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Human-caused and a newly formed El Nino are combining to , scientists say.

No other major city – defined as the 25 most populous in the United States – has had any stretch of 110-degree (43.3-degree) days or 90-degree (32.2-degree) nights longer than Phoenix, said weather historian Christopher Burt of the Weather Company.

“When you have several million people subjected to that sort of thermal abuse, there are impacts,� said NOAA Climate Analysis Group Director Russell Vose, who chairs a committee on national records.

For Phoenix, it’s not only the brutal daytime highs that are deadly. The lack of a nighttime cooldown can rob people without access to air conditioning of the break their bodies need to function properly.

With Tuesday’s low of 94 F (34.4 C), the city has had nine straight days of temperatures that didn’t go below 90 F (32.2 C) at night, breaking another record there, according to National Weather Service meteorologist Matt Salerno, who called it “pretty miserable when you don’t have any recovery overnight.�

On Monday, the city also set a record for the hottest overnight low temperature: 95 F (35 C). During the day, the heat built up so early that the city hit the 110 mark a couple minutes before noon.

Dog parks emptied out by the mid-morning and evening concerts and other outdoor events were cancelled to protect performers and attendees. The city’s Desert Botanical Garden, a vast outdoor collection of cactus and other desert plants, over the weekend began shutting down at 2 p.m. before the hottest part of the day.

In the hours before the new record was set, rivers of sweat streamed down the sunburned face of Lori Miccichi, 38, as she pushed a shopping cart filled with her belongings through downtown Phoenix, looking for a place to get out of the heat.

“I’ve been out here a long time and homeless for about three years,� said Miccichi. “When it’s like this, you just have to get into the shade. This last week has been the hottest I ever remember.�

Some 200 cooling and hydration centers have been set up across the metro area, but most shut down between 4 p.m. and 7 p.m. due to staffing and funding issues.

The entire globe has simmered to record heat both in . Nearly every day of this month, the global average temperature recorded before 2023, according to University of Maine’s Climate Reanalyzer. U.S. weather stations have broken more than in the past seven days, according to NOAA.

Rome reached an all-time high of 109 (42.9 degrees Celsius), with record heat reported throughout Italy, France, Spain and parts of China. Catalonia smashed records reaching 113 (45 Celsius), according to global weather record keeper Maximiliano Herrera.

And if that’s not enough, smoke from , and have caused problems

In addition to Phoenix, Vose and others found less populous places such as Death Valley and Needles, California; and Casa Grande, Arizona, with longer hot streaks, but none in locations where many people live. Death Valley has had an 84-day streak of 110-degree temperatures.

The last time Phoenix didn’t reach 110 F (43.3 C) was June 29, when it hit 108 (42.2 C). The record of 18 days above 110 that was tied Monday was first set in 1974.

“This will likely be one of the most notable periods in our health record in terms of deaths and illness,� said David Hondula, chief heat officer for the city. “Our goal is for that not to be the case.�

Phoenix City Parks and Recreation workers Joseph Garcia, 48, and Roy Galindo, 28, tried to stay cool as they trimmed shrubs. They work from 5 a.m. to 1:30 p.m. to avoid the hottest time of the day.

“It gets super hot out here and sometimes we have to take care of the public,� said Galindo, adding he sometimes find people passed out on the grass. “A lot of these people aren’t drinking water.�

Retired Phoenix firefighter Mark Bracy, who has lived in the city most of his 68 years, went on a two-hour morning climb Tuesday, up and down Piestewa Peak, which is 2,610 feet (796 meters) high.

“I’ve been going up there regularly since I was in the Cub Scouts, but it was never this hot back then,� said Bracy. “We’ve had hot spells before, but never anything like this.�

Dr. Erik Mattison, director of the emergency department at Dignity Health Chandler Regional Medical Center in metro Phoenix, recalled a hiker in his 60s who was brought in last week with a core body temperature of 110 degrees (43.3 C).

“Heat makes people sick. Heat makes people die,� Mattison said.

“And it’s not just older people," he added. "We’ve seen professional athletes fall ill in the heat during training camp.�

Phoenix’s heat wave has both long and short-term causes, said Arizona State University’s Randy Cerveny, who coordinates weather record verification for the World Meteorological Organization.

Long-term high temperatures over recent decades are due to human activity, he said, while the short-term cause is high pressure over the western United States.

That high pressure, also known as a heat dome, has been around the Southwest cooking it for weeks. When it moved, it moved to be even more centered on Phoenix, said National Weather Service meteorologist Isaac Smith.

The Southwest high pressure not only brings the heat, it prevents cooling rain and clouds from bringing relief, Smith said. Normally, the Southwest’s monsoon season kicks in around June 15 with rain and clouds. But Phoenix has not had measurable rain since mid-March.

“This heat wave is intense and unrelenting,� said Katharine Jacobs, director of the at the University of Arizona. “Unfortunately, it is a harbinger of things to come.�

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Borenstein reported from Washington. Follow Seth Borenstein and Anita Snow on Twitter at and

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