REPORTAGE. Heatwave: “The task is immense” in Marseille, which wants to go green to adapt to global warming

The Bouches-du-Rhône is regularly subject to extreme heat. For several years, the city of Marseille has been trying to “cool down” with a policy of planting trees in particular.

Forty departments are placed on orange heatwave alert on Monday, August 12, for the second heatwave of the summer that is once again hitting Bouches-du-Rhône, one of the departments most exposed to global warming. And yet, Marseille has fallen far behind in the face of rising temperatures. The Phocaean city has few parks and greenery in its center.

The left-wing municipality, in power for four years, is working to re-green the highly artificial city. “Concrete, concrete and more concrete”, plague a Marseillaise on the Old Port. Very few trees on the horizon and almost no green space nearby.We are suffocating, we have no more shadeshe adds. It was full of trees and they removed everything to build.”

In Marseille, 60% of trees have even disappeared since the 1970s according to Nassera Benmarnia, deputy in charge of green spaces and the return of nature to the city.There has been a denial of global warmingcriticizes the elected official. We have built and concreted everything, to the detriment of nature. The task is immense.” The town hall has therefore launched a “tree plan” like many other towns with the ambition of planting more than 300 000 trees, shrubs and bushes in six years.

It also begins a long renovation of the rare parks in the city center, including Longchamp Park and its monumental fountain.A few months ago it was clayexplains Nassera Benmarnia on site. We planted on both sides 3 500 Mediterranean species and around 120 shrubs to create islands of freshness.” Work that will take time before having an effect : it takes between three and five years for a tree to provide enough shade, and up to 10 years before it fully plays its cooling role.

Similar developments are underway in some of the more than 450 schools in Marseille. Here too, the same objective : “De-concrete and plant”explains the assistant in the courtyard of the Saint-Louis Gare school, in the 14th^e district of Marseille. Part of the concrete in the courtyard has given way to a square of planted earth and a few young trees.

Each time, the municipality has chosen species capable of withstanding the increasingly hot Marseille climate and the increasing number of drought orders, which force the city to water less. “We use as many Mediterranean species as possible, we replace the lawn with creeping ivy, which uses much less water, and the flower carpets become much more resistant perennial plants.”

“Every tree that is planted brings a benefit.”

Marc Saudreau, researcher at INRAE

to franceinfo

The Phocaean city is now on the right track, according to Marc Saudreau, a researcher at INRAE ​​who participated in the project. Cooltrees on cooling cities with trees. “The main effect is a shading effect, we limit, or even cancel out, the energy input from the sun on buildings and on the ground, he explains. In addition, trees have the ability to transpire, and by transpiring, they cool their foliage and will therefore cool the air mass. Trees allow the city to stay at 40 degrees and not go up to 50 degrees. But in no case will they allow the temperature to be lowered by 10 or 20 degrees. They are not air conditioners, they just prevent overheating.”

Hence the importance of combining greenery with other measures to cool the city. : paint buildings white to absorb less heat and de-artificialize soils. Earth is better than concrete, which heats up more and retains heat for longer.