San Francisco hit 90 degrees on Friday, the first day of spring. It was even warmer in Livermore and Redwood City, San Rafael and Santa Rosa.
As these kind of spikes have become more common, there has been a rush to add air conditioning in the region. More than half of the San Francisco area’s homes now have air-conditioning, a first for the famously cool region, which federal estimates show passed that milestone just a few years ago.
It is not just the Bay Area: The United States has become a lot more air-conditioned in recent years, both fueling climate change and taming its day-to-day consequences. About 93 percent of occupied American housing units had primary air-conditioning in 2023, according to the most recently published federal data. Eight years earlier, about 89 percent did.
Those air-conditioners will face fresh tests this week as the heat wave that had enormous swaths of the United States sweating last week threatens more unpleasantness.
Parts of Utah could reach into the 90s, and it might be only a few degrees cooler in Denver. Oklahoma City is bracing for a day of mid-90s, and St. Louis will not be far behind. The National Weather Service is warning that, thanks to “an anomalous ridge,” midweek could have some parts of the country confronting temperatures that are 30 to 40 degrees above average for this time of year.
“It doesn’t take too many heat waves before you see a lot of air-conditioning, even in places like the Pacific Northwest,” said Lucas W. Davis, a business professor at the University of California, Berkeley, who has studied the spread of air-conditioning.
Air-conditioning, of course, has long been seen as essential in many parts of the United States, particularly the Deep South and the Southwest. Much of the recent embrace of air-conditioning has occurred in places where generations of residents rarely felt a need to do more than open a window.
Compared to other countries, Dr. Davis said, the United States was “oddly obsessed with air-conditioning.” And as heat records have fallen — or been challenged — more frequently, people have scrambled to install air-conditioning systems in new places.
“It is increasing rapidly because people are increasingly in these heat waves, where there are actually threats to life,” said Dorit Aviv, an assistant professor at the University of Pennsylvania and the director of the Thermal Architecture Lab there.
“People associate cooling with air-conditioners and they think in order to cool people you need to cool the space by blowing air into it from a mechanical system,” she added.
And so the air-conditioner installers have kept busy.
The San Francisco area had 613,000 housing units with primary air-conditioning in 2015, a figure that soared 47 percent by 2023. The share of homes in greater Seattle with primary air-conditioning rose about 19 percentage points between 2019 and 2023. Denver posted a double-digit increase over less than a decade.
Metro Atlanta, by comparison, saw its share of homes with primary air-conditioning decline slightly — from 99.1 percent to 98.8 percent — between 2015 and 2023. Locales like Dallas, Miami and New Orleans remained similarly stable, with scarcely detectable ticks up or down over recent years.
Dr. Aviv said one issue is that relatively few American buildings, especially in places historically unaccustomed to intense heat, were designed to embrace alternate cooling methods, such as external shading. But the nation’s air-conditioner-centric culture did not help, she acknowledged.
“It’s all about the body’s heat balance, and there are multiple ways we can achieve that,” she said. But, she said, “This country is so saturated with air-conditioners that we think one control — the temperature on the thermostat — is going to make us feel better.”
Still, many experts, including Dr. Aviv and Dr. Davis, point to substantial evidence that systems save lives, increase labor productivity and improve educational outcomes.
But air-conditioning carries substantial costs. It demands far more electricity than, say, fans, and a United Nations report in 2023 said that some 10 percent of the world’s greenhouse gas emissions in 2050 could come from trying to keep cool.
Although air-conditioning units have become more efficient over time, they are still require significant amounts of energy — federal statistics have shown that air-conditioning was tied to about 19 percent of residential electricity consumption in the United States — and often employ planet-warming chemicals.
Industry officials and researchers expect air-conditioning to continue to gain greater global reach, despite fears about climate change and occasionally intense political debates in places like France. Outside of the United States, Dr. Davis said, greater adoption of air-conditioning can largely be traced to rising incomes and lower costs attributable to declines in electricity pricing and more efficient units.
“The world getting hotter is part of it,” he said of the mounting global interest, “but by far the bigger part of it is that the world is getting richer.”
Those are long-term debates, though. In the United States this week, emergency officials are bracing as people get hit with temperatures more common to July — if they venture from their chilled indoor spaces.
At Luke Air Force Base in Glendale, Ariz., near Phoenix, the temperature at the biennial air show went as high as 103 degrees on Saturday, according to Capt. Alyssa Letts, a public affairs officer for the base. At the last air show, the high had been around 78 degrees.
Hundreds of thousands of people attend the two-day show, perhaps feeling temperatures around 20 degrees warmer than the official numbers because of the asphalt. On Saturday, the show’s first day, around 400 people sought medical help because of heat-related illness, and 23 of them were hospitalized, Captain Letts said. Most people who sought medical help were among groups at higher risk for heat-related injuries — those who were under 12, over 60, pregnant or had another medical condition.
She said the base is encouraging people who are at higher risk to take extra precautions during the “unprecedented temperatures.”
“It is a historic first for how hot it is at this time of year in March, and we’re trying to make sure the public understands that when they come out to this air show,” she said.
Alan Blinder is a national correspondent for The Times, covering education.
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