2023-08-25 13:37:56
Hot days, followed by nights when the temperature does not cool: this definition of a heat wave is even exacerbated for the inhabitants of urban areas, who experience the phenomenon of urban heat islands (ICU).
What is an urban heat island?
Temperatures in dense urban areas are higher than in surrounding rural areas, especially at night. From the beginning of the 19th century, the British meteorologist Luke Howard observed in London a difference of up to 3.7°C between the city center and the countryside. Research continued into the 20th century, but it was accelerating climate change that increased interest in studying UHIs and how to adapt to them.
The gap between town and country can be observed in all seasons, but “we talk about it a lot during the summer period, because there can be differences of 8 to 10°C at night, for example between the heart of Paris and the outer suburbsexplains Sandra Garrigou, climate plan and adaptation project manager at the Paris Region Institute. In summer, this leads to health problems, the body does not cool off and therefore does not recover. »
See also: City life tested by extreme heat What is this excess heat in cities due to?
The causes are multiple, and linked to the specificities of the elements of an urban environment:
the height and density of the building: the higher and tighter the buildings, the more they block the passage of the wind and reflect the sun’s rays, creating a “canyon effect”; the building materials: stone, brick or concrete accumulate heat from the air and the sun’s rays during the day, to release them at night, like storage heaters do. This is also the case for mineral or bituminous soils; the lack of vegetation: lawns, shrubs and even more so trees are thermal regulators, both through shade and through the effects of evapotranspiration. Plants use solar energy to extract water from the soil through the roots and release it through evaporation into the atmosphere, rather than accumulating and releasing it at night. Rivers or bodies of water also serve as thermal regulators; human activities: the use of heating or air conditioning (which releases hot air outside), car traffic, industrial activities and the treatment garbage…
In Ile-de-France, a work cartography was carried out by the Paris Region Institute, on a very fine scale for each “morphological island”: block of houses, squares, parks…. Different databases and geographical surveys have been cross-referenced to define the type of land use, the density and the height of the building, and to define whether these blocks, according to their characteristics (large complexes, pavilions, tertiary, etc.), are likely to generate heat.
To measure the risks associated with UHIs, these approaches have been enriched with indicators on the vulnerability of the population, such as age (proportion of people over 65 or under 5), the presence of retirement homes, the access to care (general practitioner, hospital emergencies) or the proportion of low-income households. Indeed, in well-to-do neighborhoods, residents have more means to leave the city during heat waves than those living in more modest neighborhoods.
Which cities are the most affected by the phenomenon?
On a national level, the Mapuce project, coordinated by the National Meteorological Research Center (CNRM), modeled heat island effects in 42 major urban areas in France. The researchers used a city model that meshes the territory into rectangles with sides of 250 meters and simplifies the shapes of the streets and buildings, while preserving the surfaces (footprint, facade, height).
The model then simulates the interactions between the surface and the atmosphere and calculates the temperatures at 2 meters above the ground, for a meteorological situation favorable to a strong urban heat island. “What we look at is the difference between the city and the surrounding countryside, says Robert Schoetter, responsible for sustainable development research at the CNRM. There is a time range from 4:00 a.m. to 6:00 a.m., when the ICU is strongest. »
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Results, published in the journal Science of the Total Environment in October 2020, reveal inequalities between French cities. The Paris conurbation is by far the most exposed to the UHI phenomenon, with many areas where the temperature is 4 to 6°C higher than in the Ile-de-France countryside, and peaks in the central districts of Paris. But other large cities, such as Lille and Lyon, show a higher maximum intensity of 4.9 ° C and 4.7 ° C.
The researchers found a very clear positive correlation between the size of the population of urban areas and their ability to retain heat. The geographical location and the immediate proximity to the coast (less than 10 kilometers) also play a role: mountainous and semi-continental climates favor the intensity of UHIs, unlike cities subject to Mediterranean or oceanic climates.
What solutions to cool cities?
Like the factors of vulnerability, the solutions can be multiple and their effects can be added. One of the solutions is to reintegrate nature into the city: remove waterproofing from surfaces, plant trees, revegetate floors, facades or roofs (with species that are resistant and adapted to the heat and available water), discover rivers or rivers…
Buildings and urban planning are also crucial: integrating summer comfort into construction or renovation criteria, choosing light coverings or white roofs, which reflect the heat. On a neighborhood scale, adapt to wind corridors, build public fountains or shadehouses.
Finally, to improve the comfort of the inhabitants and limit the health risks of heat waves, other actions can be put in place: supporting the elderly and vulnerable, shifting working hours or public services (swimming pools or parks). ” Mapping tools are decision-making aids for elected officials, to understand that there is a problem identified in particular areas », summarizes Sandra Garrigou, for whom the fight against UHIs “is a cross-cutting issue, linked to the development of cities for which elected officials have the tools”.
Read the forum: Article reserved for our subscribers “Suffering the effects of heat waves is not inevitable, but a political choice”
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