On the steps of the dock of the Confluence, in the eponymous district of Lyon, about thirty trees, oaks, neem trees, honey locusts and Chinese hackberries, have appeared this spring. Even very close to the river, the very mineral character of this space made the heat unbearable there until then. “Now, people are picnicking in the shade of the trees. It’s a modest development but which brings comfort”, rejoices Béatrice Vessiller, vice-president of the metropolis of Lyon, delegate for town planning and the framework. of life.
This plantation is an example of the efforts being made to try to make the city less stifling as climate change causes temperatures to rise. The trend is general. According to Météo France projections, by 2100, the number of heat wave days – where the temperature is more than 5°C above the norm for several days – would be multiplied by 2 in a scenario of low gas emissions. greenhouse effect and by 5 or 10 in more pessimistic scenarios. The south-eastern quarter of France is the most exposed to this increase. In Lyon, we can expect one or two months of heat waves in the summer.
However, the capital of Gaul, like the other cities, is particularly sensitive to this rise in mercury due to the phenomenon of urban heat island (ICU) which leads to the temperature in the city being higher, especially at night. , than in the surrounding countryside. Several factors contribute to this. “Materials such as bitumen or concrete, exposed to the sun, store heat during the day and release it at night, which prevents the air from cooling”, explains Valéry Masson, researcher at Météo France. Urban morphology can also block the circulation of winds. Finally, heat emissions related to human activities are also involved. Between the city center and the Monts du Lyonnais, about 25 km away, there is a temperature difference of 5 or even 10 degrees during heat waves.
Temperatures of 38.1 degrees
The consequences are first of all sanitary, the heat severely testing the organisms. In 2003, during the heat wave, a peak of more than 80% in the number of deaths was observed in Lyon. Heat waves are often accompanied by an increase in air pollution, and they raise fears of water shortages. They weigh on economic activities, test infrastructures and tarnish the attractiveness of the city, transformed into a furnace. In mid-June, the Lyonnais suffered temperatures of 38.1 degrees during the day and 24 at night. Line A of the metro has suffered serial damage, municipal services have had to adapt their schedules and racetracks postpone planned races.
These already perceptible impacts explain why many municipalities are considering the issue of adaptation to heat. “Until recently, the challenge of mitigating climate change was at the forefront and questions of adaptation were little addressed. The multiplication of heat waves means that they have an immediate interest in asking themselves this question”, notes Morgane Colombert, project director at Efficacity, an R&D institute on the ecological transition of cities. Lyon, along with Paris and Grenoble, has a certain lead in this area, as it began mapping heat islands on its territory in the 2010s and added, in 2017, an “adaptation” section to its climate plan dealing with in particular the fight against these UHIs. The ecologists, who arrived at the head of the metropolis and the city in 2020, show their desire to accelerate on the subject.
“We have reinforced ambitions around the greening of the city and the management of water resources”, says Béatrice Vessiller. The requalification of rue Garibaldi, the first stage of which was completed under the previous term of office, is emblematic of this strategy. 600 trees were planted over 800 meters, also with low plant strata. Light concrete has replaced the bitumen and a rainwater collection basin has been built.
Up to 300,000 more trees
By 2030, the metropolis intends to increase the canopy index of its territory, i.e. the share of its area covered by the crowns of trees, from 27% to 30%, by increasing its heritage by 100,000-300,000 specimens – so many natural conditioners. “The shade of the tree first prevents part of the ground from being exposed to the sun’s rays and accumulating heat, explains Pascal Goubier, deputy director of plant heritage at the metropolis. Then, the tree draws water in the soil and transforms it into vapor at the level of the leaves – we speak of evapotranspiration – which generates a drop in temperature. The nature plan voted last June allocates 44 million euros for the all-round revegetation of the metropolis. The city, for its part, has programmed a plan of 141 million euros of investment in this area by 2026.
Greater Lyon is also working on the issue of water, the presence of which, in the form of fountains, misters or in non-artificial soils, helps to lower the ambient temperature. It plans to de-waterproof 400 hectares on its territory by 2026, replacing the bitumen with porous coatings or grass, and requires builders to respect open ground coefficients on each plot, via its local development plan. town planning. “The permeability of the soil allows rainwater to infiltrate and then evaporate, which contributes to cooling. This water is also the resource for the evapotranspiration of trees”, explains Frédéric Ségur, from the ecology department of The city.
These “green and blue” solutions are part of the adaptation axes recommended by Ademe and are acclaimed by local authorities. “They are multi-benefit, which often makes them more economical”, underlines Frédéric Ségur, recalling that they allow to play on several levels: air quality, access to nature, management of stormy rains. But faced with the rise in mercury, it is also necessary to bet on “grey” solutions, mobilizing engineering sciences, to improve the insulation of buildings or select coating materials that store less heat. On the sidewalk of rue Béchevelin, in the 7th arrondissement, Lyon is, for example, testing an anti-heat paint. “Our Ecoréno’v plan takes thermal comfort in summer much more into account than before,” adds Béatrice Vessiller. This aid scheme has made it possible to renovate nearly 20,000 homes since 2015.
Encourage the natural ventilation of buildings
Behavioral changes are also encouraged to limit heat emissions related to human activities. Greater Lyon, for example, prohibits air conditioning in these concerted development zones, encouraging builders to favor the natural ventilation of buildings. While ambitions are high, adapting to the heat poses many challenges. Technical first. The metropolitan teams are thus striving to determine the best tree species to plant to resist climate change and the measures to be taken in their first years to ensure their sustainability. Cultural too: the collaboration of the metropolis and the municipalities is essential.
Finally, adapting also poses a problem of scale for communities. “Public space is 30% of our territory”, recalls Béatrice Vessiller. Each portion of this green space can help create cool islands, spaces where residents can take refuge to better withstand the heat and for which the government announced in June the establishment of a fund of 500 million euros. Lyon already publishes an interactive map of its cool places, indoors or outdoors. “But to have an effect on the temperature on the scale of an entire city, you need to vegetate a lot”, notes Valéry Masson. The metropolis therefore offers aid to condominiums and social landlords to encourage them to go in this direction. On the heat front, in Lyon as elsewhere, victory will require the mobilization of all players.