The concept of sustainable cities has been gaining ground for years, but their spread remains too fragmented across the world.

Today, approximately 56 per cent of the world's population lives in cities. That amounts to 4.4 billion people worldwide, and the figure will continue to rise. By 2050, according to United Nations and World Bank projections, it should approach 70 per cent. This is a dynamic that has inevitably intensified, over the decades, the problems afflicting urban centres, starting with the largest ones: pollution, congestion, overdevelopment, social exclusion.

And yet cities are increasingly attractive, primarily for economic reasons. India, China and Nigeria are the countries with the highest concentration of inhabitants in urbanised areas. As a result, it is on these territories that more than 80 per cent of global gross domestic product is already generated today. The growing share of people choosing to live in urban centres, however, demands a radical rethink of how those centres are conceived.

The expansion of the population, especially in major cities, poses a long series of challenges, all tightly bound to another battle: the one for sustainability. Which in turn entails numerous dimensions: social, environmental and climatic. One concrete example is enough to illustrate the scale of the issue. The progressive urbanisation of the coming decades will drive growing demand for housing, which will clearly need to be met. If that response is also to be sustainable, these homes will need to be built in ways that minimise the impact in terms of natural resource use and greenhouse gas emissions. They will simultaneously need to be as energy-efficient as possible. They will also need to provide sufficient thermal insulation to withstand the extreme heatwaves that climate change will inevitably bring — and which will be even more intense in cities. Finally, they will need to be affordable, to avoid creating "social ghettos" and amplifying the already vast inequalities present in our societies.

This immense mass of new city dwellers will also need to get around — to go to work, to shop, to take children to school, or for any other activity. If they were to do so in private vehicles, as is still often the case today, the result would be chaos. A response in terms of sustainable mobility will therefore be needed, but a transition plan will also be required for those who already live in cities, since even for them moving freely alongside hundreds of thousands of new fellow residents could prove impossible.

More than a transition, what seems necessary is in fact a genuine ecological revolution. The reality is that our cities were conceived in a completely different era from the present one. One need only think of roads, which struggle to accommodate ever-larger cars. Or green spaces, which are often far from sufficient, yet will be crucial for countering the urban heat island effect produced by the overwhelming presence of concrete, buildings and asphalt.

Or consider the need to address the exposure of coastal cities to rising sea levels. Since 1985, the surface area of urban zones exposed to a flood risk of more than 0.5 metres has increased by 76,400 square kilometres. That is 50 times the area of a city like London, suburbs included. As a result, around 1.81 billion people worldwide now live in areas at high risk of flooding, including river floodplains.

So are sustainable cities a utopia, or are they truly achievable? The answer is complex. The change required, as noted, touches so many aspects that it must reshape the very morphology and conception of urban centres. To the point that some authors question whether the current economic and social system is genuinely compatible with the need to act on the environmental and climate crisis. In other words, the very way in which city dwellers occupy urban space must be rethought, to avert the risk of a process that generates places that are (even more, in many cases) unliveable.

New Delhi, the capital of India, is in this respect one of the most troubled megalopolises in the world. It is the second most populous city on the planet (after Tokyo in Japan), home to 25 million people. To grasp the scale of its urban demographic explosion, one need only note that it could reach 36 million in the coming years. Similar situations are found in Shanghai, Mexico City, Mumbai, São Paulo and Osaka.

But concretely, what does it take to create sustainable cities? One might think it is enough to plant trees and add a few more green spaces. In reality, however, the necessary interventions are numerous: it is essential, for example, to de-seal land surfaces, to allow water to be absorbed more easily and quickly in the event of flooding. To rethink building architecture, even in something as simple as painting facades in light colours to reflect solar radiation. To use rooftops for solar panels and greening projects.

