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.

There’s one simple antidote: urban trees. Authorities around the world have planted more trees to counteract the heat.

But how effective is this? How much hotter would our cities be without trees?

To find out, we analysed data from nearly 9,000 cities around the world, home to about 3.6 billion people. As our new research shows, trees almost halve how much heat is trapped by the urban heat island effect.

This cooling is welcome. But it is far from even. Wealthier, suburban and humid cities have more trees on average.

Trees act like natural air conditioners. They shade the ground and stop asphalt and buildings from heating up in the first place. They also cool the air by releasing water vapour from their leaves in a process called transpiration, lowering surrounding temperatures. They can make a noticeable temperature difference, especially on sizzling summer days.

Trees offer a simple way to counteract urban heat. This matters. More than half the world’s population (55%) now live in urban areas according to the........

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