U.S. may see 30-fold rise in extreme heat exposure by 2100

Humans living in the United States will experience a 30-fold increase in exposure to extreme temperatures by the year 2100, according to an analysis published Monday in the journal Proceedings of the National Academy of Sciences.

To help local leaders and policy makers better prepare for the impacts of heatwaves and cold spells, scientists at Arizona State University set out to quantify human exposure to extreme temperatures during 21st century.

It's not the first time researchers have measured the impacts of climate change on temperature extremes in the United States. The study, however, is one of the first to look at how urban development and population changes, in addition to greenhouse gas emissions, influence exposure to extreme temperatures.

"What was missing in our prior research was a human element, one directly tied to the climate results," study co-author Matei Georgescu, an associate professor at Arizona State and a senior scientist at the Global Institute of Sustainability, told UPI in an email.

If temperatures rise dramatically in the middle of a desert, but no one is there to feel the relentless midday sun, does it matter? It doesn't, Georgescu said, if one's goal is to understand human exposure to temperature extremes.

"This work was particularly focused on communicating impacts that people experience," he said.

The work was also focused on identifying the drivers of humans exposure to temperature extremes. Georgescu and his colleagues wanted to better understand the influence of the built environment and human movement patterns, and how people in different places experience temperature extremes.

"As we know, a particular temperature in one city may seem comfortable to its residents, but that same temperature may seem harsh to another city's residents," Georgescu said.

A mild day in Scottsdale might inspire residents in Fargo to crank up the air conditioning, for example. Likewise, on a 90 degree day, the dry heat of Reno feels less punishing than the humid heat of New York City.

To quantify the experiences of extreme weather in Fargo, Scottsdale and elsewhere, researchers used the metric person-hours. If 10 people are exposed to extreme temperatures for 10 hours, that's 100 person-hours.

Quantifying human exposure to extreme temperatures can help policy makers prepare for -- and potentially prevent -- negative impacts on human health and infrastructure, like local power grids.

"For example, you will recall the Northeast Blackout of 2003 that affected some 50 million residents during the hottest time of the year," Georgescu said. "The entire Northeast was in the dark, but more importantly, essential services, including air conditioning and transportation, were no longer available."

The new research showed that while urban development influences human exposure to extreme temperatures, the major drivers are greenhouse gas emissions and population trends. This means that humans will be exposed to extreme temperatures the most in places where both temperatures and populations are rising.

In absolute numbers, the obvious cities like New York, Los Angeles and Atlanta will see the largest increase in person-hours exposure. The largest relative changes in person-hours related to heat exposure will be seen in Sun Belt cities, such as Austin, Texas, and Orlando, Florida.

CDC data suggests extreme cold is responsible for more fatalities each year than extreme heat, but as temperatures continue to rise and more people move south and west, Georgescu expects the pendulum to swing the other way.

But that doesn't mean the dangers of extreme cold will go away. Some cities, including Denver, are expected to experience an increase in human exposure to extreme heat and extreme cold in the coming decades.

In total, by the end of the century, people in the United States will experience 5.2 billion person-hours of extreme temperature exposure.

Moving forward, Georgescu and his colleagues plan to fine tune their models to provide local policy makers with more precise predictions. Researchers also hope to figure out how these changes in extreme temperature exposure translate to mortality outcomes.

They also want to help identify solutions, he said.

"The immediate next step is to characterize how these heat-health impacts may be reduced with commonly proposed thermal adaptation strategies such as cool and green roofs, street trees and incorporation of other engineered materials," Georgescu said.

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