Hotter in the City

On a summer afternoon, the center of a large city can be several degrees hotter than the surrounding countryside. Scientists call this the urban heat island effect. Asphalt and concrete absorb sunlight during the day and release it slowly at night, so the city never fully cools. Tall buildings trap warm air and block breezes, while air-conditioners, vehicles, and factories add heat of their own. The effect is not merely uncomfortable. Higher night-time temperatures deprive people of restful sleep and strain the bodies of the elderly and the sick, and the extra demand for cooling raises electricity use, which, if the power comes from fossil fuels, adds further to the warming. Remedies exist, and several are surprisingly simple. Planting trees along streets provides shade and cools the air through evaporation; painting roofs in light colors reflects sunlight instead of absorbing it; and preserving parks and open water creates pockets of cooler air. The difficulty is that these measures compete for the same scarce urban land that developers would rather build on, and their benefits, though large in total, are spread thinly across many residents rather than captured by any single owner.