Cooling the City

AOn a summer afternoon, the centre of a large city can be several degrees warmer than the farmland around it. This effect, known as the urban heat island, was first described in the early nineteenth century by the amateur scientist Luke Howard, who recorded temperatures in and around London. Today it is understood to result from a combination of factors: dark surfaces such as asphalt absorb solar radiation, tall buildings trap heat between them, and vehicles and air-conditioning units release waste heat directly into the streets.

BThe consequences extend beyond discomfort. Higher night-time temperatures prevent the body from recovering after a hot day, and public health researchers have linked heatwaves in dense urban districts to increased hospital admissions, particularly among elderly residents. Warmer cities also consume more electricity for cooling, which in turn can raise emissions if that electricity is generated from fossil fuels.

CAmong the many proposed remedies, green roofs have attracted particular attention. A green roof is covered with a layer of soil and vegetation, usually planted over a waterproof membrane. Plants cool their surroundings through evapotranspiration, releasing water vapour that absorbs heat, while the soil layer insulates the building below. Measurements on hot days have found planted roofs to be considerably cooler at the surface than conventional roofs nearby.

DHowever, the effect at street level is less clear. Because most green roofs sit high above pedestrians, the cooling they provide may barely be felt by people walking below, and several researchers argue that trees planted along streets offer greater benefit for the same cost. Green roofs also require maintenance, and in dry climates they may need regular irrigation, which reduces their environmental appeal.

EFor these reasons, many urban planners now favour a combined approach. Reflective "cool" roofs, which are painted in light colours, can be installed cheaply on buildings that cannot support the weight of soil, while green roofs are reserved for sites where their additional benefits, such as absorbing rainwater and providing habitat for insects, justify the expense. No single measure, it seems, can cool a city on its own.