Phoenix residents, prepare to sweat

Study: Even more hours of extreme heat to unfold

This summer is setting records and redefining extreme heat in Phoenix.

On Thursday, the National Weather Service in Phoenix recorded the 21st day this year when temperatures reached 112 degrees or above. That broke a nearly decade-long record during a summer likely to end as the hottest since record-keeping began.

More than half of those days were in August, according to Derek Hodges, a weather service meteorologist. By contrast, there were only 13 days last year where temperatures reached or exceeded 112 degrees.

That threshold is a key element in a new study, which defines “extreme heat” in Phoenix as 112 degrees or above. The study’s authors predict Phoenix and Tucson residents will be

exposed to more hours of extreme heat as the remainder of the century unfolds.

Ashley Broadbent, an assistant research professor at Arizona State University’s School of Geographical Sciences and Urban Planning, was one of the co-authors of the recently published paper, titled “The motley drivers of heat and cold exposure in 21st century U.S. cities.”

“When we think of climate change, we usually think about global temperature change,” Broadbent said. “But I wanted to know what that means for people and for our cities.”

This paper is one of the first to explore human exposure to extreme temperatures. It merges city data on greenhouse gas emissions, urban development-induced temperature changes and population growth.

“It’s the simultaneous combination of these three factors that give us this really interesting, fine-grain picture of how human exposure is going to change in the future,” Broadbent said. “It’s important to know how bad things could get. Think of this paper as a warning because what we simulate is the worst-case scenario.”

The researchers collected data from 47 cities across the country, including Phoenix and Tucson.

One of the study’s key elements is that the predicted effects of heat exposure are based on each city’s local definition of extreme temperatures, not on a single figure.

“The local definition of extreme temperatures is important because humans acclimate to their environment,” said Matei Georgescu, a co-author of the study and an associate professor at ASU. “It’s natural for a person who has experienced an extreme temperature to eventually acclimate to it.”

Georgescu says his own life is a case study for this acclimation. Since moving to Arizona from New York a decade ago, he says his body has adjusted to the region’s dramatically different climate.

“A 90- to 95-degree day may have been extreme to me when I was in New York City, but that, during the summer for a Phoenician, is a cool, comfortable day,” Georgescu said. “I now look forward to those days.”

The local definition of extreme heat is based on city data collected from 2000 to 2010, which shows the 99th percentile of local air temperature at 3 p.m. On average during that time span, Phoenix and Tucson experienced these extreme temperatures five days a year.

“The good news is that the extreme heat events of Arizona, which are already very extreme, are not going to increase as much as the extreme heat events in cooler climates,” Broadbent said. “The predicted change in the Arizona cities was not as bad as some of the cities across the eastern Sunbelt. But given we already have a heat problem in Arizona, we should take that seriously.”

Out of all the cities studied, Phoenix had the hottest extreme heat threshold at 112 degrees.

Tulsa, Oklahoma, came in second with a temperature five degrees cooler.

Tucson’s heat temperature threshold was ranked seventh overall, at 103 degrees.

But while the two Arizona cities had among the hottest temperature thresholds, neither were expected to have the greatest number of extreme heat days by the end of the century.

The study predicts that by 2100, Phoenix and Tucson will see an increase from five days of extreme heat per year to as many as 45 and 60, respectively. Even though the definition of extreme heat may be higher in Phoenix, the Florida cities of Tampa, Orlando and Miami are predicted to have the most extreme heat days, with more than 80 a year.

To describe the extreme temperature exposure that residents of these cities would be facing, the study used a metric dubbed “person-hours.” When one person is exposed to one hour of extreme heat, that equals one person-hour. Similarly, if 10 people were exposed to 10 hours of extreme heat, that would equal 100 person-hours.

Anton L. Delgado is an environmental reporter for The Arizona Republic and azcentral.com. Tell him about stories at anton.delgado@arizonarepublic.com. Follow his reporting on Twitter at @antonldelgado.

Environmental coverage on azcentral.com and in The Arizona Republic is supported by a grant from the Nina Mason Pulliam Charitable Trust. Follow The Republic environmental reporting team at environment. azcentral.com and @azcenvironment on Facebook, Twitter and Instagram.