Imagine Paris at 122 degrees Fahrenheit, or 50 Celsius.
The asphalt streets would melt in spots, making it virtually impossible for ambulances and buses to pass. The lights and fans could cut out in neighborhoods if underground cables burned or junction boxes shifted. Cellphone service might go down as antennas on boiling rooftops stopped working. Trains would halt as outdoor rails swelled, keeping nurses, firefighters and electricity engineers from reaching their jobs when they were most needed.
Those are situations city officials are already planning for.
“A heat wave at 50 degrees is not a scenario of science fiction,” said Pénélope Komitès, a deputy mayor who oversaw a crisis simulation two years ago based on those presumptions. “It’s a possibility we need to prepare for.”
France has recently experienced its second heat wave of the summer, with temperatures reaching record highs last week in the southwest and heat alerts covering three-quarters of the country. In Paris, this has become the new normal. Eight of the 10 hottest summers recorded in the city since 1900 occurred since 2015.
In 2019, temperatures in Paris hit a record, nearing 109 degrees. Scientists say it will get worse, particularly since climate change is warming Europe at more than twice the global average.
In 2022, city officials asked climate scientists if Paris might experience heat waves that reach 50 degrees in the near future.
Their answer was yes, possibly, by the end of the century, or as soon as around 2050 if greenhouse gas emissions increase exponentially. But the scientists’ modeling showed that scenario was unlikely if global pledges from the Paris climate accord were met and the rise in warming was kept below 2 degrees Celsius.
“I don’t think we should bet on that as a society,” said Alexandre Florentin, a green city councilor and environmental engineer who spent more than a decade working at Carbone 4, a leading French climate change mitigation and adaptation firm.
He led a committee of city lawmakers, from all political parties, to examine the capital’s vulnerabilities to extreme heat waves. They published their report, Paris at 50°C, in 2023, separately from the crisis simulation.
They found that there were temperature thresholds that could cause widespread breakdowns, leading to a cascade of crippling domino effects.
During an interview with a hospital director, for example, Mr. Florentin learned that the medical center’s air-conditioning system was designed to work only when the outside temperature was about 109 degrees or lower.
Any higher and it would break down and the hospital would be forced to close its operating rooms and send urgent cases to other hospitals. “What would happen if they have the same problem?” Mr. Florentin said. “He didn’t have an answer.”
He added, “As long as that threshold is passed, we face domino effects.”
Another important finding was the vulnerability of schools, should a heat wave hit during the school year — like in late June.
“The classes will close, and that will have rippling consequences all through society,” Mr. Florentin said. “If their parents work at a hospital or the electricity facility, there will be bigger problems” — meaning understaffing at crucial times.
His strongest recommendation was for the city to invest more in green and shaded yards and to transform schools into “passive” cooling centers with designs that allow for more air circulation or geothermal cooling systems, not electricity.
Paris is particularly ill-adapted to heat waves. A 2023 study published in the London-based medical journal The Lancet deemed it the European capital whose residents were most exposed to heat-related deaths.
The city has the highest population density in Europe, and those people are packed into buildings without insulation and with zinc roofs built for the city’s historically moderate winters and summers, explained Franck Lirzin, author of the 2022 book “Paris in the Face of Climate Change.”
Many of its main squares are paved in stone and ringed with asphalt roads, transforming them into radiators that help increase the city’s temperatures by as much as 10 degrees Celsius compared with the countryside nearby.
Just under 15,000 people died from heat-related causes in 2003 during a heat wave that hit France that August. Many were older adults living in apartments that had zinc roofs with no insulation or air-conditioning, according to reports by national lawmakers and the national public health agency.
In response, the country drafted its first national heat wave plan and introduced a system of registering isolated older or disabled people, so that they could be checked on during heat waves.
Given the surprising speed of climate change, the lessons of 2003 already seem outdated. “The climatologists tell us the 2003 heat wave will be considered a cool summer soon,” Mr. Florentin said. “We must prepare for much worse.”
The city’s emergency simulation presumed a two-week heat wave, with temperatures surging to near 115 degrees and forecasts for 122.
City workers focused on two Parisian neighborhoods, shuttling elementary- and middle-school children to climate shelters set up in an abandoned train tunnel and an underground parking lot.
That drill was followed by a tabletop exercise to see how firefighters, police officers, hospital staff members, the Red Cross and others would interact and respond.
The big lesson from the exercise was that “Parisians are not ready,” Ms. Komitès said.
Some are trying to change that.
A nonprofit group focused on sustainable food has organized “Eating at 50 degrees” events around France, with chefs working on menus sourced locally that require no ovens or stovetops, which exacerbate the heat.
Another group, Health in 2050, has been bringing doctors, pharmacists and medical scientists together to discuss how they can prepare for the health crises and new diseases a hotter climate will bring to France.
The Odéon — Théâtre de l’Europe is organizing an event in September in Paris to discuss how theaters and museums can adapt for climate crises.
In May, Prime Minister François Bayrou passed a decree requiring all workplaces to create an extreme heat plan.
The city government has doubled down on its own adaptation plans — pulling up asphalt parking places and the center of roads to plant trees — 15,000 last winter alone, said Dan Lert, deputy mayor in charge of the city’s ecological transition and its climate plan.
“Our first line of defense is massively plant,” Mr. Lert said in an interview. “The best natural air-conditioners in Paris are trees.”
Where the city cannot plant trees, officials are putting up more shade structures and water misters to offer solace during hot days. They opened three bathing sites in the Seine river this summer, so people have places to safely cool down during heat waves.
Another key part of the defense plan is insulating the city’s buildings, so they can better resist heat waves. Since 2023, the number of private housing units being fitted with insulation increased to 7,000 annually from 1,500 annually, with an aim to reach 40,000 by 2030, Mr. Lert said.
But the challenge is daunting. There are one million private apartments in Paris, few with insulation, he said.
“It’s a race against time,” Mr. Florentin said. “There is going to be a lot of change. The question is what percentage of change we want and prepare for, and what percentage we just suffer through.”
Catherine Porter is an international reporter for The Times, covering France. She is based in Paris.
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