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Listen to part of a lecture in an environmental science class. Professor: Today we're discussing the concept of 'urban heat islands,' a phenomenon where cities are significantly warmer than surrounding rural areas. This happens because materials like concrete and asphalt absorb and retain heat during the day and release it slowly at night. Additionally, human activities, such as transportation and industrial processes, generate waste heat. One major consequence is increased energy demand for air conditioning, which in turn leads to higher greenhouse gas emissions. To mitigate this, some cities have implemented green roofs—rooftops covered with vegetation. Green roofs not only provide insulation, reducing the need for cooling, but also absorb rainwater and improve air quality. However, they can be expensive to install and require regular maintenance. Another strategy is increasing urban tree canopy, which provides shade and cools the air through evapotranspiration. But trees need space and water, which can be scarce in dense urban areas. So, while these solutions are promising, they require careful planning and investment.