Cities as climate laboratories

After the war, European cities began a two-pronged movement. They expanded and became more functional, compartmentalising uses between living, working, leisure and shopping areas, with little regard for the spaces left for nature and communal areas. This bias has given rise to a number of problems that cities are now facing. “Europe is now the world champion in urban sprawl. Since 2000, it has even overtaken North America,” warns Hélène Chartier. “But this has not improved people’s quality of life.

The vulnerable city

By extending distances, building residential areas and generalising the use of the car that goes with them, we have created cities that emit CO₂, at the mercy of heatwaves, extreme rainfall and urban pollution. Today, they are responsible for 60% to 80% of energy consumption and 75% of carbon emissions. According to Hélène Chartier, the main mistake made by the modern European city is that it was built against the grain of its history. “If we look at European city centres as we know them, from Prague to Paris, Rome or London, we see fairly compact models,” she points out. By expanding, Europe’s major capitals have built up their own sources of vulnerability. “It is estimated that nature and permeable spaces have shrunk by 30% since 1990. By removing nature from cities, we have ultimately removed their best allies in reducing the effects of climate change. Permeable soils and vegetation are natural spaces for sequestering CO₂, absorbing water and providing coolness.

Changing trajectories

The means of changing this harmful trajectory are well known: optimising space, rebuilding common spaces with free movement, desartifying and revegetating city centres, densifying rather than spreading out by building the city on top of the city. “Climate experts such as the IPCC and IPCC estimate that a return to a good urban planning model could reduce carbon emissions by 25% by 2050,” points out Hélène Chartier. “Urban renaturalisation is proving to be a key lever for promoting resilience and adaptation to climate change, and for ensuring the preservation of the water cycle and the management of natural risks in local areas”. But the city is also a place where there is a clash between the desire to make the area attractive, the economic interests of property developers, the desire for individual housing and the need for mobility. Contradictory forces that need to be channelled.

When cities copy themselves

Faced with this situation, metropolises are organising themselves with their own resources and specificities, greening the space formerly reserved for cars, rehabilitating common spaces and hybridising the uses of buildings. There are many ways of doing this, including shade structures in busy streets in Singapore, Tel Aviv and Barcelona. In Paris, London and Milan, the principle of “children’s streets” frees lanes from car congestion. Rehabilitation of infrastructures such as Piazzale Loreto in Milan, a roundabout transformed into a crossroads square with shops, cultural activities and a new green space at its centre. The introduction of “superblocks” in Barcelona, pushing car traffic to the periphery of blocks of 4 or 5 streets in order to give the central space back to pedestrians, cyclists and community uses. “Cities are copying each other with different budgetary capacities. And that’s great,” says Hélène Chartier. “Every time we add greenery and shade to the city, we manage to bring the temperature down by 3 or 4 degrees during heat peaks, as was measured after the installation of green corridors in Medellín, Colombia.

Planning the sustainable city

But cities don’t change on their own. It needs constraints. In Paris, the bioclimatic Local Urban Plan, adopted in 2024 after four years of consultation, symbolises this integration of climate issues into planning. It sets out a strict framework for preserving green spaces and reducing soil sealing. It limits demolition in favour of rehabilitation and conversion of buildings, imposes low-carbon building materials and makes housing densification conditional on social and environmental criteria. “We have no choice. We have to regulate. And we have to do it quickly, without giving anything away” concludes Hélène Chartier. The fact remains that these regulations must ensure that the city remains an inclusive place that leaves no one behind. Otherwise, it will have failed in its mission. “That’s why C40 is talking about inclusive climate action and encouraging cities to include social housing production targets in their bioclimatic planning,” concludes Hélène Chartier.