Since Wednesday, an intense heat wave has spread over the entire French territory. Carcassonne, Cognac, Millau, Dinard, Saint-Brieux… Many cities have broken heat records for the month of June, approaching or exceeding 40 degrees, while minimum temperatures do not drop below 20°C at night. In anticipation of this heat wave and a few days before the second round of legislative elections, government spokeswoman Olivia Grégoire announced on Tuesday the launch of a “cities renaturation program”: 500 million euros to fight against heat waves. This plan must make it possible to “co-finance, alongside local authorities”, devices such as “islands of freshness”.
According to the Intergovernmental Panel on Climate Change (IPCC), by the end of the century, 75% of the world’s population could be exposed to deadly heat waves, especially in cities, against 30% today. The intensification of these extreme climatic phenomena therefore seriously questions the resilience of urban residential areas. Marjorie Musy, research director at Cerema (centre for studies and expertise on risks, the environment, mobility and development), specialist in urban and building microclimatology, explains why cities today are not not adapted to these heat waves. For the researcher, restoring these spaces has now become an emergency.
L’Express: How did you welcome the announcement of this city “renaturation” plan, of which Cerema is one of the partners? What will it consist of?
Marjorie Musy: We currently have no information on the plan and how the resources will be allocated, but it is a good thing that we are emphasizing the quality of life in the city. With each heat wave, we ask ourselves the question: how to make cities more livable “climatically”? The latter concentrate nearly 80% of the French population and therefore expose a lot of people to the effects of urban heat islands. In some cities, the evening temperatures are 7, 8 or even 10 degrees higher than those of the surrounding countryside. However, this urban heat island effect increases the impact of the heat wave and we know that there are more health risks in the city because of this amplification.
Concretely, why are cities warmer?
Solar radiation is trapped in the shape of the streets, in the materials (concrete, bitumen, etc.) that store this heat. The latter have a lot of inertia, so it takes practically an entire night for them to restore it. With the amount of facade, ground, that we have in town, the surface rising in temperature is very important. The whole environment is warmer. Added to this are anthropogenic heat emissions (even if in France the phenomenon is not yet extreme), linked to traffic or air conditioning. In cities where air conditioning is important, we will easily add one or two degrees. It’s simple: by cooling the inside, you warm the outside.
There is also the issue of noise pollution and air quality. In town, when our accommodation overlooks a boulevard or a noisy street, we have trouble ventilating at night because we can’t sleep, and in addition we breathe in exhaust pipes. Buildings would be more habitable in periods of high heat if cities were calmer. The building would not be air-conditioned and therefore would not aggravate the heat problem if it could be cooled at night by ventilating it naturally. It is high time to anticipate the living conditions in the city. It is urgent to act because renaturing takes time. Afterwards, by doing so, we will only gain a few degrees while the heat waves will become more and more frequent and difficult. In the city as in the countryside, it will necessarily be necessary to get used to the heat and to adopt strategies to prevent the buildings from rising in temperature.
What are the solutions to bring more freshness?
We must renature the cities, that is to say bring back nature in the form of revegetation, but not only. You have to think about things in opposition to surfaces built of concrete, metal or bitumen. Vegetation, and in particular the planting of trees and anything that can draw water from the soil, is undoubtedly one of the best solutions. If we want to have an effective nature, that is to say a vegetation which refreshes, which is the objective of the plan since we are told of it at the time of a heat wave, the question of the soil and water is essential.
There is also talk of better ventilating the city by keeping corridors unbuilt, but this is difficult in existing cities. And during a heat wave the wind is weak. It is also possible to shade the streets with stretched fabrics where one cannot put vegetation. And then it is necessary to renovate the buildings taking into account summer comfort. In addition to fighting against thermal colanders, we must ask ourselves the question of other types of housing sensitive to heat, in particular those which have been very glazed and have no solar protection. The attic apartments, for example, must be renovated with great care.
In Barcelona, the municipality has launched a major project updating the Cerda plan [plan de réforme, d’aménagement et d’extension urbaine de la ville de Barcelone datant du 19e siècle, NDLR] in order to make pedestrian and renature one street out of three. It is a strong initiative. We can renature if we remove surfaces that are currently dedicated to the car. The inhabitants living in these streets will thus have a cooler and calmer environment. In France, for example, there could be streets closed at night so that there is less noise.
Do you observe an awareness of the public authorities on the subject?
Yes, there is a fear of reliving the heat wave of 2003. There have been heat waves since but they have been less deadly because awareness of the problem has worked. Today, I even see small communities asking themselves questions on the subject. However, we are not immune to relive such events.
As in Lyon or Bordeaux, many cities plan to plant a lot of trees. But it’s a long and technical process, you don’t transform a city in the snap of your fingers. It takes resources, time to study to make the right diagnoses, know the priority areas and find the most suitable solutions. You can’t plant trees everywhere, you have to choose the plant palettes carefully so that they hold up over time and the fact of reducing the number of vehicles in town can create tension for some drivers.
This is a subject that has entered the minds of elected officials but, unfortunately, it took time. And since it takes time to find solutions… When we were talking about it ten years ago in western towns, people said: “finally climate change is good, we will have a little warmer in summer. There, that’s it, we really realize the consequences.