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Listen to part of a lecture in an environmental science class. Professor: Today we're going to talk about a surprising solution to urban flooding: green roofs. Now, traditional roofs are made of materials like asphalt or concrete, which are impermeable. When it rains, water runs off quickly, overwhelming storm drains and causing floods. Green roofs, on the other hand, are covered with vegetation and a growing medium, like soil. They act like a sponge, absorbing rainwater and releasing it slowly through evaporation and plant transpiration. Studies show that green roofs can retain up to seventy percent of rainfall during a storm. But their benefits go beyond flood control. They also reduce the urban heat island effect by providing shade and cooling the air through evapotranspiration. Additionally, they improve air quality by filtering pollutants and carbon dioxide. Some cities, like Toronto, have even passed bylaws requiring new buildings to include green roofs. However, installation costs are higher than traditional roofs, and maintenance, such as watering and weeding, is ongoing. Yet many experts argue that the long-term savings in stormwater management and energy costs justify the initial investment.