Apart from a few lower members of the animal kingdom, no-one other than human beings build cities.

They are totally artificial constructs and in them we live artificial lives. We travel differently, eat different food, receive water and energy through pipes and wires, live in different kinds of buildings, do different jobs.

All of these things come with an environmental price-tag. Given that the world's urban population is expanding at such a rate, it is worth asking what are the numbers on that price-tag, and whether they are higher or lower than the environmental cost of living a rural life ...