A related podcast segment discusses the tradeoffs between urban living and fertility, and an online video presentation accompanies the issue.

Cities are now home to half of the world’s 6.6 billion humans. By 2030, nearly 5 billion people will live in cities. This special issue explores the enormous implications of the mass embrace of city life.

...How will cities evolve? Batty (p. 769) shows that in spite of the apparently amorphous growth of urban sprawl, resilient patterns emerge. He advocates the use of complex systems analysis in future urban planning. Preparing for natural disasters, and recovering from them, will also challenge planners—especially because many of the world's largest cities lie on coasts and are vulnerable to flooding as the climate warms (p. 748).

Someday, cities may grow their own crops and raise their own livestock in vertical farms (p. 752). Next-generation hybrid cars could help cut greenhouse gas emissions (p. 750). A more distant dream is a “supercity” that relies on superconducting electricity cables and liquid hydrogen for its energy needs (p. 753). Futuristic concepts, perhaps, but the time has come for a radical rethink of our concept of cities and their place in the global environment.

—Ash et. al. in their introduction to the special issue, “Reimagining cities”