"Isolated green roofs are expensive insulation," Ms. Hoffman (Director of Earth Pledge) said. "But when you have a whole community of green roofs, it changes the microclimate of the area and reduces demand for energy. Think about one sidewalk in front of a building. That doesn't make a transportation path. But if everyone has one in front of their property, you have a way to walk around the city. Only a citywide effort can achieve that."