Hotter buildings increase the use of air conditioning, which leads to higher energy use and more pollution from the burning of fossil fuels. Meanwhile, higher temperatures outside cook the pollution, creating smog. Plants on the roof will cool the air by evapotranspiration, a process in which plants secrete water through their pores and as the water evaporates it cools the air. Plants also take in carbon dioxide, release oxygen, and filter the air to some extent. According to computer models conducted by the Lawrence Berkeley National Lab, which is participating in the heat island project, widespread use of reflective surfaces and green roofs could reduce summer temperatures in cities by several degrees.
Green roofs are quite common in European countries, where they have had problems with flooding.
In fact, rooftop gardens aren't a new idea at all. The first historical accounts of rooftop gardens date back to 500 B.C., when the Hanging Gardens of Babylon were in place. During the Renaissance, Pope Pius II had a roof garden built at his summer residence in Pienza, Italy. In more recent history, settlers on the American Plains built sod houses to insulate themselves from temperature extremes. The technology that has made green roofs a realistic solution for today's structures was developed in Germany, where some cities use them to combat flooding problems caused by stormwater runoff.