This chart explains why many demographers worry about a population bust, not a population bomb. Replacement rate fertility is 2.1. In country after country, economic development has led to declining fertility rates but the rates drop right past 2.1 into population decline. Some countries in Europe have rates in the 1.0-1.5 range. Assuming there are not reversals in low fertility countries, and assuming the global trend follows the lead of developed nations, about thirty years from now there are going to be significant economic problems. Some are already becoming evident in countries that have had prolonged low fertility.
Source: Carpe Diem