THE GLOBAL BABY BUST: Why 4 Billion On the Way Up is Different on the Way Down. “There is a difference [between] a world population that is 4 billion and strongly growing versus 4 billion and shrinking. When I write about global population collapse there are many people who will say that they lived in a world with 4 billion people and it was fine. This like saying they were in an airplane that was flying up through 10,000 feet and then 20,000 feet and those altitudes were all fine. The difference will be when the plane at 30,000 feet has lost engine power and will start plummeting uncontrollably. I am saying hey the world has lost three out of four engines for population growth and this is not good.”