Population: some boom, some decline

While on average fertility levels continue to decline, considerable variations exist across and within regions. Among more developed countries, rates are often below replacement levels, i.e., about 2 births per woman, with some populations already shrinking. The average level for Europe, for example, is well below replacement at 1.5 births per woman. A notable exception to this trend is the US, where fertility has been close to the replacement level for a number of years. Fertility rates in less developed countries, in contrast, are often well above replacement at 3 or more births per woman. In sub-Saharan Africa, for example, the average fertility level is 5 births per woman. Again, a notable exception to this general pattern among developing countries is China, where fertility is 1.8 births per woman.

Some demographers expect that world fertility will remain above replacement for some time to come, pointing out that nearly all of Sub-Saharan Africa and most of South and West Asia have high fertility levels. Others, however, see below-replacement fertility becoming the global norm in coming decades. Average world fertility, they note, is about half the level it was in the 1950s, with fertility declines near replacement in such diverse cultures as Algeria, Iran and Vietnam. Lower mortality, urbanisation, increased education, especially girls, the growing employment of women, later ages at marriage and the greater availability of modern methods of contraception are strong, universal forces that point toward low levels of fertility. Also, they draw attention to the fact that today approximately 45 per cent of the world’s population resides in countries with below-replacement fertility versus less than 1 per cent of world in the 1950s.

Over time, relatively small differences in fertility rates can lead to enormous differences in population size. For instance, if fertility rates eventually settled at about one-quarter child above replacement, i.e., 2.35 births per woman, world population would more than double to 14 billion by the century’s end and continue growing thereafter. On the other hand, if fertility rates settle at about one-quarter child below replacement fertility, or 1.85 births per woman, world population would be around 5.5 billion by the end of the century and continue declining thereafter. In other words, replacement fertility in the long term is essentially the tipping point for population change: above it growth and below it decline.

As noted earlier, this uncertainty about the path of fertility is a central and challenging question of this century given its impact on the future size of world population. Many demographers making long-range population projections see fertility levels eventually fluctuating around the replacement level. To do otherwise, as noted above, would lead in the long term to either extremely large, expanding populations or very small, shrinking populations. Assuming fertility rates gravitate to replacement during the coming decades and subsequently fluctuate closely around it, world population could in due course stabilise at around 9 to 10 billion.

Stabilisation of world population is perhaps the paramount issue of the 21st century. Without global population stabilisation, humankind will find it enormously more difficult to deal with the critical issues facing the planet, such as global warming, biodiversity, the environment, energy, food/water supplies, migration and security. The path to population stabilisation requires sustained and critical attention and informed policymaking at all levels. Today’s decisions not only affect human well-being, but also the quality of all life forms on Earth in the coming decades and beyond.