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Listen to part of a lecture in an environmental science class. Professor: Today we're discussing the impact of urban heat islands, a phenomenon where cities are significantly warmer than surrounding rural areas. This occurs because concrete and asphalt absorb and re-radiate heat, while vegetation, which provides shade and releases moisture through transpiration, is often scarce. One major consequence is increased energy consumption for air conditioning, which in turn raises greenhouse gas emissions. Additionally, elevated temperatures can worsen air pollution, as heat accelerates the formation of ground-level ozone. To mitigate these effects, cities have started implementing green roofs, planting street trees, and using reflective materials on buildings. However, these strategies can be expensive and require long-term maintenance. Interestingly, studies show that even small parks can create cooler microclimates, benefiting nearby neighborhoods. So, while large-scale solutions are ideal, local efforts also matter. Now, what is the primary cause of the urban heat island effect?