The city that cooled itself

On an August afternoon the difference between two streets a kilometre apart was eight degrees. One had trees; the other had been resurfaced the previous year in black asphalt.

The urban heat island is usually described as a city-wide effect, which hides what matters. Average city temperature runs two or three degrees above the surrounding countryside, and that figure is undramatic. Within the city, differences between streets are far larger, and they track three measurable things: how much of the surface is planted, how dark the surface is, and how much the street is shaded during the hours when the sun is highest.

All three are decided by ordinary maintenance rather than by climate policy. A resurfacing contract chooses a colour. A planting budget decides whether a felled tree is replaced within the year or at all. A parking layout decides whether the shade falls on cars or on the pavement where people walk.

This is why the most effective heat programmes in several cities have been administrative. One required that every resurfacing specification state the albedo of the material chosen, which did not mandate anything; it simply made the choice visible to the person signing. Another moved the tree budget from the parks department to the roads department, on the grounds that roads were where the heat was and parks already had trees.

Heat kills more people in most countries than floods or storms, and it does so unevenly, concentrated in exactly the streets that have the least planting and the oldest housing. Treating it as a maintenance question rather than a climate question is not a retreat. It puts the decision where it is actually being made.