To a planet of seven billion people, getting to nine billion sounds like a lot. But population experts are convinced that the number of people will begin to decline around the middle of this century.
And there are examples already, like Japan, where the overall population is dropping. Which sounds great if you've had a hard time finding a seat on the train, but has potentially dire implications for the labor force and old-age pensions.
Darrell Bricker and John Ibbitson explore in Empty Planet: The Shock of Global Population Decline. John Ibbitson is our guest.