Urban trees cool the world’s cities more than we thought – but we can’t rely on them alone

Cities and towns are usually 1–3°C hotter than the surrounding countryside, because asphalt, concrete and brick absorb heat from the sun and radiate it slowly. Some cities can be as much as 7°C hotter. This effect is known as the urban heat island. This can be dangerous, especially in hot countries. In very hot conditions, dehydration and heat exhaustion become real risks. If it gets too hot, it can be lethal.