According to a Spanish study published by The Lancet, more than 4% of summer mortality can be attributed to heat islands in 93 European cities. Among them, Toulouse. The planting of trees is recommended, the uprooting obviously prohibited.

More than 4% of deaths in cities are due to urban heat islands during the summer, according to a study by the Barcelona Institute for Global Health (ISGlobal)published by the newspaper The Lancet January 31. The authors estimate that 2,700 of these deaths could have been canceled if the tree cover represented 30% of the surface of these cities, instead of 15% currently.

The study was conducted in 93 European cities including Toulouse. It is based on the existence of heat islands and the temperature difference between towns and surrounding countryside.

In the city, the concrete and asphalt with its impermeable surfaces, the low vegetation and the evacuation of the air conditioning networks of the buildings raise the mercury. Trees with their shade, but also thanks to evapotranspiration (rejection of absorbed water), temporize this phenomenon.

According to the study, tree cover must represent at least 30% of the surface of each neighborhood to have an effect. An emergency in health terms because exposure to heat has been associated with the premature mortality of the most vulnerable people, especially the youngest and oldest, with cardiorespiratory diseases and hospitalizations.

The phenomenon is proven for heat waves, but also occurs with moderately high temperatures in summer. Given global warming and ongoing urban growth, this effect is expected to worsen. over the next few decades.

The study revealed the death rate of residents over the age of 20 in 93 European cities (a total of 57 million inhabitants), between June and August 2015, and collected data on daily rural and urban temperatures for each city. In Toulouse, the 26 deaths could have been stopped that year according to the researchers.

First, they revealed premature mortality by simulating a hypothetical scenario without an urban heat island. They then revealed the temperature reduction that could be achieved by producing 30% tree cover and the associated mortality that could be negated.

The results show that, from June to August 2015, cities were on average 1.5^ohC warmer than the surrounding countryside. A total of 6,700 premature deaths could be attributed to warmer urban temperatures, representing 4.3% of total mortality during the summer months and 1.8% of mortality over the year.

A third of these deaths (2,644 precisely) could have been avoided by increasing the forest cover by up to 30%, and thus the temperatures. Overall, the cities with the highest excess heat death rates fell to Southern and Eastern Europe.

Study highlights substantial benefits of planting more trees in citiesalthough the authors acknowledge that this may be difficult in some cities due to their design, and that tree planting should be combined with other interventions such as green roofs or other temperature reduction alternatives.

“Our results also demonstrate the need to preserve and nurture the trees we already have, as they are a precious resource and it takes a long time to grow new trees. It’s not just about increasing the number of trees in the city, but also to know how they are distributed”, says Mark Nieuwenhuijsen, research director at ISGlobal.

The Metropolis of Toulouse, in collaboration with Météo France, offers a map of urban heat islands. The temperatures in the different districts of the city and in the surrounding municipalities are revealed and updated every fifteen minutes.

The city is carrying out a revegetation campaign with two main poles: the development of five large existing parks (Le Touch, l’Hers, la Garonne, le Canal, la Margelle) but also the tree plan: 100,000 trees will be planted in 10 years in Toulouse. The municipality wants to create islands of freshness in the neighborhoods. At the level of the agglomeration, the elected officials also smoke initiatives. At the Union, an urban micro-forest project is in progress.

Planting trees has an effect on global warming. But the benefit is not immediate, it takes time for the trees to grow. The greening of cities also brings “many other health benefitsincluding longer life expectancy, fewer mental health problems and better cognitive functioning,” recalls Mark Nieuwenhuijsen.

Today, cold is the cause of more deaths than heat in Europe. But as one of the study’s authors, Tamara Lungman, points out, spies based on current emissions from heat-related illnesses and deaths to become a greater burden on our health services over the coming decades.