Published on 21 August 2018


Building heat-resistant cities

European cities are not known for withstanding the heat. On the contrary, the materials used to construct cities, the layout, the roads, only aggravate the heat bubbles existing in urban areas. Solutions exist to burst these bubbles, but it requires us to rethink urban architecture and greening zones.

There could be a 10° C difference between the outskirts of a city and the city centre.

France and Europe will be able to breathe a bit easier with the decrease in temperatures set to follow this long and difficult heatwave. Cities suffered the most during this period, with temperatures that exceeded 35° C, with no real relief at night. Actual bubbles of heat are formed in urban areas, as there can be a dozen degrees difference between the outskirts of a city and the city centre.

"In the summer, average temperature variation of about 4° C can be observed at night between Paris and the coolest areas of the region, but in hot weather, the phenomenon can double in intensity and the variation can reach as high as 8° C," says Aude Lemonsu, CNRS researcher at the Météo-France research centre.

Air conditioning trees

Cities already house 54% of the world's population and will house 66% by 2050. However, European cities, apart from those on the Mediterranean basin, are not designed for these heatwaves. With global warming increasing, this adaptation will become necessary, and it will require that we rethink and redraw cities to make them more resilient.

The first solution is revegetation. Wooded areas are 2° C to 8 ° C cooler than urban centres. Isolated tree plantations, urban parks and natural spaces help reduce the intensity of heat islands by bringing shade and freshness to public areas.

According to Amandine Crambes, urban planner at Ademe, one must count on the air conditioning effect of trees. On France Info television, she explained that a mature tree releases 450 litres of water per day due to evapotranspiration. "The effect is comparable to four air conditioners," affirmed Crambes.

Thermal renovation

Aware of this challenge, many large cities have embarked on "The Million Tree Initiative".  It aims to plant one million trees in urban areas. New York has already completed its project, while Paris is committed to planting 20,000 trees within city limits by 2020.

It will also be necessary to rethink the materials and colours used. If it is 26 ° C, "a dark roof can reach up to 80 ° C, but the same roof would reach 45 ° C in a lighter colour, and vegetation will not go above 29 ° C," said Crambes. Thus, in the summer of 2017, Los Angeles repainted white roofs to improve the reflection of the sun's rays.

Crambes explained that it is now necessary to consider that the thermal renovation of buildings does not only consist of keeping houses warm in winter, but also protecting them from the heat in the summer. A novelty for French cities which had, only until recently, cold weather.

A return to open air

In a report by Ademe in October 2012, intended to "combat urban heat-island effect", the agency calls on the construction of buildings, parks, and neighbourhoods to account for wind corridors that will have a natural cooling effect in cities. "Combined with a strategy of good wind circulation in the summer time, green spaces, and profiting from fresh bodies of water, these are all things that are able to refresh the city more effectively," the agency writes.

Finally, there is a need to limit the concrete in our concrete jungles. Asphalt has a high thermal inertia: it accumulates a lot of heat during the day and restores it slowly at night. By removing these layers, cities will be better able to cool. And returning the earth to the open air will allow cities to benefit from the effects of water evaporation.

Ludovic Dupin @LudovicDupin

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