Green roofs—rooftops covered with a layer of vegetation—are getting a lot of credit for providing environmental benefits. They have been found to reduce storm water runoff from buildings, conserve energy by moderating rooftop temperatures, restore fragile ecosystems and beautify urban spaces. From Toronto to New York, cities are investing billions in green infrastructure programs that rely on this kind of technology and ongoing research is helping refine its application. From a purely aesthetic viewpoint, these oasis in the sky are wonderful expressions of human ingenuity with lofty aspirations that speak to a primal need for symbiosis. Below is a gathering of a heavenly horticultural mosaic. Check out the WorldChanging website for more info.