The lake that cools a city

How Lake Geneva turned into one of Europe’s most ambitious climate adaptation projects

When we think about the future of cities, we often imagine smarter buildings, greener transportation, or solar panels covering rooftops. Rarely do we think about what lies beneath the surface. Yet in Geneva, Switzerland, one of the world’s most ambitious urban energy projects is almost entirely invisible. Beneath the streets, parks and riverbanks, a vast network of underground pipes is quietly transforming the way an entire city heats and cools itself—using nothing more than the naturally stable temperature of Lake Geneva. As heatwaves become longer, more frequent and more intense across Europe, the Swiss city has found an elegant answer to one of the defining challenges of climate change: how to keep buildings comfortable without dramatically increasing electricity consumption or greenhouse gas emissions. The project is called GeniLac, and it offers a glimpse of what climate adaptation could look like in the cities of tomorrow.

Cooling a city with a lake

Forty-five metres below the surface of Lake Geneva (Lac Léman), the water remains at a remarkably constant temperature of around 7°C (45°F) throughout the year. Unlike surface waters, which warm dramatically during summer, these deeper layers remain stable thanks to the enormous thermal mass of the lake. Engineers realised that this naturally cool water could become a renewable energy resource. Instead of relying on thousands of individual air-conditioning systems, GeniLac pumps cold lake water through a network of underground pipes. Inside connected buildings, heat exchangers transfer unwanted indoor heat to the lake water without mixing the two systems. The slightly warmer water – its temperature increased by no more than 1.5°C, in accordance with strict environmental regulations – is then returned to the lake. During much of the year, this process requires no conventional refrigeration at all. Engineers refer to it as free cooling: the lake itself provides enough cooling capacity that energy-intensive compressors become unnecessary. The result is extraordinary. Compared with traditional air-conditioning systems, connected buildings can reduce cooling energy consumption by up to 80 percent, while eliminating the need for chemical refrigerants that can contribute to global warming if released into the atmosphere. Perhaps even more remarkable is the fact that GeniLac does far more than cool buildings. During winter, the same network becomes a source of renewable heating. Water at 7°C may not sound warm, but for modern heat pumps it represents a valuable source of thermal energy. Building-level heat pumps extract heat from the lake water and amplify it to the temperatures needed for radiators, underfloor heating and hot water. The same pipes that remove heat from offices during July can help warm apartments during January: it is a simple but powerful example of circular energy thinking: one infrastructure serving two opposite needs across the seasons.

The video sums it all up:

From the United Nations to schools, to airports

The origins of the project date back to 2009, when a much smaller system known as GLN (Genève-Lac-Nations) was developed to serve the international organisations clustered around Geneva’s diplomatic district. The first major customer was the Palais des Nations, the European headquarters of the United Nations. There is a certain symbolism in that choice: the institution that regularly publishes reports warning about climate change became one of the first buildings to reduce its own cooling emissions using the lake beneath its windows. Soon afterwards, other international organisations followed, including the World Health Organization, the World Trade Organization, the International Telecommunication Union and the World Meteorological Organization. As the benefits became evident, the city decided to think bigger: today, GeniLac is expanding well beyond diplomatic compounds: schools, universities, theatres, municipal buildings, hospitals, residential blocks and cultural venues are progressively joining the network. In Geneva’s Jonction district alone, approximately 90 buildings were connected during 2025, avoiding an estimated 10,600 tonnes of CO₂ emissions every year.
One of the most significant developments in 2026 is that Geneva Airport has officially become part of the GeniLac network, marking a major milestone in the project’s expansion from a district energy system into a city-wide infrastructure. As part of its CAP2030 decarbonisation strategy, Geneva Airport is replacing its traditional fossil-fuel-based heating and cooling systems with energy supplied through GeniLac. The project includes the construction of a 3,800-square-metre energy centre, developed in partnership with Services Industriels de Genève (SIG), which will distribute renewable heating and cooling throughout the airport’s terminals and facilities. Once fully operational, the airport expects to eliminate the consumption of approximately 2 million litres of heating oil every year, reducing its direct CO₂ emissions by around 60%. It is one of the clearest examples of how district energy systems can decarbonise not only residential neighbourhoods but also highly energy-intensive infrastructures.

The most interesting part of it all is that, unlike spectacular wind farms or vast solar parks, GeniLac is almost invisible, as it most impressive engineering works are buried underground. The heart of the system is a pumping station known internally by engineers as “the cathedral,” located beneath the Vengeron area near the motorway. From there, kilometres of underground pipelines extend beneath roads, parks and even sections of the Rhône River. Residents rarely notice the infrastructure unless construction work is temporarily underway. Once completed, the city returns to normal: trees are replanted, streets reopened and daily life resumes, while the hidden network continues operating silently beneath everyone’s feet. This invisibility may actually be one of the project’s greatest strengths. Rather than changing how people live, it changes the infrastructure that supports urban life.

Why district cooling is gaining momentum

Geneva is not entirely alone: around the world, cities are increasingly exploring district cooling systems that produce cooling centrally rather than building by building. Projects using seawater already operate in cities such as Toronto, Stockholm, Helsinki and parts of the Middle East. Others exploit rivers, aquifers or underground geothermal resources. The principle is always similar: instead of each building running its own air-conditioning units, entire districts share efficient thermal infrastructure. The International Energy Agency (IEA) has repeatedly highlighted district heating and cooling networks as essential tools for decarbonising cities, particularly as electricity demand for air conditioning is expected to soar in coming decades. According to the IEA, global cooling demand could more than triple by 2050 if current trends continue. Without more efficient alternatives, cities risk entering a vicious cycle: higher temperatures drive greater air-conditioning use, which increases electricity demand and, in many regions, greenhouse gas emissions – further accelerating climate change; systems like GeniLac help break that cycle.

Perhaps the most interesting aspect of Geneva’s project is philosophical rather than technological: the underlying technologies – pumps, heat exchangers, insulated pipes and heat pumps – are not revolutionary, most have existed for decades. The innovation lies in treating an entire city as a connected energy ecosystem rather than a collection of isolated buildings. Instead of asking every office block or apartment complex to solve its own heating and cooling challenges, the city provides shared infrastructure that benefits everyone. This approach reflects a broader shift occurring across Europe, as it shows how the energy transition is increasingly moving away from isolated devices towards integrated systems: district heating, smart electrical grids, thermal storage, wastewater heat recovery and renewable energy networks that work together. In this context, lakes, rivers and underground geology are no longer viewed simply as natural features. They become strategic assets in building climate resilience.

Lessons beyond Switzerland

Geneva’s geography is undoubtedly unique. Not every city sits beside one of Europe’s largest alpine lakes, yet the broader lesson extends far beyond Switzerland: every city possesses resources that can be reimagined – whether seawater, rivers, abandoned mines, wastewater networks or geothermal reservoirs. The challenge is not merely technological but political, financial and cultural.

GeniLac itself required long-term planning, substantial public investment and democratic support. In 2022, Geneva’s citizens endorsed the project’s expansion through a public referendum, illustrating how climate infrastructure increasingly depends not only on engineering excellence but also on public trust. As cities worldwide grapple with rising temperatures, Geneva offers an encouraging reminder that climate adaptation does not always require futuristic inventions. Sometimes the smartest innovations are those that quietly work with nature rather than against it.