Down to Earth

Urban heat: How to keep a city cool?


Often dubbed a silent killer, heatwaves claim more lives than any other natural disaster. Cities bear the brunt of the pain, heating up twice as fast as the global average. Buildings, transport, pollution and few trees all contribute to an "urban heat island" effect, making cities typically 3–8 degrees warmer than rural areas. 


The trap of air conditioning

Air conditioning is the most obvious solution to beat the heat, but the more we use these devices, the warmer urban areas become.

''If we release hot air into the atmosphere, we will increase the temperature of the city. This undeniably creates a vicious circle that will force you to use more air conditioning and will release more etc, etc.'' warns Brice Tréméac, director of the Laboratory of Cold, Energy and Thermic Systems, based in Paris.

There are, however, other ways to cool a city that don't rely on the flick of a switch.

A forest in the city

In Aubervilliers, on the outskirts of Paris, a former car park has been transformed into what's been dubbed an ''urban forest''.

Over the past few months, 70 native trees have been planted, selected for their capacity to cool as well as their ability to survive in a city environment.

Andrej Bernik is one of the architects leading the project. He explains: ''Trees cool the air in two ways: on the one hand they provide shade and on the other hand they draw water from the ground and evaporate it through the leaves. This lowers the temperature. It's like a natural air conditioner.''

France's meteorological service has offered to measure the impact of the trees. Initial results suggest a temperature drop of two degrees in the zone where the trees have been planted.

'A technique as old as the hills'

It's no coincidence that the buildings in many cities across Spain, Greece or northern Africa are painted white.

This traditional technique has for many years helped keep homes cool by reflecting sunlight during soaring summer temperatures. The same principle is now being applied in France, where roofs are often coated in black bitumen which acts like a heat sponge. Picard, a leading chain of frozen food products, is testing whether painting existing roofs white could impact temperatures inside its stores as well as reducing air conditioning and refrigeration costs.

According to Daniel Simon, the technical director of Cool Roof France, it's not just any ordinary white paint.

''It has the peculiarity of being very, very, very white. It reflects 95 percent of the sunlight and this light will no longer be transformed into heat but will be sent back into the atmosphere, it will return to where it came from,'' Simon explains.

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