The City That Cannot Cool Down

AOn summer nights, a strange geography appears on thermal maps of any large city: the countryside cools, but the urban core glows on, sometimes seven or eight degrees Celsius warmer than fields only twenty kilometres away. This is the urban heat island, first measured in London two centuries ago by the amateur meteorologist Luke Howard, who noticed that the city's nights were consistently warmer than those of the surrounding villages. What Howard recorded with thermometers on carriage rides, satellites now photograph directly — and the pattern he found has grown with the cities themselves.

BThe causes are not mysterious; they are built into the streets. Asphalt and dark roofing absorb up to ninety per cent of the sunlight that strikes them, storing heat all day and releasing it slowly through the night — which is why the heat island is strongest not at noon but after sunset. Buildings interrupt the wind that would otherwise carry warmth away, and their canyon-like streets trap radiation, bouncing it between walls instead of releasing it to the sky. Cities also make their own heat: every air conditioner, engine and refrigerator pumps warmth into the streets. Ironically, air conditioning cools interiors by heating the city — one study in Paris estimated that during a heatwave, the machines alone raised outdoor night temperatures by around two degrees.

CThe missing ingredient, compared with the countryside, is water. Rural land is a vast evaporation machine: plants draw moisture from the soil and release it from their leaves, and this evaporation consumes enormous quantities of heat, exactly as sweating cools skin. A city sealed in concrete and drained by pipes has almost nothing to evaporate. Rain that falls on farmland lingers in soil for weeks of cooling; rain that falls on a car park is gone down a drain within minutes. Researchers who compare neighbouring districts find the effect written clearly on the map: a large park can be several degrees cooler than streets a short walk away, an island of coolness inside the island of heat.

DThe temperature difference is not merely uncomfortable; it is dangerous and expensive. Heatwaves kill more people in most developed countries than storms and floods combined, and the deaths concentrate where the heat island is strongest — dense districts with little green space, which are often also the poorest. Night temperature matters most of all: bodies recover from hot days during cool nights, and it is precisely the cool night that the heat island abolishes. During the European heatwave of 2003, mortality in Paris was markedly higher in neighbourhoods with the least vegetation. The heat island, in other words, is not experienced equally: it is a map of income as much as of temperature.

EBecause the causes are physical, the remedies are too, and none of them is exotic. Los Angeles has painted streets with pale, reflective coatings; the results were immediate — surfaces up to six degrees cooler — though the glare and cost remain debated. Cities from Singapore to Medellín have planted trees on a massive scale, and the corridors of greenery in Medellín have been credited with lowering local temperatures by around two degrees. Rooftops can be painted white, or better, planted: a green roof both reflects sunlight and evaporates water. None of these measures abolishes a heatwave. What they do, study after study finds, is shave off the few degrees at the top — and in heat emergencies, the top degrees are the ones that kill.