How do you produce a global sporting event, with millions of people swooping down on one city, in the age of global warming?

That is the test for the Paris Olympics this summer.

The organizers say they’re putting the games on a climate diet. These Olympics, they say, will generate no more than half the greenhouse gas emissions of recent Olympics. That means tightening the belt on everything that produces planet-warming emissions: electricity, food, buildings, and transportation, including the jet fuel that athletes and fans burn traveling the world to get there.

An event that attracts 10,500 athletes and an estimated 15 million spectators is, by definition, going to have an environmental toll. And that has led those who love the games but hate the pollution to suggest that the Olympics should be scattered around the world, in existing facilities, to eliminate the need for so much new construction and air travel. That’s why Paris is being watched so closely.

It is making more space for bikes and less for cars. It’s doing away with huge, diesel-powered generators, a fixture of big sporting events. It’s planning guest menus that are less polluting to grow and cook than typical French fare: more plants, less steak au poivre. Solar panels will float, temporarily, on the Seine.

But the organizers’ most significant act may be what they are not doing: They aren’t building. At least, not as much.

Instead of building new showpieces for the games (which generates lots of greenhouse gas emissions from the manufacturing of concrete and steel), the Paris Olympics are repurposing many of the city’s existing attractions, including the Grand Palais, the plaza known as the Concorde and even a swimming pool built for the 1924 Paris Olympics.

It’s not without controversy.

One notable emissions-reduction effort, a decision to forego conventional air-conditioning at the athletes’ village, has raised concerns. Instead, the buildings will rely on a cooling system that uses water pulled from underground. Several Olympic teams, are considering bringing their own air-conditioners.

Still the hope is that experiments like these will offer a template for other Olympics in the future, and other cities worldwide. The few new buildings that are being built, including the athletes’ housing, as well as a swimming complex and an arena, are using less cement and more wood. They have solar panels and greenery on their roofs.

The new buildings are also meant to have a life far beyond the Olympics. They’re designed to be used by local residents for decades to come and, the leaders of the Paris 2024 organizing committee say, revitalize the city’s suburbs. “We set for ourselves ambitions that have never been set for any event before, let alone have this scale,” said Georgina Grenon, who is in charge of the games’ environmental efforts.

Critics counter that, while much of what Paris is doing is commendable, particularly the limits on new construction, to truly address the climate crisis requires more than paring back emissions here and there. “We need to fundamentally rethink these huge mega events,” said Cesar Dugast, co-founder of a climate analysis group called Eclaircies. “Instead of concentrating all the events in a single city, it could be envisaged to distribute them around the world.”

‘It’s a big test’

There’s a more immediate risk facing the Olympics: climate change itself. Rising global temperatures are making Paris summers dangerously hot. That has heightened concerns about how to protect athletes and fans in late July and August.

City officials say they have planted thousands of trees in recent years to temper summer heat. They are erecting misting towers to spray the air. The search is on for wide umbrellas under which fans can wait. “We have solutions. We are preparing,” said Dan Lert, the deputy mayor in charge of preparing the city for heat. “It’s a big test.”

One major thing that sets the Paris games apart from previous Olympics is that it has imposed a limit on the total emissions it will produce. The target: Generate no more than half the greenhouse gas emissions of the 2012 Olympics, which were held in London.

London was chosen as a benchmark because the organizers there also sought to reduce emissions, and they measured them. Estimates like these are based on standard measures of, for instance, how much carbon dioxide is produced by the amount of cement used in new buildings.

The Paris organizers say they will offset those emissions by purchasing “carbon credits” to help fund emissions-reducing projects worldwide. The games organizers have not said what projects the games will fund and at what price. In any case, the market for carbon credits can be murky, with some projects not delivering on their promise.

Refashioning a historic city

What Paris is doing shows what can be done to remake an old city for a new global climate. It also shows what the limits are.

The Place de la Concorde, an 18th century plaza where guillotines were once erected during the French Revolution, this summer will be home to Olympic events like skateboarding.

The plaza is also now home to an unassuming metal box designed to spur an electricity revolution. It contains a high-powered electrical outlet connected to the national grid, enabling every big event on the plaza to throw off the shackles of diesel.

