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The reality of global warming has been proved beyond doubt and the rise in temperatures has been even more noticeable in urban areas. We are now talking about urban heat islands, which contribute towards climate change, increasing atmospheric pollution and resulting in a deterioration of the quality of city life. As we will see in this article, the reintroduction of water resources and green spaces into urban areas acts as a natural heat regulator and allows the temperatures to drop by several degrees

The annual average temperature on the surface of the globe has increased by 0.6° since the beginning of the 20th Century and the IPPC’s 2007 report predicts a rise in temperature of 2° to 6° between now and the end of the 21st Century, with irreversible consequences for the health of humans, animals and plant species, also affecting the quality and availability of water resources, the agricultural sector… And this heating process is particularly noticeable in urban areas. We talk about Urban Heat Islands (UHI). In fact, in the cities the temperatures are, on average, higher by 1.5 to 2.5° C than the temperatures recorded in less urbanised environments.

The impact of these UHIs cannot be denied: they change the urban climate and lead to health problems. The air pollution, reinforced by the phenomenon of atmospheric inversion and a change in the physico-chemical composition of the atmosphere, leads to a deterioration of the quality of life, and an increase in allergies and respiratory problems. For some time now a number of studies have been carried out on the subject, but on account of the recent awareness of global warming, greater attention is now being paid to urban heating.

A number of different factors influence the urban climate, such as the height of the buildings and the spaces between them, the width of the streets, the construction material used or even the type of surface cover (waterproofing, plant cover, water). The heat flux and urban pollutant concentration created by human activities also have a major influence on this heating process.

A plethora of solutions have been recommended by the scientists for reducing this urban heating process, ranging from the development of public transport systems (common carriers) to improving the energy efficiency of the buildings and using specially adapted construction materials. In any event, however, the urbanisation process is depriving us of natural heat regulators such as water and plants. Their reintegration will help to cool our cities down.

 

The presence of water in the city acts as a direct source of cooling through a humidifying process.

A body of water counteracts any changes in temperature because, when removing the heat from the atmosphere while converting to water vapour, it reduces the ambient temperature. The effect will be even more noticeable when the air is dry. In the traditional Mediterranean way, we can see the importance of water in places such as Aix-en-Provence, the town of a hundred fountains, or in Seville, where the patios and gardens are bestowed with fountains, water jets and misters.

As evaporation is proportional to the surface area of contact between air and water, then priority must be given to any process that creates little droplets, such as misters and sprays. We can see that the efficiency of induced evapo­ration (misting, sprinkling) is grea­ter than that of natural evaporation.

Nevertheless, the water resource is a commodity that has to be used sparingly. We can cut down on consumption by substituting rain water for drinking water, a “free” resource that is not very widely exploited. This is the case in Geneva, Switzerland, where the La Grange and Eaux-Vives parks are irrigated with recovered rain water combined with water from a lake. This new resource, which never dries up, can also be used to irrigate the left bank quays. In the same way, at Orly, in the Ile-de-France, a large rainwater storage tank has been installed under the market place, this being used to clean the area, supply a fountain and irrigate the surroun­ding green spaces.

"As evaporation is proportional to the surface area of contact between air and water, then priority must be given to any process that creates little droplets, such as misters and sprays"

There has also been a tendency for open air storage systems to be developed. These fixtures, available in different models, have many advantages and they are far easier to maintain than underground structures; for example, the drainage water and run-off allow for the soils to be wetted and for the water to filtrate from the surface and recharge the aquifers. Another form of water storage is the green roof system, which holds back the rain water and thus further evapotranspiration (and cooling) is induced.

 

The development of the green spaces in the city is another way of reducing the urban temperatures.

As a result of the phenomenon of evapotranspiration, the interchanges of gases and water vapour that exist between the plants and the atmosphere humidify the air. This phenomenon works better with deciduous plants, with larger leaves. As far as the lawns are concerned, the dew point phenomenon accentuates the effect. It has been noted that green spaces of small- and medium-sized areas have a far more efficient cooling effect than the large parks.

Another idea put forward for combating urban heating is to have plants growing next to the buildings. This would help to create sha­de and protect the building from the sun during the summer months. Having deciduous trees would allow for a cooling effect to be created in the summer, but in the winter, when the leaves are gone, they would still allow the sun to get through and warm the buildings.

The plants can also be part of the buildings, whose roofs and walls represent the largest reserve of available surface area in the urban infrastructure. These living walls and roofs have a number of virtues: they promote evapotranspiration, and thus help to reduce the phenomenon of urban heating. They reduce rainwater runoff and help to prevent the ground from becoming saturated. Finally, they improve the thermal and sound insulation of the buildings.

“These living walls and roofs have a number of virtues: they promote evapotranspiration, and thus help to reduce the phenomenon of urban heating”.

Since the 1980s, the city of Stuttgart, a pioneer in this area, has made it compulsory for the contractors, when constructing a new building, to cover (some of) the roofs with plants. In Stuttgart, the summer tempera­tures have dropped by 2° in 20 years. In Lyons, the deputy mayor in charge of town planning is pushing forward with the idea by expe­rimenting with innovative hori­zontal and vertical plant cover projects. The experiments have been conducted by the Agengy Rheinert, Franco-Ger­man archi­tects based in Lyons, who have been imple­menting green roof projects for almost 30 years: “In order to back up this hypothesis, we have set up an innovative system for the continuous moni­toring of heat levels locally. We have attempted to prove that covering the roofs with plants reduces the summer heat island effect, caused by the large surface areas of sheet metal, asphalt and tiles being exposed to the sun”, explains Mr. Reinhart. After only a few weeks of random sampling of the temperatures of different elements exposed to the same level of sunlight, the first results speak for themselves: with an air temperature of only 20°C, black sheet metal heats up to 50°C and red tiles to 40°C, whereas the temperature of a roof extensively covered with plants to a depth of 7 cm, measured from the waterproof membrane, is 17°C when irrigated and 19°C without irrigation (recorded on 03/05/2012 at 12.00 midday).

 

Apart from lowering the temperatures, the reintroduction of nature into the city environment is generally beneficial for biodiversity. The different environments created, from green spaces to wet zones, allow for the development of different species of flora and fauna and the presence of rare species, with plants providing a natural habitat for these varieties.

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