Plus d'arbres sont nécessaires pour refroidir les villes européennes en cas de vagues de chaleurmais ils doivent être plantés près des maisons

More trees are needed to cool down European cities in heat waves, but they must be planted near homes. Heat waves this summer claimed the lives of tens of thousands of people in Europe’s capital cities as they experienced record temperatures. In London, emergency wards filled with elderly patients who fainted in the heat. Dry weather caused subsidence and left homes with cracks. In Paris, temperatures exceeded 40°C (104°F) in late June, and people struggled to carry on with their lives. In Madrid, heat waves burned thousands of hectares on the outskirts of the city. Thami conducted preliminary research on the effects of extreme heat on European cities this summer. The study examined how many buildings have adequate shade from trees. The results were striking. In London, Paris and Madrid, more than 90% of buildings did not have enough shade around them to prevent urban heat island effects. These were among the worst results in the cities studied, but even the best performers had a majority of residents without adequate heat protection, cities such as Hamburg (56% unprotected) and Barcelona (70% unprotected). This builds on past research showing how major cities around the world, including in Europe, tend to struggle to reach useful levels of tree canopies to create shade close to homes and workplaces. It suggests that homes should have a view of three trees and be within a 300 m (984-foot) walk of a park. Urban heat islands occur when cities lack sufficient shade due to high rates of tree removal, sparse planting, and poor tree health. There are vast amounts of asphalt, clay and concrete, which are nearly perfect materials for soaking up and holding heat from direct sunshine. Unshaded asphalt can reach more than 70°C (158°F) on heat wave days. Even after sunset, cities stay hot because roads, buildings and sidewalks begin to radiate their absorbed heat back at us as they cool. The urban heat island effect can add significantly to ambient temperatures, making a day that is already very hot (perhaps 35°C [95°F]) even more dangerous in high-asphalt, low-shade locations (more than 40°C [104°F]). Tree shade is an effective strategy for addressing urban heat, with additional mental and physical health benefits to local residents. Trees prevent solar radiation from reaching heat-absorbing materials such as asphalt and also keep the sun off our bodies. Tree shade has been shown to reduce the heat absorbed by pedestrians by up to 18°C. Healthy trees also release water through their leaves as they photosynthesize, which absorbs heat as it evaporates. This evaporative cooling effect adds an extra few degrees of cooling, in addition to shading people and preventing heat island effects, even during heat waves as long as trees still have access to water. Planting more trees in cities in the wake of this summer's heat waves will help cool urban areas. The think tank Create Streets Foundation found the temperature on an exposed paved area in London during one heat wave was 55°C (131°F), but was 25–30°C (77–86°F) in a shaded area meters away. Looking at a city such as London, you might argue that it already has plenty of trees: around 8 million. But trees are only effective tools against urban heat when they’re close to homes. A lush, shady park 800 m (one-half mile) away isn’t going to prevent dangerous localized heat island effects for you and your neighbors. The other crucial factor is that thresholds of shade cover must be reached for trees to offer substantial cooling. A few isolated trees creating small islands of shade aren’t enough. Adding more tree canopy has been shown to be effective at preventing urban heat islands when it reaches 30–40% overall shade cover.