Economics for a Crowded Planet: Which One would that be?

Jeffrey Sachs evidently believes the earth is overcrowded. I can't believe I'm reading this. His book is called: Common Wealth: Economics for a Crowded Planet. Which one is the crowded planet?

Sachs presents “four compelling reasons why the poorest countries need to speed the demographic transition,” “a list of seven requirements to enable family planning programs to accelerate the decline in fertility."

He has missed out on the news, even supported by the United Nations, that underpopulation is likely to be the big new problem on the horizon, not overpopulation.
To be fair, I am only quoting from a review in the NYT, and have not read the book itself. I'm not likely to, unless somebody pays me. Life is too short.