This is the new hit of the summer, composed with a little more intensity each year by global warming. In France, there are now few summer seasons without a marked heat wave. After an early episode in mid-June in the Rhône, our country was hit again, Sunday August 15, by extreme heat, with five departments in vigilance orange heat wave. “By the end of the century, heat waves will be more frequent, more intense, and spread over a period from May to October”, warns the High Council for the climate in his last report. A problem that primarily affects cities, where the urban heat island (UHI) causes the thermometer to climb a few more degrees.
This phenomenon refers to the temperature difference between a city, which is warmer, and the surrounding countryside, which is cooler. It is therefore a relative measurement, expressed in degrees: we speak for example of a heat island of + 3 ° C. It is caused by a set of disturbances related to urban planning and our way of life: buildings that block air circulation, building materials that store heat, lack of water and green spaces. , air conditioning which rejects heat in the street, automobile traffic, industry, etc. It is most important at night, when the countryside cools off and buildings or roads reject the heat stored during the day. It is also at night that it poses the biggest problem of public health, by preventing the body to recover after the furnace of the day.
>> Five questions to understand the urban heat island and the means to deal with it
Using data of the MApUCE project, a very detailed climate simulation developed by researchers from CNRS and Météo France, franceinfo scrutinizes the degrees of exposure to ICU in 42 urban areas (some, such as Marseille, have not been studied). These data are more precise and more complete than the thermography generally used to speak about the subject. “Thermography only gives the surface temperature, not the air temperature. However, there may be a difference of 20 degrees, it does not give the same information at all”, explains Valéry Masson, urban climatologist at Météo France. This is the first time that this data has been presented to the general public.
A problem that first hits the most populous cities
The urban areas modeled for the MApUCE project form a representative panel, with more or less large cities and different geographical situations (coastal, river, continental). Before looking at these results, a little reminder: the ICU is not the only determinant of the temperature of a city, which depends first of all on the ambient temperature of the region. A heat island of + 2 ° C in Lille or Montpellier does not result in the same final temperature if it is 15 ° C in the North and 21 °C in the Hérault.
The intensity of the urban heat island is classified into four categories: negligible (less than 2 ° C), not negligible (between 2 and 3 ° C), high (from 3 to 6 ° C) and very high (more of 6 ° C). For scientists, action must be taken as soon as the 2 ° C mark is exceeded. “We call the 2 to 3 ° C class ‘tipping zone’. This means that if they are not taken into consideration in the years to come in terms of development, they risk becoming block zones. intense heat “, with a guard Julia Hidalgo Rodriguez, researcher in urban climatology at the University of Toulouse.
How to read this graph: each point represents a city. The size of the point depends on the size of the city and its color indicates the category of its maximum ICU. The value of the latter is displayed by clicking on the point.
As shown in this graph below, for which we have retained the main city of each agglomeration and the maximum value of the ICU, the more the municipality is populated, the more the heat island is strong. This is particularly obvious in the case of Paris. “There is an urban continuum much larger than for other cities and also an urban roughness [la hauteur des bâtiments] very important “ which disrupts the flow of air, analyzes Erwan Cordeau, in charge of studies on climate, air and energy at the Paris Region Institute. Montpellier is the only agglomeration of more than 400,000 inhabitants to present a negligible heat island, thanks to its proximity to the sea.
How to read this graph: each city is placed according to its population on the y-axis (vertical) and according to its maximum UCI on the x-axis (horizontal). These points form a straight line which shows that the more populated the city, the stronger the ICU. Some cities, such as Belfort (with a particularly strong UCI for a rather weak population) or Montpellier (with a particularly weak UCI for a large population) escape this logic.
The ICU is not limited to large cities. “The phenomenon also exists for medium and small-sized cities, which urban planning actors and the population are not always aware of., note Julia Hidalgo Rodriguez. In the Toulouse metropolis, in the peripheral municipalities, some tell us that they are not concerned, that it is not a problem. We have to deconstruct that. ” Smaller towns such as Belfort (46,000 inhabitants, +4.31 ° C) or Beauvais (56,000 inhabitants, +3.75 ° C) are also concerned.
>> MAPS. Visualize the heat island in your urban area
Main cities overheated
Always focusing on the main cities of each agglomeration, we see that a majority of the population of ten of them is exposed to a strong or very strong heat island. Here again, Paris stands out with 100% of its population concerned. Lille (88%) and Lyon (83%) also show impressive proportions.
How to read this graph: in Paris, 10% of the population is exposed to a very strong UHI and 90% to a strong UHI. In Lille, 88% of the population is exposed to a strong UHI and 12% to a weak UHI.
The elderly and higher social categories on the front line
The heatwave of 2003, with its 15,000 deaths, demonstrated this: the elderly are the most vulnerable to the heat. In some of the main cities in our panel, however, a large number of them live in an area with a high UHI. This is particularly evident in the Parisian metropolis – of the 170,330 people aged 75 and over, only 196, living near the Bois de Boulogne and Vincennes, do not live in an area where the intensity of the heat island is strong or very strong – but also in Lille (91%), Grenoble (82%), Lyon (79%) or Reims (75%).
How to read this graph: in Paris, 8% of people aged 75 and over are exposed to a very strong UHI and 92% to a strong UHI. In Lille, 91% of this population is exposed to a strong UHI and 9% to a significant UHI.
We also looked at the social characteristics of the people living in the heat island of these urban areas, focusing on the main city and its neighboring municipalities, where the ICU is strongest. Our data show that executives and higher intellectual professions are more exposed than employees and manual workers, a situation which can be explained by the fact that the ICU is more intense in the city center, where housing is more expensive, than in the suburbs.
However, it should not be deduced from this that the former suffer more from the heat than the latter: their financial resources allow them to cope more easily (renovation of housing, air conditioning, holidays outside the city, etc.). “Who can move in August and leave the hot zone? Who stays at home in Paris or in Seine-Saint-Denis? There is this question of being a captive of a territory”, observes Erwan Cordeau, from the Paris Region Institute. This heat island specialist also points out that low-income people often live in old, poorly insulated housing, which they cannot afford to renovate, and overcrowded.
How to read this graph: on the panel considered (main cities + neighboring municipalities), 40% of employees and workers are exposed to a strong UHI and 1% to a very strong UHI, against respectively 66% and 4% for executives and higher intellectual professions.
Methodological note: the MApUCE project is the result of a collaboration between seven laboratories, the National Center for Meteorological Research, the Interdisciplinary Solidarity, Societies, Territories Laboratory of Toulouse and the Toulouse Architecture Research Laboratory, the Interdisciplinary Studies Laboratory urban areas of Aix-en-Provence, the Laboratory of Information, Communication and Knowledge Sciences and Technologies of Vannes, the Technical, Territories and Societies Laboratory of Marne-la-Vallée and the Coastal, Environment and companies from La Rochelle. The National Federation of Urban Planning Agencies, made up of 52 agencies, was also part of this interdisciplinary research partnership.
The data are the result of a simulation, carried out with an atmospheric model, at the scale of squares of 250 m by 250 m, by Robert Schoetter, researcher at Météo France. The characteristics of cities are modeled using 64 indicators, including the height of buildings, the number of inhabitants or land use. You will find the details of this process in these two scientific articles (here and to be).
To complete them with socio-economic and demographic information, we used data from INSEE, at the scale of l’Iris. This unit, which has 2,000 inhabitants, is the finest available to INSEE. Franceinfo would like to thank Erwan Bocher, Julia Hidalgo Rodriguez, Valéry Masson and Najla Touati for their invaluable help in producing these infographics.
–
