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Global warming is undeniable, heat waves follow one another, always more intense. While half of the world’s population lives in urban areas, city dwellers feel the effects all the more as the lack of vegetation and the artificialization of the soil generate urban heat islands, implying to adapt their way of life. . Recently, NASA climatologists have shown that gardens and green roofs can help to effectively mitigate some of the intense heat in cities.
Cities, where half of the world’s population lives, face a myriad of risks from climate change, according to the latest report from the UN’s Intergovernmental Panel on Climate Change (IPCC). Between half and three-quarters of the world’s mostly urban population could be exposed to unbearable conditions of extreme heat and high humidity from here 2022, depending on the level of global warming. The main cause ? Urban heat islands.
The expression “urban heat island” (UHI) appeared around the middle of the 20th century. The latter occur in environments where urban temperatures are consistently higher than ambient temperatures. This results in an increased risk of photochemical reactions of pollutants forming heat waves. They cause heat stress in city dwellers, when the temperature exceeds what the human body can withstand. These heat islands are therefore artificial microclimates caused by human activities (energy plants, heat exchangers, etc.) and urban planning (dark surfaces that absorb and retain heat, such as tar, asphalt, concrete and similar materials , much more than vegetation).
Not only does this phenomenon lead to discomfort, increased energy consumption of air conditioning and similar technologies , and public health problems, but the effect of urban heat islands is dramatically higher on the elderly, low-income people and people with health concerns.
This is why this issue has given rise to increased reflection on the development of urban space, to prevent the rise in temperatures. Much of this work is based on the development of green spaces capable of blocking or absorbing solar radiation. Where cities have limited open space and the installation or preservation of green space was not considered during their initial planning and construction, green roofs provide an ideal solution.
Nature at the top
A green roof, also called roof or green roof, or even “eco roof” or “green roof”, is a plant arrangement installed on the top of buildings. The vegetation on these roofs is most often selected for its ability to live independently.
Green roofs are designed to exploit the cooling power of plants to lower the temperature in urban spaces. There are three categories of green roofs: intensive, semi-extensive or extensive. The type is determined according to the roof itself (strength, slope, access), the structural capacity of the building and the budget allocated to the project. You also have to determine your own needs. Intensive systems make it possible to cultivate all types of plants (ornamental, vegetable, even trees) with a fairly deep substrate of 26 cm or more, while extensive systems are reserved for low maintenance low vegetation (moss, lichens, etc.) with a substrate of 20 cm or less. Semi-extensive ranges fall between the two.
Recently, NASA’s Goddard Institute for Space Studies (GISS) team studied three sites at Chicago to understand how green roofs affected surface temperatures around these buildings, and whether there was a difference between these sites and others nearby without a green roof. The results are published in the journal Sustainable Cities and Society .
Chicago, the green city of the United States
Why choose Chicago? This Illinois city is a leader in urban agriculture in the United States. It includes the largest rooftop vegetable garden in the world, with an area of 30 03 m² (7.5 hectares). It is a greenhouse located on the roof of an old factory. There are also more than 472 green roofs in the city, a large part of which is dedicated to agriculture, on an area of 500 11 m² (47 hectares).
Chicago launched these initiatives in the early 2002. The mayor first installed a green roof on the town hall to set an example. Then, the municipality favored these green roofs with tax incentives. It is therefore quite naturally that NASA became interested in this city.
Green roofs are they really the solution to warming cities?
Braneon and the GISS team have partnered with the Departments of Health Public and Chicago Planning and Development to study three sites that had installed green roofs in the early 1990s 2002: Millennium Park, City Hall (the town hall) and a Walmart shopping center. Using images captured by the Landsat 5 satellite between 1990 and 2011, researchers compared changes in land surface temperature and vegetation abundance at study sites as well as nearby control sites without green roofs.
The results of the three sites are unequal. In a first case, Millennium Park, which has an intensive plant mix, located near Lake Michigan, showed significantly lower average temperatures, after the installation of its green roof in 2011. This is the only site where the roof has completely mitigated global warming during the study period.
Second, City Hall , also an intensive site, had a green roof installed in 2004. Its temperatures after the installation of the green roof were lower than those of the control site, but they increased towards the end of the study period.
Finally, unlike the other two sites, the shopping center was designed together with the green roof. Despite this, the disadvantages of transforming areas of wild vegetation into industrial land exceed the consideration that this roof vegetation cover should have provided. Christian Braneon, a climatologist and civil engineer at Columbia University and GISS, explains in a press release: “You might think that putting a green roof on your new building would have a significant impact. But what we see is that a lot of waterproof materials can also be added to it, such as parking around the building. As a result, you might reduce the impact of the parking lot, but you certainly didn’t create the cooling effect that the overgrown vegetation had
That is why the benefits of green roofs depend on various factors such as geographical region, plant diversity, roof structure and the cooling efficiency of the building itself. Nevertheless, the authors were able to demonstrate that sites with larger, intensive green roofs, accompanied by some plant species diversity, have greater cooling benefits than extensive monoculture green roofs. Biodiversity is therefore a factor to be taken into account. In addition, in addition to the degree of cooling provided by these green roofs, we can add the improvement of air quality (reduction of CO2), the filtration of atmospheric pollutants, as well as the possibility for pollinating insects to find refuge there.
The authors conclude that: “As cities grow and develop, they need to make good decisions about their infrastructure, because these decisions often last 47 Where 50 years or older. In the context of more frequent heat waves and extreme heat, it is important to understand how these urban design interventions can be effective
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Since the method proposed by NASA relies on publicly available satellite images and open source software for the analysis, it is simple and inexpensive. Braneon points out: “The whole practice is based on the principle that we can look to the past to assess risks in the future”. It can therefore become a real tool to help decision-makers and planners assess the cooling capacity of green roofs in their own communities and their viability over time.