Our estimates indicate that world population would grow by an additional 40% if global fertility rates had moved instantaneously to replacement in 2005. Nonstable and stable momentum contribute roughly equal shares to world population momentum. Taking natural logarithms shows that nonstable momentum accounts for about 53% of total world momentum, and stable momentum contributes roughly 47%.
The value for nonstable momentum reflects a country’s recent trend in fertility. In a stable population where fertility has been constant for a long time, nonstable momentum is nonexistent. But countries that have a history of fertility decline, especially a recent and sharp decline, will have larger values for nonstable momentum. The value for stable momentum is dictated by a population’s current level of fertility in relation to mortality. A high (low) net reproduction rate corresponds to a large (small) value for stable momentum. This decomposition gives us a new way of thinking about the determinants of overall population momentum. In addition, it allows us to integrate disparate strands of the population momentum literature and see how the various kinds of momentum that researchers have considered fit together into a single analytic and empirical framework.