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Listen to part of a lecture in an environmental science class. The professor is discussing the concept of urban heat islands. "Now, urban heat islands occur when cities replace natural land cover with dense concentrations of pavement, buildings, and other surfaces that absorb and retain heat. This effect increases energy costs, air pollution levels, and heat-related illness. One key factor is the albedo, or reflectivity, of surfaces. Dark roofs and roads have low albedo, meaning they absorb more sunlight and re-radiate it as heat. In contrast, cool roofs—painted white or covered with reflective coatings—can significantly reduce surface temperatures. Another strategy is increasing vegetation, such as green roofs or street trees. Plants provide shade and release moisture through transpiration, which cools the air. However, the effectiveness of these strategies depends on local climate and urban density. For instance, a city with a dry climate might benefit more from shade than from evapotranspiration, since moisture is scarce. So, while urban heat islands are complex, targeted mitigation can make a measurable difference." What does the professor identify as a primary cause of urban heat islands?