It's already been the hottest summer on record in Tucson, and now the weather service is warning of excessive heat over the Labor Day weekend, the first weekend of September. 

 Josh Galemore / Arizona Daily Star

If Tucson felt too much like hell last month, there’s a reason. Temperatures set a string of records as the sun bore down mercilessly and the monsoon went AWOL.

July 2020 was not only Tucson’s hottest July on record, it was the city’s hottest month in the 125 years that records have been kept.

Last month’s average temperature of 91.5 degrees was 4.5 degrees above normal, and 0.9 of a degree above the previously hottest month on record, July 2005, the National Weather Service says. In all, city temperatures broke seven records last month and tied seven others.

That came two months after May became Tucson’s hottest May on record, with an average temperature of 80.7 degrees, scoring nearly 10 degrees above the normal May average. An additional 12 temperature records either fell or were tied that month.

May through July 2020 was also the hottest three-month period on record. (June was only the 17th warmest June on record.)

Tucson’s monsoon has also been dismal, with 0.66 of an inch of rain officially recorded since the June 15 start of the season — representing the fourth-driest monsoon on record as of last week.

A normal monsoon would have produced 2.91 inches of rain by this time, said John Glueck, a National Weather Service senior metereologist.

Most immediately, the searing heat and the lack of monsoon rains is due to a strong mass of high pressure air over this area that has stuck around, Glueck said.

“We’ve had little pieces of moisture here and there. But for the most part, the orientation where the high is at is keeping the bulk of monsoon moisture we normally get south of the border,” Glueck said. “We are just heating up.”

This summer’s extreme heat is no surprise to Gregg Garfin, a longtime University of Arizona climate scientist who for many years has been tracking the steady rise of temperatures in this area due to human-caused climate change.

Like many scientists, Garfin is reluctant to attribute any short-term burst of unusually hot weather like the one Arizona is enduring now to a long-term trend such as climate change triggered by heat-trapping greenhouse-gas emissions.

But this weather is consistent with what you would expect to see in this hot arid region due to climate change, said Garfin, deputy director of UA’s Institute for the Environment.

“It’s not surprising to me that as we move into the future, we are going to continue to break records,” Garfin said, adding, “For sure, this is an unusual year.”

The record warmth this summer is not a good sign for the future, said former UA Institute for the Environment director Jonathan Overpeck, who is now at the University of Michigan.

“Temperatures are increasing relentlessly in the Desert Southwest due to humans — mainly the burning of fossil fuel. This warming means drier conditions as well, and the drying acts to drive yet more warming — a vicious cycle,” said Overpeck. “The failure of the monsoon made the drying and warming even worse this year, but the ultimate cause of the record warmth is mostly human, meaning it’ll get worse and worse until we stop burning the fossil fuels.”

“Worse means drier, less water in our rivers, more dust storms and more severe wildfire, all things that are already visibly worse than just a couple decades ago. The warming is causing the Southwest to aridify, and only we can stop it,” Overpeck said.

Wildfires such as the Bighorn Fire north of Tucson leave the ground charred and unable to absorb water, which can increase flood risks. “Even a light rain can produce devastating flash floods and mudflows, often with little warning,” Pima County officials warned. In this July 2020 video, a debris flow oozes down the Cañada del Oro Wash.

Speakers at Wednesday’s public “webinar” on the Bighorn Fire also cited the record heat as a factor helping to drive the fire’s ability to burn about 120,000 acres in the Catalina Mountains from June 5 through July 22.

Other factors included this summer’s prolonged dry weather and constant high winds that spread the fire, speakers said at the webinar, sponsored by Tumamoc Hill’s Desert Laboratory, the UA’s Arizona Institutes for Resilience and philanthropist Sarah Smallhouse.

Garfin noted that computer models looking at long-term temperature trends — trends supplied to the U.S. government for its 2017 National Climate Assessment — forecast that the number of 100-degree-plus days in Pima County could rise to nearly 100 by 2100, if the world doesn’t takes strong steps to curb greenhouse emissions.

If emissions are allowed to keep rising unabated, the number of 100-degree days in the county could reach nearly 140 by 2100, the data presented by Garfin shows. The 1961-1990 average was about 40 100-degree days.

So far this year, Tucson has had 67 100-degree days with at least one very hot month left in the summer.

For the next three months, the weather service predicts there is around an 80% chance that temperatures in Arizona and the Southwest will be above normal.

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Contact reporter Tony Davis at tdavis@tucson.com or 806-7746. On Twitter@tonydavis987