Empty half the Earth of its humans. It's the only way to save the planet | Kim Stanley Robinson

There are now twice as many people as 50 years ago. But, as EO Wilson has argued, they can all survive - in cities

Discussing cities is like talking about the knots in a net: they're crucial, but they're only one part of the larger story of the net and what it's supposed to do. It makes little sense to talk about knots in isolation when it's the net that matters.

Related: The 100 million city: is 21st century urbanisation out of control?