Diesel generators are the dirty secret of sports events. Typically they are trucked in to provide a stable source of power.

The Paris games have also cut a special deal with the electric utility stipulating that there be enough wind and solar energy in the grid to produce all the energy that the games consume.

When it comes to emissions, transportation is another headache. Paris has already been limiting space for cars and making space for bikes, and it’s using the games to accelerate that shift.

But the Olympics, with its huge crowds, threatens trouble for how Parisians get around their city, with many making plans to flee on vacation.

Pierre Rabadan, a former pro rugby player who is now Paris’ deputy mayor for sports, lifted his shoulders against a wind and walked briskly out of the tram stop in front of the city’s new basketball arena, at the top of Rue de la Chapelle. He pointed to an almost finished bike lane along the road, carved out of what had been a wide boulevard devoted to automobiles.

Since the election of Anne Hidalgo as mayor in 2014, Paris has added some 600 kilometers of bike lanes. Around 10 percent have been dubbed Olympistes, a play on “piste,” the French word for track.

“The problem is we built the city around cars,” Mr. Rabadan said.

Another problem is that the city’s metro system is bursting at the seams. Trains are already crowded, and workers are rushing to complete new extensions of two lines in time to serve the games.

To make room for Olympics visitors, the city has urged people to stay off the trains or work from home.

Tarzan and the old pool

Key to the organizers’ climate strategy is to build as little as possible, which is why it is tapping into a leftover from the 1924 Paris Olympics: the Georges Vallerey swimming pool.

It’s getting a new air filtration system, as well as a new roof that lets in light but keeps out heat and cold. Old wood roof beams have been repurposed as countertops. The wooden bleachers, installed at least 40 years ago, remain. Sturdy stucco walls reveal the pool’s age.

“We need not trash everything or destroy everything and put it in the bin,” said Mr. Rabadan.

The pool holds history. It is where Johnny Weissmuller, an American swimmer, won a gold medal in 1924. He went on to play Tarzan in a string of Hollywood movies, Mr. Rabadan is keen to point out.

Roughly 95 percent of the venues to be used in the 2024 games are old buildings or temporary structures. For example, several temporary pools will be built for the games, then taken apart and re-installed in communities that have a dearth of public pools.

‘A laboratory’

The Olympics, Ms. Grenon said, offer “a laboratory,” particularly when it comes to the buildings designed from scratch.

A new aquatic center, on the edge of a highway in the northern suburb of Saint-Denis, is a showpiece of Douglas fir and pine. Its 5,000 square meter roof curves like a wave: The architects designed it that way to shrink the size of the building, reducing the energy needed to heat the space.

The pool is 5 meters deep only where necessary to have greater depth for diving, and shallower where it’s not. That too saves water and energy it takes to heat the water. Some of that heat will come from a nearby data center. The venue’s 5,000 seats are made from recycled plastic.

The goal, said Cécilia Gross, one of the architects, was “to do better with less.”

Rising nearby is the biggest new Olympics project: the 128-acre Athletes Village complex that is to morph into a mixed neighborhood for 6,000 residents afterward. Its builders say its emissions are at least 30 percent less than a conventional project of its size.

Timber has a starring role here, too. The village is a cluster of mostly wood-frame buildings.

While timber has its own environmental costs depending on how it’s grown, it is considered far more sustainable than concrete.

In the village, a small patch of sidewalk is paved with oyster shells that can be watered from an underground reservoir and cool the sidewalk on hot days. One experimental building is to recycle all its water. To cool the grounds, 9,000 trees have been planted, including local varieties like oaks and elms that can survive in a hotter future.

Then there’s the unconventional air conditioning.

A network of pipes, using water cooled by first being sent underground, will cool the interiors of the buildings in a technology known as a geo-exchange system. In New York City, St. Patrick’s Cathedral uses something similar, but using air instead of water. A smattering of U.S. universities are also switching to geoexchange.

Along with shade trees, insulation and a breeze from the river, the builders say indoor temperatures can be kept cool enough for the Paris summers of the future. However, the games organizers say, Olympic teams are still free to bring their own air conditioners.

The United States, Canada and Norway said they would. Australia and Ireland have too, according to press reports. The mayor, Ms. Hidalgo, in an interview with Reuters, urged teams to “trust the science.”