Where to live?

To see which areas of the world have physical conditions that could theoretically accommodate an extra billion people sustainably, we overlaid maps of seven variables from The Atlas of Global Conservation7. We ruled out regions with extreme or high water stress; other arid areas; tundra and ice; centres with species unique to a region; and regions with population densities that exceed 100 people per square kilometre, namely much of Europe, the Middle East, India and China and the western United States.

That leaves large areas of South America; parts of southern Canada and the northern and eastern United States; south-central Africa; parts of Asia north of the Himalayas and from the Black Sea to north China; and scattered parts of Oceania (see 'Habitable zones'). Some moist tropical areas could support crops such as cacao, coffee, oil palm, rice and maize (corn). But development should be prohibited in biodiversity hotspots such as Borneo, northern Queensland in Australia and parts of the Amazon basin.

Society must think globally, plan regionally, then act locally.