Could miniature forests help air-condition cities?

CITY AIR is in a sorry state. It is dirty and hot. Outdoor pollution kills 4.2m people a year, according to the World Health Organisation. Concrete and tarmac, meanwhile, absorb the sun’s rays rather than reflecting them back into space, and also displace plants which would otherwise cool things down by evaporative transpiration. The relentless spread of buildings and roads thus turns urban areas into heat islands, discomforting residents and exacerbating dangerous heatwaves, which are in any case likely to become more frequent as the planet warms.

A possible answer to the twin problems of pollution and heat is trees. Their leaves may destroy at least some chemical pollutants (the question is debated) and they certainly trap airborne particulate matter, which is then washed to the ground by rain. And trees cool things down. Besides transpiration, they provide shade. Their leaves have, after all, evolved to intercept sunlight, the motor of photosynthesis.