Population Growth

Global population growth is shaped by births and deaths, though migration can also impact regional populations. Another factor is also at play: population momentum. This concept explains why the age structure of a population can cause it to grow even as fertility declines below replacement level. In essence, even if the fertility rate is declining, there is still a large absolute number of people of reproductive age in the population, resulting in more births than deaths. For example, in Nigeria, the UN projects that if the fertility rate dropped to replacement level today, the population would continue to grow for the rest of the century, with 124,000 more people than today in 2100 (an increase of 57 percent). In contrast, fertility rates in Japan have been below replacement level since 1959, leading to a much older population, but population size only started to decline in 2005. If all fertility rates were at replacement level, then the world population would still hit 9 billion in 2039: only two years later than current projections. Short of an unprecedented disaster, the population will continue to grow.