme heat in US may increase by 30 times by 2100 end, sunbelt cities to be most hit: Study
(Getty Images)

The residents of major US cities could experience greater exposure to extreme heat by the end of the 21st century than at present. The number of “person-hours” of exposure — where one person hour is equal to a single person exposed to one extreme heat hour — will increase by up to 30 times by the end of the 21st century, according to a research team from Arizona State University (ASU). This accounts for the rising temperatures and population growth during the 21st century.

Researchers found that the average annual heat exposure at the start of this century in the US was about 5.2B person-hours. Assuming a worst-case scenario of peak global warming, population growth and urban development, the annual heat exposure would rise to 150B person-hours by the end of the century, a nearly 30-fold increase, the authors explain. 

"Our study demonstrates how cities across the US will experience extreme heat and cold temperatures during the 21st century. The number of person-hours of exposure is projected to increase by as much as 30 times. We reveal the US cities where the largest numbers of people will experience hot and cold extremes (that is, population heat and cold exposure) as the climate changes. Large changes in population heat exposure are projected in cities across the US sunbelt. This surge in exposure is driven by the combined effects of climate change and population growth occurring in tandem," study author Ashley Broadbent, an assistant research professor at ASU’s School of Geographical Sciences and Urban Planning, told MEA WorldWide (MEAWW).
According to the research team, previous projections of future temperature extremes in the US have not accounted directly for warming due to urban development or its interactions with greenhouse gas (GHG)-induced climate change. Such projections have also defined extremes using a fixed temperature threshold, but threshold temperatures at which excess mortality occurs vary regionally, they add.

The current report, therefore, analyzes how three key variables would affect human exposure to extreme temperatures from the beginning of this century to its end. The authors examined the simultaneous contributions of greenhouse gas-induced warming, urban development and population growth to projected changes in population-weighted exposure to extreme heat and cold — that is, population heat and cold exposure — in 47 major metropolitan areas in the US across the 21st century. The experts explain that the combined effect of these three drivers will substantially increase the average heat exposure across the US, but heat exposure is not projected to increase uniformly in all cities across the US. Accordingly, there will be hotspots where heat exposure grows sharply. 

To describe how these three variables would affect temperatures, and in turn populations, the team used a metric they dubbed person-hours to describe humans' exposure to extreme heat and cold. "It’s an intuitive metric. For example, when one person is exposed to one hour of extreme temperature, that exposure equals one person-hour of exposure. Likewise, if 10 people are exposed to 10 hours of 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,” says Broadbent.


The study concentrated on three key factors: climate change brought about by greenhouse gas emissions, urban-development induced impacts arising from the growth of cities and population change in individual cities (Getty Images)

The analysis, published in the Proceedings of the National Academy of Sciences (PNAS), reveals that population heat exposure would increase by a factor of 12.7-29.5 by the end of the 21st century, relative to the beginning of the century, under a high-intensity greenhouse gas emissions and urban development pathway. This is substantially greater than in previous projections, emphasizes the team.


The largest absolute changes in population heat exposure during the 21st century are projected to occur in major US metropolitan regions such as New York City, Los Angeles, Atlanta, Washington DC, Denver, Chicago, Miami and Dallas. These cities will make up approximately 60% of the total increase in projected heat exposure across the US. "Most notably, the broader New York City region could experience an absolute increase in annual population heat exposure of 24.6 ± 4.4 billion person-hours, more than twice the absolute increase in population heat exposure than in any other metropolitan region in our sample," reveals the report.  

Of the 47 metropolitan regions considered, 29 are projected to experience a 10-fold or more increase in population heat exposure by the end of the 21st century. The authors categorized these locations with the largest relative change in population heat exposure, that is, changes relative to start-of-century, as the “worst-impacted” cities. Of the 29 worst-impacted cities, 23 are located in the US sunbelt. The results, for example, show that the largest relative increases in population heat exposure are projected to occur in cities across the sunbelt such as Orlando, Austin, Miami and Atlanta, driven primarily by simultaneous GHG-induced warming and rapid population growth.


"The increase in exposure is quite large if you look at it relative to the start of the century. Some cities across the sunbelt, according to our projections, will have 90 times the number of person-hours of heat exposure. For example, cities in Texas that see substantial population growth and strong GHG-induced climate warming could be markedly affected,” explains Broadbent.


Of the 47 metropolitan regions considered, 29 are projected to experience a 10-fold or more increase in population heat exposure by the end of the 21st century (Getty Images)

While the average temperature in the US will be warmer in the future, the study also finds that cold exposure will increase slightly compared with the start of the century, primarily because of population growth. The end-of-century population cold exposure is projected to rise by a factor of 1.3-2.2, relative to start-of-century population cold exposure. “While there is a general decrease in the number of projected extreme cold events by the end of this century, the number of individuals exposed to extreme cold is projected to increase, as population growth means that the total number of person-hours of cold exposure will go up. Cold is currently more of a national health problem than heat, but our results suggest that by the end of the century heat exposure may become a larger health problem than cold exposure," says Broadbent. 

However, cold exposure will not disappear completely as the climate warms. According to one of the team’s simulations, Denver is projected to have more extreme cold at the end of the century compared with the beginning. "That’s the interesting thing about climate change. We know the average temperature is going to increase. But we know less about how the extremes are going to change, and often the extremes are the most important part of our daily lives," writes Broadbent.

Based on their findings, the team suggests that one way to prepare for increased heat exposure is to reduce greenhouse gas emissions on a global scale, which would reduce the number of hours people are exposed to extreme temperatures. Other options include localized infrastructure adaptation that provides buffering effects against rising temperatures. This includes measures such as planting trees, providing shade and cooling areas, and constructing buildings using materials that absorb less heat.

"Without greenhouse gas mitigation efforts, most US cities will need to prepare for more extreme temperatures and a large increase in human heat stress during the 21st century. For many cities, heat stress will become a much larger public health problem than cold stress by the end of this century. Cities should consider investing in local heat adaptation measures (for example, urban greening initiatives and/or cooling infrastructure) and other initiatives (cooling centers and A/C access) to protect heat vulnerable communities," Broadbent told MEAWW.

According to study author Matei Georgescu, an associate professor in ASU’s School of Geographical Sciences and Urban Planning, while there are several takeaway messages from this work, a central one concerns the future resiliency of cities. "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. 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 modeling community being integrated into their environmental adaptation plans? If so, how, and if not, why not?” says Georgescu.


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