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Narrator: Listen to part of a lecture in an environmental science class. Professor: Today we're going to discuss the concept of 'green roofs'—that is, roofs of buildings that are covered with vegetation. Now, green roofs are not a new idea. In fact, they have been used for centuries in cold climates, where the sod roofs helped insulate homes. But in recent years, they've gained popularity in urban areas for several reasons. First, they help manage stormwater runoff. In cities, rain falls on impervious surfaces like concrete, and the water quickly runs off, often overwhelming sewer systems. Green roofs absorb much of this rainwater, releasing it slowly back into the atmosphere through evaporation and transpiration. Second, they reduce the urban heat island effect. Dark rooftops absorb heat, raising the temperature of the city compared to surrounding rural areas. Green roofs, however, reflect less heat and also cool the air through evapotranspiration. Finally, they provide habitats for birds and insects, which is especially important in dense cities where green space is limited. However, green roofs do require maintenance—irrigation in dry periods, weeding, and occasional replacement of plants. So they're not entirely 'low-maintenance,' but the benefits often outweigh the costs.