Arizona State University

Phoenix set a record last week for days with high temperatures in the triple digits.

Now, research in the journal PNAS on 47 U.S. metro areas suggests heat exposure across the 21st century might exceed expectations.

The authors project that 29 of the 47 metropolitan regions studied will undergo a 10-fold (or more) increase in population heat exposure by the end of the 21st century. Of the 29 worst-impacted cities, 23 lie within the Sunbelt.

The impact of excess heat hinges on how many people feel it and for how long.

Co-author Matei Georgescu on the School of Geographical Sciences and Urban Planning at Arizona State University said that's why it's important to look past global averages and ask how heat affects specific people and places over time.

"Nobody lives on the globally averaged planet. People need information at the local level, at the municipal level so that they can plan accordingly," Georgescu said. 

Such planning could include reducing greenhouse emissions and ensuring power grids can stand up to higher demand. But studies often leave out factors like urban heat island effects, and how well people and cities have adapted to heat. 

When lead author Ashley Mark Broadbent and colleagues included such factors in their high-emissions scenarios, Phoenix numbered among the hardest-hit cities — though near the bottom.

"Phoenix is very unique in many ways because it's already extremely hot. And so there are physical limits to how hot it can get. Nevertheless, Phoenix is continuing to get hotter," said Georgescu.