Deforestation. Desertification. Species extinction. Global warming. Growing threats to food and water. The driving issues of our times are the result of one huge problem: Us.

As the population continues to grow, our problems will increase. And this means that every way we look at it, a planet of ten billion people is likely to be a nightmare.

Stephen Emmott, a scientist whose lab is at the forefront of research into complex natural systems, sounds the alarm. TEN BILLION is...