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　　For many people who live in cities，parks are an important landscape．They provide a place for people to relax and play sports，as well as a shelter from the often severe environment of a city．What people often overlook is that parks also provide considerable environmental benefits．

　　One benefit of parks is that plants absorb carbon dioxide and give off oxygen．According to one study，an acre of trees can absorb the same amount of carbon dioxide that a typical car emits in 11，000miles of driving．Parks also make cities cooler．Scientists have long noted what is called the Urban Heat Island Effect：building materials such as metal and concrete（混凝土）absorb much more of the sun's heat and release it much more quickly than trees and grass．Because city landscapes contain so much of these building materials，cities are usually warmer than surrounding rural areas．Parks and other green spaces help to lessen the Urban Heat Island Effect．

　　Unfortunately，many cities cannot easily create more parks．However，cities could benefit from many of the positive effects of parks by encouraging citizens to create another type of green space：rooftop gardens．While most people would not think of starting a garden on their roof，human beings have been planting gardens on rooftops for thousands of years．Some rooftop gardens are simple container gardens that anyone can create with the investment （投资） of a few hundred dollars and a few hours of work．

　　Rooftop gardens provide many of the same benefits as other urban parks and garden spaces，but without taking up the much-needed land．In the summer，rooftop gardens prevent buildings from absorbing heat from the sun，which can significantly reduce cooling bills．In the winter，gardens help control the heat that materials like brick and concrete release so quickly，leading to savings on heating bills．Rooftop vegetables and herb gardens can also provide fresh food for city settlers，making their diets healthier．Rooftop gardens are not only