In the summer of 2023, mercury suffocated France and thermometers rose to over 40°C in some areas. In the cities, the temperature was all the more unbearable as the “urban heat islands” are present in some particularly dense corners.
To transform these hot areas into “islands of freshness”, the most popular solution is vegetation. Like Bordeaux, which last July announced the planting of 20,000 trees in the city by 2025. According to the UN, greening in urban areas would reduce the need for air conditioning by 30%.
Carbon storage, reduction of air pollutants, soil restoration, preservation of droughts or floods, its virtues are numerous. Provided you use this solution the right way. Choice of plant species, number and maturity, disposition, soil desensitization, water cycle management are all factors that can affect its effectiveness.
In addition, to put nature back in the city, some opted for a “two-in-one” solution by vegetating the walls and roofs of buildings, like the Quai Branly Museum in Paris which has been wearing its green dress facing the Seine for years. This solution therefore has a dual interest, allowing both to regulate indoor temperatures (and therefore avoid the air conditioning) but also to store rainwater.
This last advantage, allowing to regulate the rainwater, can be very useful for the municipalities faced with the saturation of their purification plants for example. In Paris, the Regional Biodiversity Agency is studying the capacities of about thirty green roofs: the first results of the project “Grooves” – Green Roofs Verified Ecosystem Services – published in 2018, highlight the effectiveness of these devices especially in terms of stormwater storage.
White paint to lower the temperatures of roofs and streets.
This solution was borrowed from Altantic after having already proven itself in Los Angeles. Since then, it has flourished around the world.
Concrete roads and sidewalks are hot sinkholes, absorbing up to 95% of solar radiation during the day and returning it at night. They are therefore largely responsible for heat islands.
To remedy this, some municipalities repaint the asphalt with a white or light gray coating where the sun’s rays are reflected. Like the eco-district of Clichy-Batignolles in the Paris region for example.
In Doha, blue was chosen to coat the roads with “cooling” paint, which lowers the temperature of bitumen by 15 to 20°C, according to the Qatari capital. A solution that would also have the ability to reduce air pollution and fuel consumption.
Disengagement of public spaces and roads to revive soils and limit flood risks
Several cities have started to let the soil breathe, but also limit flooding. In Lyon, the pedestrian Garibaldi street, a major axis of the city, has been redesigned to fight against heat islands, but also to integrate the management of rainwater and soil health.
The city has thus redeveloped the street by unpacking, creating valleys and recovering rainwater for reuse – especially for the targeted mechanical watering of municipal parks and gardens. In addition to revegetating, according to the City, these devices lowered the street temperature by 9°C during the heatwaves.
The school courses “oasis”
Usually made of concrete, children’s games and little vegetation, some municipalities are starting to rethink their school classes. The goal: to transform them into “oases” with water, nature and air.
This is particularly the case in Paris. Supported by ADEME and its RECRE project, the City intends to renovate its schoolyards to replace asphalt with a draining coating limiting heat capture, add water fountains, trees in the ground, educational gardens and vegetable gardens…
In the event of a heatwave, the municipality has also begun opening these schoolyards, spaces of freshness, to the inhabitants of the district.
Urban grasslands to bring back biodiversity
In cities, some abandoned areas find a second wind thanks to the design of meadows. Home to hundreds of plant species and thus making it possible to bring back a little biodiversity in an urban environment, this device combines the advantages of disengagement, the island of freshness and finally positive effects for fauna and flora.
In addition, this solution has the advantage of requiring little maintenance from the municipalities: planting different species and mowing two to three times a year. In the Rhône-Alpes region, for example, the design of drought-resistant multi-species grasslands was undertaken years ago.
It is estimated that in an urban meadow, plant diversity can reach nearly 50 species per square meter. Enough to attract many insects of all kinds, but also birds for example.
English