When cities overheat
Urban overheating is becoming a recurring phenomenon, resulting from the cumulative effect of climate change and the Urban Heat Island (UHI) phenomenon. Climate change is leading to an increase in the intensity and duration of heat waves (hot air masses causing high temperatures for several consecutive days) in different parts of the world. These meteorological phenomena themselves reinforce the UHI, a climatic effect causing a difference in temperature between the centre of urban areas and outlying or natural areas, which can be greater than 10 °C during heat waves. The effect is even more pronounced at night, when the heat stored by artificialised land and buildings is released into the air, keeping the temperature high. Faced with the various consequences (social, environmental, economic) and climate projections, local and regional authorities are now faced with the need to adapt their cities in order to combat this phenomenon. Below is an overview of the challenges and solutions.
Factors related to urban morphology, surface types and human activity
The phenomenon is now well known: the mineral surfaces of a city (concrete, stone, road surfaces, etc.) are heated by the sun’s rays. They absorb and store part of this energy, which is released at night by convection and infrared radiation. Urban forms contribute to the phenomenon by blocking wind circulation and trapping heat (proximity between buildings, limited opening of urban structures to the sky). Similarly, certain urban activities (transport, industrial activities, etc.) contribute to the increase in temperature by emitting heat.
Environmental and human health issues
There are many consequences, from social to environmental and economic. Health risks are increased, especially for the elderly, infants and young children, women over 45 and people with chronic diseases or psychiatric disorders. They can result in sunstroke, dehydration and hyperthermia. Beyond the health aspect, UHIs affect the well-being and comfort of populations: the use of outdoor spaces and buildings can become uncomfortable for users, affecting daily life and the economy (reduced productivity at work, sleep disorders).
The challenges of combating UHI are also environmental. Local climatic variations linked to UHIs can affect local plant and animal biodiversity, threatening the survival of certain species or, conversely, favouring the proliferation of invasive species. There is also a risk of increasing energy demand in buildings and transport during the summer months due to air-conditioning, which exacerbates climate change.
Diagnosing and modelling the phenomenon
Faced with these challenges and climate projections, local and regional authorities are now faced with the need to adapt their cities in order to combat this phenomenon. The challenges of urban overheating are integrated at each stage of the development process, from the strategic vision (planning on a regional and urban scale) to development projects, buildings and public spaces (in the specifications, in the design phase, construction site, etc.). All scales (urban area, neighbourhood, block, building) are intertwined and interact: a building can both play a role in the UHI (wind blockage etc.) and suffer its effects.
Diagnosis is a prerequisite for adapting strategies to take action on UHIs. Detailed mapping of the phenomenon requires specific tools (measurement of surface temperatures by thermal infrared radiation etc.), as the measurement networks of weather stations are usually not dense enough to characterise this precisely. These quantitative measurements must be accompanied by qualitative surveys of users, both to identify their feelings and perceptions and to raise their awareness of vulnerabilities related to UHIs.
On the scale of urban projects, modelling the phenomenon makes it possible to anticipate its effects and adapt the urban fabric accordingly. It is useful for guiding design choices, whether urban planning, architectural, vegetation, soil treatment or facade materials. This is the aim of an experiment conducted by the University of Burgundy, the Centre for Expertise in Energy Efficiency (C3E), Bouygues Construction R&D and Linkcity in the Pont des Tanneries district of Dijon. The goal was to measure the temperature changes caused by a cooling design in the district. The exercise had a concrete impact on certain choices: limitation of impermeable coverings, addition of drop shadows, limitation of radiation towards the facades, proper positioning of plantations and modification of urban forms, in particular on the “hot spots”, to allow better ventilation.
Drawing on nature and ecosystem services
There are many levers for action, including drawing on nature and ecosystem services. For example, the city of Lyon is implementing its 2017-2030 Canopée plan to increase the number of trees in the area. Over 300,000 trees will be planted with the aim of increasing the canopy area from 27% to 30% by 2030. Trees cool the ambient air by intercepting the sun’s rays, shielding mineral surfaces from the sun’s rays and by evapotranspiration (release of water into the atmosphere in the form of water vapour). There are also open spaces associated with them, which allow rainwater to infiltrate and can themselves be vegetated.
Nevertheless, the choice of species requires careful attention and a forward-looking vision: is it best to pick species that are drought-resistant (in anticipation of climate change impacts) but that sometimes provide little shade? Is it best to focus on local species or others that will be better suited to the climatic changes of coming years? There are several tools to guide the process, such as SESAME, developed by the centre for studies on risks, the environment, mobility and urban planning (Cerema) with the city and metropolitan district of Metz, which analyses a wide choice of species according to the ecosystem services provided in different urban situations.
On a micro-local scale, initiatives are taking shape, such as the project Lisière d’une Tierce Forêt, which aims to transform a very inorganic car park located in front of a residence for young workers into an island of freshness. The 72 trees planted were chosen for their ability to transpire and asphalt was removed from the car park to make way for a sample of draining pavement. A water storage basin to feed the trees during periods of water stress reinforces the system and reflective and permeable materials have been installed throughout the site. The measurements show a 2.5 °C drop in the average temperature over 24 hours, reaching a 6 °C drop around 1 pm. In addition to its impact on the UHI, the project restores the natural water cycle and contributes to well-being and social cohesion by providing a pleasant outdoor space to users of the residence.
This rationale of maximising the impact of an intervention in the urban environment and a holistic vision of the challenges (social, environmental, etc.) are at the heart of the resilience paradigm, which is asserting itself as an effective framework for thinking about adapting the urban fabric to the challenges of our time (shocks, stress, irreversible developments such as climate change and the mass decline in biodiversity). Diversity – another key principle related to resilience – prompts actors in the urban fabric to combine solutions and activate a wide variety of drivers to respond to a hazard such as UHI. As such, the urban organisation (reduction in traffic, prolonged opening of cool places during heat waves), the design of buildings (buildings with a through layout, bi-orientation, high performance insulation, including solar protection), the urban morphology (bioclimatic urbanism, open blocks, ventilated streets) and the infrastructures (fountains and misters, choice of materials with a high albedo and low thermal inertia) are also keys to combating UHIs. There are therefore many existing solutions and the documentation is expanding (e.g. the ADEME’s “Refreshing Cities” guide published in May 2021) to help stakeholders choose the most relevant ones according to different climatic and urban contexts.