第36回 の解答

Cooling the Concrete Jungle

Cities are often several degrees warmer than the surrounding countryside, a phenomenon known as the urban heat island effect. It occurs because asphalt, concrete, and brick absorb sunlight during the day and release the stored heat slowly at night, while air conditioners, vehicles, and factories emit additional heat. Limited vegetation also means there is less evaporation, a process that naturally cools the air. The effect is often most pronounced after sunset, when rural areas cool quickly but densely built districts remain warm, depriving residents of relief from the daytime heat.

Rising urban temperatures are not merely uncomfortable. They increase electricity demand for air conditioning, which in turn releases more heat and greenhouse gases, creating a vicious cycle. Higher temperatures can also worsen air pollution and pose serious health risks, particularly for elderly people and those living alone. To address the problem, many cities are experimenting with cool roofs, which are coated with reflective materials, and green roofs covered with plants. Some, including Tokyo, have also studied ways of preserving wind corridors, open spaces that allow cooler breezes from the sea or rivers to flow into the city.

Such measures can be effective, but researchers caution that their benefits vary. Green roofs, for instance, help little if they dry out during heat waves, and reflective surfaces at street level can bounce heat onto pedestrians. Moreover, cooling projects are sometimes concentrated in wealthier neighborhoods, while poorer districts with few trees remain the hottest. Many experts therefore stress that individual measures work best when combined with long-term urban planning that preserves parks, plants trees where they are most needed, and limits excessively dense development.