Cooling the City

A On a hot afternoon, the centre of a large city can be several degrees warmer than the countryside around it. Scientists call this an urban heat island. The effect is strongest at night: fields and forests cool down quickly after sunset, but brick, concrete and asphalt release the heat they have stored during the day, keeping streets warm long after dark.

B Several features of cities combine to produce the effect. Dark roofs and roads absorb most of the sunlight that falls on them. Tall buildings trap warm air and block the breezes that might carry it away. Cities have fewer trees and less open soil, so less water evaporates to cool the air. Finally, cars, air conditioners and factories all release heat directly into the street.

C The consequences go beyond discomfort. Hot nights prevent the body from recovering after a hot day, and doctors have linked heatwaves to sharp rises in hospital admissions, especially among older people and those who work outdoors. Higher temperatures also increase the demand for air conditioning, which raises electricity use and, where power comes from fossil fuels, adds to air pollution. In this way, heat and energy use can feed each other.

D One of the simplest responses is to change the colour of surfaces. Light-coloured or specially coated roofs reflect far more sunlight than dark ones. Engineer Marta Silva, who studies building materials, points out that a white roof can stay dramatically cooler than a black one on a sunny day, and that this also lowers the temperature inside the building. Some cities now require new flat roofs to be reflective, and painting existing roofs white is a cheap project that neighbourhoods can carry out themselves.

E Trees offer a different kind of cooling. Their shade stops surfaces heating up in the first place, and the water they release through their leaves cools the surrounding air. Urban planner Daniel Osei argues that trees are the most cost-effective tool available, because they also reduce flooding, clean the air and improve people's mental health. His work suggests the benefits are greatest when trees are planted along streets where people actually walk, rather than gathered in a few large parks.

F Water itself can help. In Seoul, an elevated motorway was removed to uncover a stream that had been buried under concrete, creating a corridor of cooler air through the city centre. In Dhaka, campaigners have called for the city's remaining canals to be cleaned and protected for similar reasons. However, climate scientist Hana Kim warns that water features must be designed with care: shallow, still water warms up quickly and may encourage mosquitoes.

G No single measure solves the problem, and each city must choose the mix that suits its climate and its budget. What researchers agree on is that planning ahead is far cheaper than reacting to a crisis. As heatwaves become more frequent, the question for city leaders is no longer whether to cool their cities, but how soon they can start.