Îlots de chaleur et la couleur des toits

Des surfaces réfléchissantes pour lutter contre le réchauffement A défaut de parvenir à contrôler les émissions de gaz à effet de serre, recouvrir les toits et les trottoirs de surfaces réfléchissantes pourrait contribuer à faire baisser les températures, selon une nouvelle étude. Alors que le réchauffement climatique fait l'objet de nombreuses préoccupations, les recherches visant à déterminer des moyens de le réduire se multiplient. Une étude publiée dans la revue Environmental Research Letters suggère ainsi que recouvrir les toits et les trottoirs de surfaces réfléchissantes permettrait de faire diminuer les températures mondiales. En effet, selon des chercheurs québécois, il est possible d’augmenter l’albédo (rapport entre l'énergie solaire reçue et l’énergie solaire réfléchie) des zones urbaines de 0,1 environ en adoptant des toits blancs et des trottoirs clairs. Selon les calcules des scientifiques, une telle mesure architecturale pourrait contribuer à diminuer les températures moyennes du globe de 0,01°C à 0,07°C par an. Cela représenterait une réduction de 25 à 150 milliards de tonnes de CO2 rejetées dans l’atmosphère chaque année. "De quoi compenser les émissions de toutes les voitures du monde pour les 20 à 30 prochaines années", estime Hashem Akbari, principal auteur de l’étude cité par Sciences et avenir. Selon lui, cette mesure permettrait par exemple de diminuer de 20% l’utilisation de la climatisation sous les toits. "Il s’agit d’une économie d’environ 50 milliards de dollars par an. Le refroidissement direct de la Terre en réfléchissant le rayonnement solaire dans l’espace est un bon moyen de faire rentrer de l’argent dans les poches", conclut-il. Lien: http://www.maxisciences.com/r%E9chauffement-climatique/des-surfaces-reflechissantes-pour-lutter-contre-le-rechauffement_art23761.html

Canadian professor lobbies for roofing changes to fight climate change By Postmedia News and Agence France-Presse, Postmedia News One Canadian expert believes a significant piece of the global-warming puzzle is perched on the roof over our heads. The colour of urban roofs could go a long way to combat global warming and save homeowners energy costs in the process, says Hashem Akbari, a professor of civil and environmental engineering at Montreal's Concordia University. Akbari said installing a white roof - or cool roof - that would not absorb as much heat as a standard roof is a key element to fighting global warming. ``Cool roofs save you energy if your building is air-conditioned. If the building needs air conditioning, installing a cool roof may actually solve your problem and you may not need it,'' he said. ``This all would be done at zero cost because you basically do that at the time you are changing your roof. At that time . . . you just select a white roof or light roof. ``If you do that then you also improve the ambient air quality within the city and you cool the globe.'' Scientists sketched a vision this week of converting the world's cities into giant sunlight reflectors to help fight global warming, but met with skepticism from fellow academics. Gradually replacing traditional urban roofs and roads with white or lighter-coloured materials would yield a cooling benefit that, over 50 years, would be the equivalent of a reduction of between 25 and 150 billion tonnes of carbon dioxide, researchers said. At the top end of the scale, this equals the emissions of all the world's cars over the same period, their study published in Britain's Institute of Physics' journal Environmental Research Letters stated. Light-coloured materials help reflect the sun's rays rather than absorb and convert them into heat, a phenomenon known as albedo. Pavements and roofs make up more than 60 per cent of urban surfaces and, by trapping solar energy, are largely to blame for ``heat islands,'' where cities or districts become local hot spots. Urban heat islands also gobble up energy in air conditioning and inflict health costs through smog. French climate consultant Jean-Marc Jancovici, however, said the proposals would have only a localized effect. "If you decrease significantly the temperature in local places with something like painting the roofs in white, it doesn't ensure that you will have a decrease in the temperature in remote places," he said. Alfredo Stein of the Global Urban Research Centre at Britain's University of Manchester also predicted practical difficulties, particularly for the world's sprawling slum areas. However, Akbari - who is also founder and executive vice-chair of the Global Cool Cities Alliance - stands by the potential benefits and said another ingredient to roofing changes making a real difference falls on government and policy-makers to ``prescribe use'' for cool roofing. He said in cooler places that require more heat in the winter, but also require air conditioning in the summer - such as many parts of Canada - the net savings will still benefit the consumer. He said the savings a light roof would achieve in the summer months by keeping the home cool would more than offset the additional need for heat in the winter. ``This is a measure that saves everybody throughout the world, uniformly, in terms of energy saving,'' he said, noting that the idea is not a fix-all for climate issues. ``It improves the ambient comfort and it cools the globe. There is absolutely no negative impact for this. ``There's not going to be a silver bullet or one single technology that solves the global warming problem. If there was, we would have already done it.'' Link: http://www.canada.com/Canadian%20professor%20lobbies%20roofing%20changes%20fight%20climate%20change/6450207/story.html