Anyone who has walked from a shaded park onto a sun-baked street knows the feeling, but the difference is more than an impression. On a still summer night, the centre of a large city can be several degrees warmer than the fields beyond its edge. Climatologists call this the urban heat island effect, and they have measured it in cities all over the world.
The principal cause lies underfoot. Natural landscapes are covered in soil and vegetation, which reflect a fair share of sunlight and hold moisture. Cities, by contrast, are paved in asphalt and concrete — dark, dense materials that absorb solar energy efficiently and store it as heat. Long after sunset, this stored warmth radiates back into the air, which is why the effect is most pronounced at night rather than at midday. The geometry of the city compounds the problem: tall buildings crowd narrow streets into “canyons” that trap warm air and shut out the breezes that might otherwise carry it away.
People add heat of their own. Every car engine, factory, and air conditioner expels warmth directly into the streets — and, perversely, the hotter a city becomes, the more its residents run their air conditioning, pumping yet more heat outdoors. Meanwhile the vegetation that once cooled the land has been stripped away. Plants lower the temperature through transpiration, releasing water vapour from their leaves; as that water evaporates, it draws heat from the air, much as sweat cools the skin. Replace a forest with a car park and that natural air-conditioning disappears.
The consequences are serious. Heatwaves become deadlier in cities, straining hospitals and power grids and falling hardest on the elderly and the poor. The remedies, however, follow directly from the causes. Planting trees and creating parks restores the cooling power of vegetation; “green roofs” covered in plants do the same overhead. Replacing dark surfaces with pale, reflective ones bounces sunlight back into the sky before it can be absorbed. None of these measures is expensive or exotic; together, they can shave precious degrees off a city determined to keep its cool.