Changing these places therefore means making a decisive contribution to solving far wider problems. "The notion of a sustainable city makes it possible to take responsibility for and act at the local scale — that of a city or a neighbourhood — on global problems," noted Cyria Emelianoff, professor of geographic spaces and societies at the University of Angers in France, as early as 2007. Fundamentally, it is a form of "mitigation" of urbanisation itself. Most responses aim to "reintegrate" nature into cities: it is no coincidence that the word "green" is frequently used, as in the case of the "Capitals" nominated each year by the European Union as virtuous examples in this regard — from the first, Stockholm, in 2010, to the most recent, Guimarães, which will hold the title in 2026. Passing through Hamburg, Vitoria-Gasteiz, Copenhagen, Essen, Oslo, Grenoble and Tallinn, among others (none of which is Italian).

It was also for this reason that the C40 group was founded in 2005 — a global network of nearly one hundred mayors of the world's leading cities, united in action to address the climate crisis. It now brings together 94 major cities (plus six observers) from 49 different countries, representing 600 million inhabitants and 25 per cent of global gross domestic product. And also, illustrating the "weight" of urban centres in environmental battles, 70 per cent of global greenhouse gas emissions.

According to the network, "over the last decade, cities, regions and states have emerged as a driving force for climate solutions. Since the 2015 Paris Agreement, local leaders have turned promises into progress, cutting emissions, investing in clean transport, expanding urban forests, creating green jobs and delivering solutions directly to the communities that need them most. Around the world, from the streets of Bogotá to the world's largest clean air zone in London, cities are showing the world that a fair, resilient and sustainable future is not just a vision. It is already underway."

Virtuous examples are indeed not lacking. As in the case of Quezon City in the Philippines, where the deployment of electric vehicles is helping to limit climate-altering emissions and local pollution. Cities like London have launched plans to restrict the presence of cars, starting with the most polluting ones. In Cape Town, South Africa, a solar park will power 28,000 homes while avoiding 114,000 tonnes of CO2 emissions per year.

In Paris, two thousand kilometres of new cycle lanes have been created and there are plans to ban internal combustion engine vehicles by 2030. In Los Angeles, low-income families can benefit from a support programme to facilitate the switch to solar power.

In many cases, sustainable cities also incorporate eco-neighbourhood projects. These are places that have been transformed, or conceived from scratch, in accordance with social, environmental and climatic criteria. However, years after the launch of such initiatives, there are also those who warn of possible "distortions." In his provocatively titled book Against the Sustainable City, researcher Matthieu Adam of the French Centre national de la recherche scientifique (CNRS) explains that the main motivations driving municipalities to embark on eco-neighbourhood projects are often unrelated to ecology or to responding to the needs of city residents. The real objective is instead to attract new capital and wealthy population.

In this respect, the example of the Vauban project in Freiburg, in Germany, often cited as a model eco-neighbourhood, is emblematic. "The declared ambition to integrate ecology as a driver of growth does not translate here into a break with ecocidal accumulation," the researcher argues. According to him, such projects need to be combined with a rediscovery of the value of public goods, without growth and the accumulation of wealth as the sole guiding star.

In conclusion, it is possible to say that sustainable cities are not a utopia. On the contrary, they already exist. But in a fragmented way. As "green dots" that still clash with too much surrounding grey and black. They can take the form of a cycle lane, a building clad in vertical forests, beehives installed in parks, or new urban furnishings based on the concept of greening. But these fragments are still too scattered. Unevenly distributed, they risk even feeding the social differences that should instead be reduced. And too often they are the product of short-term thinking, haunted by the Sword of Damocles of budget cuts as well as cultural barriers.

This does not mean demanding that our cities transform themselves entirely and in a short space of time. It does, however, mean accepting that a transition is inevitable, just, and must involve everyone: policymakers, local authorities, governments, citizens, architects, businesses — each with their own role. It must become a shared cultural heritage, sustained by common political will, capable urban engineering, measurable results and adequate funding. To make sustainable cities virtuous places in which our children will live, and not merely oases populated mostly by the well-off, the transformation must take the form of a collective mobilisation.