Marianne Cohen , Sorbonne University ; Laurence Eymard , Sorbonne
University ; Romain Courault , University of Paris 1 Panthéon-Sorbonne and Serge Muller , National Museum of Natural History (MNHN)
By compiling data on more than 800 European cities, a recent scientific study estimated that Paris was one of the deadliest cities in Europe in the event of a heat wave.
To understand why, we must look at the notion of urban heat island (UHI). A phenomenon well known to meteorologists and which exacerbates the local increase in temperatures with the multiplication of heat peaks in the summer period.
Still according to this study published in the Lancet Planetary Earth, 90% of Parisians were exposed to a high intensity urban heat island (between 3 and 6°C difference) and 10% to a very high intensity urban heat island ( difference of more than 6°C) in 2021.
If today protecting urban habitats from heat peaks has become a public health and environmental issue, these issues have rarely preoccupied the supporters of urban development in past centuries.
Cities were first built to protect their inhabitants before integrating hygienist objectives . More recently, cheap oil and the private car have favored the establishment of family policies and planning that facilitate urban sprawl.
In Paris, a heat wave is defined as an episode of at least 3 consecutive days where the maximum temperatures exceed 31°C and the minimum temperatures 21°C. That of 2003 was an extreme event which raised awareness in Europe, given its geographical scope and its health impact. Since then, heat waves have followed one another and will get even worse between now and 2050 , without any real political debate on urbanization models except for exceptions .
However, heat waves represent a direct danger to the health of populations, and particularly affect neighborhoods that are already vulnerable. So let's first try to understand why the urban heat island effect is particularly harmful in Île-de-France, before seeing how we could remedy it.
The urban heat island
All artificial surfaces generate excess heat, the average temperature in town being a few tenths of a degree (small town) to several degrees (metropolis) higher than that of the surrounding countryside. For example, a difference of 4°C was observed between the center of Paris and the outlying woods during the 2003 heat wave.
To understand why, several factors must be taken into account.
The ICU increases with:
Heat due to human activities (combustion, air conditioners, heating, servers, etc.).
The nature and color of the materials: concrete, asphalt, tiles and other dark mineral and synthetic materials which absorb solar energy during the day and re-emit it at night (thermal radiation).
The height and spacing between buildings: a high building density traps hot air and limits the cooling of surfaces and walls. The tall buildings and the horizontal extensions of the metropolis cause an aerodynamic slowdown, limiting the evacuation of heat.
Conversely, the attenuation factors are:
Natural soils, vegetation and water: soil made up of gravel contains (insulating) air pockets, which limit heat absorption and its light color reflects solar radiation. Water has a strong cooling power, thanks to surface evaporation. Healthy vegetation plays the same role, through its transpiration. It can develop in all the interstices of the building, more easily and durably than water tables.
Shade: Grounds shaded by shoreline buildings, shadehouses (structures intended to provide shade) or tall trees accumulate less heat.
Light floors, walls and roofs reflecting sunlight. They therefore store less radiation than dark materials. On the other hand, the reflection of the sun can aggravate the heat of the air near the surface during the day.
Locally, the hot surfaces cause an atmospheric depression, which favors the circulation of air coming from the cooler peripheries ( nocturnal thermal breeze ).
Similarly, the relief promotes the circulation of air down the slopes during the night.
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The various parameters set out above mean that the Métropole du Grand Paris (MGP) has a very strong urban heat island. The continuous urban fabric around Paris intra muros further aggravates this situation.
In the outlying districts where the poorest households live, buildings are mixed with industrial and commercial areas, and daytime temperatures reach extreme values due to the predominant impermeable and dark coverings .
On the contrary, to the west of Paris and in the bend of the Marne where the wealthiest households live, the temperatures are lower, either close to the average, or cooler, due to the extension of residential areas with gardens. , often located near large green spaces.
Air circulation is also favored by the cool corridors entering the city (air along the Seine valley, or coming from the nearby forests on the plateaus to the South-West). Conversely, air circulation is slowed down in the heart of the city, in the northern districts and near dense suburbs, by the minerality and the height of the buildings (including many buildings on slabs).
Some urban planning models, such as garden cities or green belts, alternatives promoted at the turn of the 20th century could be sources of inspiration, thanks to larger areas of tree vegetation, permeable soils, perennial rivers and ponds, also contributing to the well-being of the inhabitants.
This is the case for Greater London, which in addition to being bordered by wetlands and large water reservoirs enjoys large parks and a hydrographic network that accentuates the freshness of the sea. Due to its less dense urbanism and its slightly more northern location, Greater London experiences much lower temperatures than Greater Paris .
As Greater Paris is already largely built, the aim here is above all to offer solutions for adapting to and mitigating heat waves and other climatic extremes, all without aggravating existing inequalities.
