Summer-like heat is stretching into spring and fall in Phoenix, research shows

Climate change is prolonging the heat season in Phoenix and around the globe, according to new research from the NASA Goddard Institute for Space Studies.

The paper, published this month in the journal AGU Advances, uses recent record-shattering heat waves in Phoenix in March 2026 and October 2024 as case studies for understanding how extreme heat is stretching into spring and fall worldwide.

During the 1980s, nearly all extreme heat events in Phoenix happened in summer months, said New York University climatologist Casey Ivanovich, one of the study’s authors.

“But as our climate is changing, we’re now starting to see extremes popping into the shoulder seasons, into the spring and fall,” Ivanovich said.

Researchers counted 183 extreme dry heat events in Phoenix from 1980 to 1989, 99% of which occurred in June, July or August.

Extreme dry heat events were much more frequent in recent years, the paper shows, with 338 events between 2015 and 2024. Of those dry heat events in recent years, 6% stretched into September and October. Meanwhile, researchers observed an increase in humid heat events earlier in the spring in recent years in Phoenix.

Ivanovich said it’s important to track the way seasons are shifting because extreme heat outside summer months can have serious human health impacts.

Heat waves in the spring, she said, are especially dangerous.

“This tends to be because people are caught off guard. They are not ready physiologically to deal with the heat; their bodies haven’t acclimated yet,” Ivanovich said.

This year’s record-shattering March heatwave led to at least nine heat-related deaths in Maricopa County, an unprecedented count for so early in the year.

Fall heat is also dangerous, Ivanovich said.

“In the fall, heat could be potentially more risky if we’ve had a long summer season where people have been repeatedly exposed to extreme heat and our bodies are already under stress,” Ivanovich said.

Ivanovich and coauthors found spring and fall temperatures are warming even more than might be expected from climate change alone. She said more research is needed to understand what other factors may be driving increased shoulder-season heat.

“There has to be something else going on, whether that’s changes in the hydroclimate, changes in land use associated with urbanization, or changes in vegetation cover,” Ivanovich said.

Ivanovich said a better understanding of shifting seasons can help inform decisions about public health responses to heat, such as when to open cooling centers.