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AOn a still summer afternoon, the centre of a large city can be six or seven degrees warmer than the farmland that surrounds it. Scientists call this difference the urban heat island. It is not caused by one single thing but by the ordinary materials of city life. Brick, asphalt and concrete absorb sunlight during the day and release it slowly after dark, so streets stay warm long after sunset. Buildings block the wind that would otherwise carry heat away, while cars, air conditioners and factories add warmth of their own. Rain, meanwhile, runs off hard surfaces into drains instead of soaking into soil, so very little water is left to evaporate and cool the air.

BTrees interrupt almost every stage of this process. A mature street tree intercepts sunlight before it reaches the pavement, and it also pumps water from the soil into the air through its leaves, a process known as transpiration. Both effects lower the temperature of the air beneath the canopy, and the second is by far the more powerful. Researchers in Melbourne found that shade alone reduced surface temperatures by several degrees, but that streets with continuous canopy and moist soil were cooler still, even at night. In other words, a thirsty tree is a much less useful tree, which is why watering young plantings through their first summers matters more than the number of saplings a city can claim to have planted.

CThe benefits are not distributed evenly. Studies in North American and European cities have repeatedly shown that wealthier neighbourhoods have more canopy cover than poorer ones, sometimes twice as much. Older, more expensive districts were often laid out with wide verges and generous gardens, while denser low-income areas were built with narrow streets and little planting space. The consequence is measurable: during heatwaves, hospital admissions for heat-related illness rise fastest in the very districts with the fewest trees. Several city governments have therefore begun to direct new planting towards the hottest and least shaded streets rather than towards the places where residents complain most loudly.

DPlanting trees, however, is easier to announce than to achieve. Beneath a modern pavement lies a dense layer of compacted material designed to stop the road from sinking, and roots struggle to penetrate it. Water pipes, electricity cables and fibre-optic lines occupy the same shallow space. Faced with these obstacles, engineers once planted small trees in small pits, where they grew slowly, produced little shade and frequently died within a decade. Newer designs use structural soil, a mixture of stone and earth that supports the weight of traffic while leaving gaps for roots and air, and this approach has roughly doubled survival rates in the cities that have adopted it. Such installations are expensive, and the cost falls in the first year while the cooling arrives twenty years later.

ESpecies choice matters as much as engineering. Fast-growing trees provide shade sooner but tend to have brittle wood and short lives, so a street planted entirely with one quick species may need replacing all at once. A single species is also vulnerable to disease: in the twentieth century, elm disease removed millions of trees from streets that had been planted almost exclusively with elms. Most urban foresters now aim for variety, recommending that no single species make up more than about a tenth of a city's stock. They must also consider the future climate rather than the present one, since a tree planted today will spend most of its life in conditions warmer and drier than those recorded in local averages.

FEven so, the arithmetic remains favourable. Economists who compare planting costs with savings on air conditioning, reduced illness and longer pavement life usually find that every unit of money spent on urban trees returns two or three times as much over the life of the tree. What the arithmetic cannot do is deliver results quickly. Cooling a city is a slow, unglamorous project of watering, maintenance and record-keeping, carried out by one administration for the benefit of another. That, rather than any scientific uncertainty, is the main reason canopy targets are so often announced and so rarely met.