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Narrator: Listen to part of a lecture in an environmental science class. Professor: Today we're going to talk about urban heat islands. As you know, cities tend to be warmer than surrounding rural areas. This is due to several factors: dark surfaces like roads and rooftops absorb more solar radiation, and there's less vegetation to provide shade and cooling through evapotranspiration. But there's a lesser-known contributor: waste heat from buildings and vehicles. Now, one proposed solution is to increase the albedo of urban surfaces, for example, by painting rooftops white or using reflective materials. This can reduce the temperature of the building and the surrounding air. However, a recent study in a mid-sized city found that while cool roofs lowered summer temperatures by about two degrees Celsius, they also increased the demand for heating in winter, because the buildings lost heat more easily. So the net energy savings were minimal. The researchers suggest that a more effective strategy might be to combine cool roofs with green roofs, which provide insulation and also absorb rainwater. But the initial cost is higher, so city planners need to weigh long-term benefits against upfront expenses.