Surrounding cities with trees may bring down urban heat

A reduction in temperature of 0.5C may not sound much but with cities getting hotter as the climate warms it could save the lives of elderly people and young children.

The urban heat island effect, which raises temperatures in cities by 2-3C because the sun’s rays are absorbed by roads, roofs and other hard surfaces, is already making life uncomfortable for millions worldwide and causing an increasing number of deaths.

Some cities, acting on scientific advice, have been combatting this by planting trees in urban areas, replacing tarmac with grass wherever possible, painting buildings white to reflect the sun, and creating as much space for vegetation as they can.

Now scientists have discovered that what grows on the land surrounding the city also makes a difference to the temperature in the centre. The reason is that the heat island effect produces a low pressure in the city centre. The hot air rises and sucks relatively cooler air from the surrounding countryside.

Studying 30 cities with different characteristics, the scientists found that those with belts of trees and large lakes on their borders drew in cooler air that helped reduce the city centre temperature by 0.5C. Grassland and arable crops had a lesser benefit. A good case for ringing cities with parks.