This week, the world's population ticks over a historic milestone. But in the next century, society will be reshaped dramatically — and soon we'll hit a decline we'll never reverse. We never know precisely how many of us are alive at any one time, but this Tuesday is the United Nations’ best estimate on when we’ll reach 8 billion human beings. Eight billion. It's a number too big to imagine but think of it this way: In the time it takes you to read this paragraph, the world's population grew by around 20 people. While the Earth's population is growing...