Rethinking urban planning in the face of climate change
Further greatly densifying the urban fabric, as provided for in the Île-de-France master plan, will have the effect of increasing the surface area of the dome of the urban heat island, and will most likely exacerbate its maximum intensity in the center, being since the air circulation may be blocked at the periphery. As Parisians have experienced, especially in 2022, the center of the metropolis will become unlivable in hot weather. This will lead to an aggravation of social inequalities, without solving the structural problems of Île-de-France, since the only way out for households is to move to the outskirts or even to the provinces for the luckiest.
In order to guarantee a satisfactory quality of life without increasing its surface area and its social and spatial disparities, the most promising avenue is therefore to exploit internal sources of cooling and to optimize air circulation at all scales.
What to preserve and improve
Existing trees, whether planted in green spaces, along alignments, or in garden cities, as well as permeable areas not covered with bitumen, sports fields not covered with synthetic surfaces, must be preserved, multiplied and extended.
The agricultural areas located outside the Greater Paris Metropolis for the most part (e.g. Plateau de Saclay, Triangle de Gonesse), must also be preserved or preserved, because the soils there are permeable and relatively cool (except for long periods of drought). At the scale of the territory, it is essential to preserve areas of low urban density, because they provide an interesting natural ground surface (individual gardens or small groups of buildings), and a potential for air circulation through low frame height.
However, the opposite is happening in the Metropolis of Greater Paris. The garden cities, because of the aging of the built environment, are threatened with destruction, whereas they should be rehabilitated and classified, because this model of town planning is relevant both from a social and bioclimatic point of view. New constructions in former residential areas classified as housing islands in the Mode of Land Use (Digital inventory of land use in Île-de-France established by the Paris Region Institute) occupy the essential part of the land, the gardens having been destroyed. The proliferation of underground infrastructures also limits the possibilities of revegetation. The facilities for the Olympic and Paralympic Games are insufficiently virtuous in this regard.
The urban projects in progress for this deadline were designed ten years ago in a carefree era that is now over. Evidenced by the recent update of the Master Plan of the Ile-de-France Region. These projects should therefore be amended to reduce the impact of heat waves (in particular prohibiting dark coatings on buildings (walls, shutters and roofs), promoting air circulation in apartments, insulating walls from the outside, spacing out buildings and surrounding them with tree-lined gardens Similarly, projections of population growth and housing needs in the Greater Paris Metropolis should be updated in the light of the post-Covid-19 pandemic era , which reduced them.
Vegetate, of course, but how?
The Metropolis of Greater Paris is particularly devoid of tree vegetation, as shown by the map of the Atelier parisien d'Urbanisme, updated in 2021 . Complementary tree plantings are therefore highly desirable, in all appropriate spaces . These plantations will have to be adapted to the climatic conditions of the coming decades . They may take different and complementary forms depending on the spaces:
- Multiplication of plantations of varied and adapted species along the avenues in order to increase the shade of the roads and sidewalks, to constitute corridors favorable to the circulation of biodiversity and to favor less air-conditioned cars. Originally, the plantations on the main boulevards were dominated by a small number of species. Out of 1,900 trees on the Champs-Élysées, for example, around 900 are chestnut trees and 560 plane trees. This type of monoculture is clearly not to be favoured, due to the limited adaptability of these species in the face of climate change, the danger that a plantation with only a few species runs in the face of possible pathogenic threats and the poverty of this type of plantation for biodiversity.
Creation and/or extension of new parks, gardens and urban squares, on the multifunctional model of the Haussmannian parks of Paris, constituting spaces of recreation (and refuge, even at night during episodes of heat waves) for the populations .
Increase in revegetation (with the inclusion of shrubby and herbaceous plants) on the mineralized squares of the metropolis, on the model of the tree plantings carried out at the Place de la Comédie in Montpellier or at the Place de la Gare in Strasbourg.
Maintenance of spontaneous wasteland, like those that have developed on the edge of the "small Parisian belt" .
Creation of new forests or dense urban groves, constituting islands of freshness, following the example of the"Miyawaki micro-forests" (of the order of a few hundred m 2 ) or projects on larger areas (1 per hectare) like the one currently being built on Place de Catalunya in Paris .
All of these plant formations, complementary and adapted to each local situation, will contribute to increasing the overall canopy index of the Greater Paris Metropolis and tending towards a real "urban forest" which constitutes the best possible adaptation of cities to coming heat waves .
It is also important, on a regional scale, to connect these vegetated zones by cool corridors, oriented in such a way as to optimize the circulation of the prevailing breezes arriving from cooler agricultural, forest or humid zones located on the periphery.
Marianne Cohen , University Professor in Geography, Sorbonne University ; Laurence Eymard , Emeritus CNRS Research Director, researcher in the field of climate and the environment, Sorbonne University ; Romain Courault , Lecturer in Geography, University Paris 1 Panthéon-Sorbonne and Serge Muller , Emeritus Professor, researcher at the Institute of Systematics, Evolution, Biodiversity (UMR 7205), National Museum of Natural History (MNHN)