Heat islands in major French cities: a risk map available district by district

Do you live in a neighborhood that's particularly exposed during heatwaves? On Wednesday, May 21, the Center for Studies and Expertise on Risks, the Environment, Mobility, and Planning (Cerema) published an interactive map of particular interest to the 44 million French people whose municipalities were closely studied. This work aims to highlight the urban heat island phenomenon, which manifests itself as a much higher temperature in the heart of cities than in the surrounding rural areas. This phenomenon is particularly problematic during heatwaves, since the concrete urban environment creates a sort of "heat bubble" that is even more difficult to bear during the day, but also at night, as the thermometer struggles to cool down. This difficulty in finding nighttime coolness is exhausting for the body and has consequences for health.

In Paris, for example, it can be ten degrees warmer in the city center than in the suburbs. Climate change, which is causing summer temperatures to rise ever higher, is further exacerbating this phenomenon, so much so that the capital could see peaks of 50°C as early as 2049 .

Cerema has provided an overview of the sensitivity to extreme heat in the 88 largest urban areas in France. The public institution used very high spatial resolution satellite images and open databases to map local climate zones (LCZs). By zooming in on the map, you can see the exposure of your own house, apartment, or workplace.

In total, according to Cerema, more than five million residents live in neighborhoods "highly sensitive to extreme heat." And an area of ​​more than 200 km2 of built-up areas in France, twice the size of the city of Paris, is "highly or very sensitive to the heat island effect" and would require "significant adaptation actions."

In addition to the capital and its 1.7 million people exposed to the heat island, Lyon, Bordeaux, Marseille, and Lille are at the top of the ranking. In France, these five cities account for nearly half of the entire metropolitan population living in highly sensitive areas, Cerema told Libération.

The smaller the city, the smaller the proportion of people affected. In cities with more than 400,000 inhabitants, these areas represent "nearly 20 % of the urbanized fabric" and two million people live in "high or very high sensitivity" areas, or 50% of the population. In cities with 20,000 to 50,000 inhabitants, only 7% of the inhabitants are affected.

This national mapping of urbanized sectors most prone to urban heat island phenomena is one of the measures of the new national plan for adaptation to climate change (Pnacc), launched in March . "Ultimately, the objective would be to cover all metropolitan and overseas urban areas with more than 20,000 inhabitants," the document specifies. Cerema offers local authorities a free pre-diagnostic service to classify urban areas according to their potential exposure, particularly in the context of urban renewal projects, the revision of an inter-municipal local urban plan or to develop climate change adaptation plans. To mitigate the heat island phenomenon, cities can de-impermeabilize the ground and plant vegetation extensively, repaint roofs white or review the insulation of buildings to avoid installing air conditioners that heat the outside air.