In summer, large built-up areas are suffering more and more extreme temperatures, which can become outright dangerous in Urban Heat Islands (also called UHI). In city councils, one question keeps rearing its ugly head: what effective means are there for combating heatwaves in the city? Solutions do indeed exist to eliminate urban heat islands, often with crossover benefits in issues like air pollution or the preservation of biodiversity. Discover five concrete illustrations along with us!
1. Incorporating green lungs in urban development
In Paris, in a heatwave, the night-time temperature difference between the city and its suburbs can reach up to 20°F (10°C) (1). The culprit is “street canyons”, where urban heat islands take root and capture and retain up to 90 % of solar irradiation.
Green spaces, trees planted in clear ground, green roofs, and plant walls act as natural air conditioners thanks to the evapotranspiration phenomenon and the ground shade from leaves, while also depolluting the surrounding air. From tarred surface to planted area, the temperature can decrease by more than 10°F (5°C) (2).
A few examples:
- In Lyon, the city has raised an urban canopy and will have planted 300.000 trees by 2030, creating urban forest across 30% of the built-up area.
- In Strasbourg, one of the solutions adopted for reducing urban heat islands is the revegetation of cemeteries.
- Plant walls and green roofs mainly reduce surface temperatures, wherein irrigation of their substrate becomes important. When the two are combined, their city cooling power can be substantial, as seen in a canyon street in London where the temperature was decreased by 8°F (4°C). (3)
2. Using water in all its forms
To bring temperatures down during hot spells, landscape feature pools, marshes, ponds and lakes, water courses, splash parks and water amusements are very useful urban amenities. Good watering of green spaces also plays a vital role: a generous water supply will maximize their carbon capture and local air cooling capacities. But make sure to pay attention to water conservation and to the flood risk when the ground is too dry! Good rainwater management and the use of automatic watering are your best friends here.
A few examples:
- "Water is Life", they say. Rome is one of many cities that have opted for the simple, low-cost solution of installing public mist sprays, offering a real-feel temperature drop of 14.2°F (7.9°C) inside the mist (2).
- In the early 2000s, Seoul brought one of its streams back into the open, whereas it was previously enclosed by a highway. It's now a real green lung for the South Korean capital, with temperatures around Cheonggye Creek falling by 6–8°F (3–5°C)! (4)
3. Reappraising ground and building surfaces
An indispensable factor in subduing urban heat islands is surface albedo value: the ability of surfaces to reflect solar radiation without taking in heat. Values range from 0 to 1; the lower the value, the higher the heat retention. The average asphalt surface has an albedo of only 0.07! Promoting infiltration, using materials with a higher thermal inertia, and choosing optimum surface colors for coatings are just some of the leads worth following.
A few examples:
- The Los Angeles “Cool Pavements” experiment applied white surface paint to the city's main roads, decreasing temperatures by 12–14°F (6–7°C). (5)
- In Milan, an overall increase of 0.5 in the albedo of urban surfaces brought down real-feel temperatures by 5–7°F (3–6°C), while the switch to white reflective coatings decreased air temperatures by 0.8–1.5°F (0.4–0.8°C). (²)
- Innovative coatings are on the increase and getting better all the time, like Amter depolluting geotextile for parking lots, or phase-change materials
- And there's no overlooking solar panels, a very useful way of replacing low-albedo surfaces since they cool off a lot quicker after nightfall!
4. Incorporating bioclimatic considerations in building design
South-facing buildings were the norm for a long, long time, but they have their downsides when it comes to long-duration heatwaves. When designing a new neighborhood, spacing and layout must be considered in order to make optimum use of prevailing winds and create natural ventilation. Thermal insulation and shade-giving features (sailcloths, awnings, porches, etc.) also have their part to play.
A few examples:
- In Lyon's “La Duchère” eco-estate, “La Sauvegarde” is a green urbanism project where climatic issues are a factor in the orientation of buildings.
- In Tel-Aviv, Bauhaus-style buildings have been adapted to local climatic constraints. Among the solutions for guaranteeing thermal comfort are narrow, shaded balconies to take advantage of the offshore winds and a raised pillar construction to allow air circulation under the dwellings.
5. Adapting individual and collective behavior
Reducing urban heat islands is a non-starter without an adjustment in behaviors and better management of city life. Between the omnipresence of car engines in downtown areas, the ill-considered dependence on air-conditioning, the poor knowledge of good gestures, and half-hearted or even non-existent adaptation of working hours to cope with heatwaves, there are so many action paths we can take to decrease anthropogenic heat emission!
A few examples:
- More and more cities are taking steps to drastically decrease city-center vehicle traffic by implementing park-and-ride facilities, increasing the coverage and frequency of public transport, and creating bikeways.
- Awareness campaigns remind people of the individual gestures to adopt to stay cool indoors or at work on company premises in order to avoid the use of air conditioning, which will otherwise increase threefold in Europe by 2050 (2).
There are going to be many more heatwaves in cities in coming decades. For the townspeople of tomorrow, it’s today's landscape development, green urban planning, and societal reform that will govern their quality of life and their good health!
(1) Agence Parisienne du Climat: Pourquoi fait-il plus chaud en ville ? Un micro climat parisien marqué par le phénomène d’îlot de chaleur urbain
(2) ADEME: Rafraîchir les villes des solutions variées
(3) Unep: Les bienfaits sur le climat
(4) Landscape performance series: Cheonggyecheon Stream Restoration Project
(5) Sud Ouest: À Los Angeles, de la peinture blanche sur les routes contre la chaleur
© Photo credit: Pixarno / Adobe Stock