"Green roofs are a wonderful solution for the environment for several key reasons. Because the rooftops in urban areas spend all day gathering heat from the sunlight, at night they create a "heat island," radiating out the heat they've absorbed. This has been shown to actually change the normal temperature of urban places. Rooftops covered in plants, however, would mitigate this effect, and instead of absorbing the heat, absorb the carbon dioxide in the air." (Quoted from article)