As heatwaves intensify across Europe, most cities are reaching for a familiar fix of more air conditioning. But in 1990s Paris, planning began for a different kind of solution: one of the world’s largest district cooling networks.

The system has 120kms (75-miles) of underground pipes distributing chilled water to museums, offices, hospitals, schools and other public buildings including the Louvre, the Grand Palais, and some luxury hotels and office districts. Instead of thousands of individual air-conditioning units, cooling is produced centrally and shared across the city like a utility.

The system circulates cold water through a network of pipes: cold river water from the Seine is pumped through one pipe, which runs right next to a secondary pipe carrying warm water from the city’s buildings. A thin metal wall separates them and a heat exchanger allows the heat from the warm city water to enter the cold Seine water without the fluids ever touching. It is similar to holding a cup of hot tea in a bowl of cold water – the liquids don’t touch, but the tea cools down.

That cooler water is then circulated through the buildings connected to the system and the Seine water is returned to the river slightly warmer than before.

The plan was formulated in the 1990s by a subsidiary of the city’s electric utility, Engie, which began conceptualising and building one of the world’s largest district cooling networks to fight the urban heat island effect and improve energy efficiency. In 2022, with help from the Paris city government, the company Fraîcheur de Paris, which translates as “the freshness of Paris”, took over the contract and began a huge, multi-year expansion of the existing underground infrastructure.

“It’s a kind of miracle solution in the era of global warming,” says Thibauld Voïta, an energy and climate expert and adviser at the Jacques Delors Institute.

The city of Paris remains the owner of the network and it is operated by the transportation company RATP and Engie through a 20-year concession contract that was renewed in 2022. It plans to triple the network’s size by 2042, extending it to all the arrondissements and reaching more than 3,000 buildings including critical infrastructure such as hospitals, schools, day-care centres and retirement homes.

Tim Guigon, a spokesperson for Fraîcheur de Paris, said: “Not all of [Paris’s buildings] have the same cooling needs, nor are they all suited to be connected to the network. The figure of 3,000 … reflects a realistic development trajectory. The ambition is to move from a historic network focused on large tertiary buildings to a city-wide infrastructure.”

Beyond the city-wide cooling effects, the hope is that Fraîcheur de Paris will stop at least some of the 2.1 million people who live in Paris from buying an air conditioner, which works by taking the heat inside and sending it into the streets outside. “Everything that requires energy releases heat, and that heat has to go somewhere,” said Sophie Parison, a researcher in Paris who focuses on urban heat and cooling solutions.

Fraîcheur de Paris does not completely avoid this problem – it sends slightly hotter water back into the Seine. But so far, there is no clear evidence that is harming the Seine’s ecology. Studies and monitoring generally show that the system’s heat exchanges create only small, regulated temperature changes that stay within environmental limits.

Pauline Lavaud, the director of climate transition in the city government, says Fraîcheur de Paris “offers much higher energy and environmental performance than individual cooling systems”.

This is backed by experts. “The energy consumption should be much less than if the same cooling were provided by modular systems,” said Charles Simpson, a senior researcher in climate change at University College London, referring to air conditioning units.

Paris is not the only city doing this. Stockholm uses sea water from the Baltic to reduce electricity usage during heatwaves and Toronto uses a cooling system that pulls water from Lake Ontario. Experts said this could work in a mega-city like London, too, but it cannot be copied and pasted.

Lavaud said: “Developing a district cooling network requires substantial investment. For the project to be economically viable and commercially attractive, a city must have sufficiently concentrated cooling demand … where a dense network can be justified.”

Cost is one big hurdle. The total value of the 20-year contract for Fraîcheur de Paris was €2.4bn (£2bn). Replicating a similar structure in London would cost at least that.

For developing economies in the global south, district cooling could be a gamechanger, but high interest rates and chaotic existing infrastructure is likely to make a city-wide retrofit unaffordable. In places where there is less existing underground infrastructure, it could be more doable.

Geography is another hurdle. The Thames, for example, does not have the ideal water flow and temperature for the project, and the city’s underground is congested with utility and Tube lines.

“Actions must always be adapted to the type of city and local issues,” said Emmanuel Gendreau, an ecologist and environmentalist at the Sorbonne. “It is crucial not to simply apply adaptations that have already worked in one city directly to another.”