One person can make a difference -- purchase your copy of For America and discover simple ways to improve your life, your country, even the world. But a simple and attractive way to cut energy costs, improve air quality, and ensure public health is to grow a garden in the sky.     Each time a city building is set up, the grass in that spot and trees that once reached up to the sky are replaced by a layer of asphalt or a tar roof . The lost green space can have a very real effect on a city's air temperature, air quality, and energy costs. On a summer day in most American cities, a tar roof can feel like the inside of an oven  . But what if we could replace the hot roof of each skyscraper or building in a city with grass or a garden?     Whether it holds vegetable gardens, wildflowers, or grasses, a green roof can lower the temperature above a building by 60 degrees, which can reduce the building's energy costs and, if used more widely, could decrease a city's total energy costs, and improve air quality and public health.     But that's not all -- green roofs are a great way to handle water management in urban areas by taking in much rainfall, provide shelters for urban wildlife, and actually extend the life of the roof itself. And green roofs also do something else: they beautify the city -- whether you're on the 3rd floor or the 30th. Now that's a brilliant achievement.      The non-profit group Earth Pledge is promoting a Green Roofs Program. To learn more about a green roof, just click here: EarthPledge.com. We can learn from the passage that green roofs   _  .
can help reduce temperature to 60 degrees can help reduce city population have a promising future in urban areas are vital in improving people's health