Many cities around the world suffer from the urban heat island effect, which is an area significantly hotter than its surroundings. One of the reasons for this is that there are lots of buildings with dark-coloured surfaces, which have a low albedo and therefore do not reflect heat well. This is particularly the case in cities that have used lots of concrete and asphalt in construction. During the summer the increased heat leads to overheating, which in turn makes more people turn on their air-conditioning, leading to more energy use and greater air pollution.
One way of cooling a building is to plant a garden on its roof!
In Chicago in particular, municipal support and encouragement for green roofs led to over 450 being planted in the city centre by 2008, with many more following. The then-Mayor Richard Daly promoted a green roof on the city hall, as well as on other city-funded buildings, such as schools and courthouses.
The plants making up a green roof naturally help with the building heating and cooling, as plants reflect heat, provide shade and help cool the air through transpiration. A flat asphalt roof would ordinarily maximise runoff and the water would collect pollutants as it ran down to the sewers, but plants intercept the water, moderating the flow of runoff to the city sewers. Plants are also useful not just in reducing the temperature and air-conditioning costs of buildings, but in reducing carbon dioxide in the surrounding area through photosynthesis, providing oxygen.