Cooling the City

A city is measurably hotter than the countryside around it. The difference, known as the urban heat island effect, typically runs to several degrees and can exceed ten on a still night. The mechanism is not mysterious. Asphalt and concrete absorb solar radiation during the day and release it slowly after dark; buildings trap that heat between them; vegetation, which cools its surroundings by evaporating water, has been largely removed; and the city adds heat of its own from vehicles, industry and air conditioning.

The consequences fall unevenly. Heat-related mortality rises sharply above local thresholds, and the burden lands on the elderly, the chronically ill and those without air conditioning — which is to say, on the poorest districts. Thermal mapping of cities from Baltimore to Delhi has repeatedly found that the hottest neighbourhoods are the least wealthy, and that the correlation is explained largely by tree cover. In several American cities the pattern maps onto historical lending practices from the 1930s with uncomfortable precision: districts denied investment then have fewer trees now and are several degrees hotter today.

Three families of remedy are usually proposed. The first is to change what surfaces do with sunlight. Painting roofs white, or surfacing them with reflective coatings, can lower the temperature of the roof itself dramatically, and studies in Californian cities have measured real reductions in indoor temperature and cooling demand. The limitation is that a cool roof cools the building beneath it far more than it cools the street, and the street is where people walk.

The second is vegetation. Trees work in two ways at once: they block radiation before it reaches the ground, and they release water vapour, which absorbs heat as it evaporates. A mature street tree can reduce the temperature felt by a pedestrian beneath it by many degrees, an effect no white roof approaches. Trees, however, are slow. A canopy planted this year delivers little for a decade and requires watering through exactly the droughts during which it is most needed, and municipal budgets are rarely structured to fund twenty years of maintenance for a benefit that arrives after several electoral cycles.

The third family concerns air and water. Narrow shaded streets aligned with prevailing winds, a feature of traditional architecture across the Middle East and North Africa, remain effective and are being rediscovered by designers. Fountains and water channels cool by evaporation. Both approaches share a weakness: they are difficult to retrofit into a city already built, and they depend on water in regions where heat and water scarcity increasingly arrive together.

The most consequential intervention may be none of these. Air conditioning saves lives during heatwaves, and there is no serious argument for restricting it. But it functions by moving heat from inside a building to the street outside, and measurements in dense districts show that the collective effect raises outdoor night-time temperatures measurably. Those who cannot afford it are therefore made hotter by the machines cooling their neighbours. Any serious plan has to reckon with a remedy that protects individuals while worsening the shared condition — which is why researchers increasingly frame urban heat as a problem of infrastructure and equity rather than of technology.