In politics it is often said: “While the future is known, it’s the past that keeps changing.” In demography, it’s exactly the opposite: while the past is known, it’s the future that keeps changing. That is simply because population’s fundamental building blocks — births, deaths and migrations — are not fixed but vary over time and place.
Before the 20th century, world population grew relatively slowly, even sometimes declining, because high rates of fertility were offset by high mortality rates, resulting in very low or even negative population growth. In contrast, during the 20th century world population nearly quadrupled, growing from 1.6 billion to 6.1 billion because while mortality rates had declined to comparatively low levels, fertility rates remained high (Table 1). The large part of that enormous demographic growth, 80 percent (an increase of 3.6 billion people), took place during the second half of the century. During that historic time, the world population growth rate peaked at 2.1 percent a year in the late 1960s, and the highest yearly increase, of more than 91 million people, occurred in the late 1990s.
Today, world population is 7.3 billion and growing at nearly half its peak rate, or 1.1 percent annually, yielding a yearly addition of 82 million people, again below its historic high. Those demographic declines are the result of decreasing fertility rates.
Since the 1960s, the world’s fertility rate has fallen from five births per woman to half that level. At the start of the transition, few demographers would have expected rapid fertility declines to occur in traditional, less developed countries. Yet, in countries as diverse as Albania, Brazil, China, Costa Rica, Iran, Lebanon, Singapore, Thailand, Tunisia and Vietnam, fertility has dropped from more than six births per woman to below the replacement level of about two births for each woman. Today, 75 countries, or 48 percent of the world population, are experiencing fertility rates below the replacement level.
Projecting the world’s future population requires summing up individual population projections for some 233 countries and areas. Each individual country projection for a future year is basically the result of the arithmetic combination of its annual expected future births minus deaths, plus immigrants and minus emigrants. Differing assumptions about expected future trends in mortality, fertility, immigration and emigration yield various projection variants or scenarios for each country.
One variant that is instructive but unlikely assumes no changes in current demographic rates of all countries. Under that scenario, world population increases rapidly, reaching more than 10 billion by midcentury and close to 20 billion by 2100 (Table 1).
A second instructive but again unlikely variant assumes all national fertility rates instantly move to the replacement level of about two children per woman. In such a fertility scenario, world population continues to grow, reaching close to 9 billion by 2050 and nearly 10 billion by the century’s end.
Because future mortality disasters and huge migration flows are extremely difficult to anticipate, such events are not usually incorporated or assumed in population projections. However, population projections incorporate recent major demographic events, such as the HIV/AIDS epidemic and large international migration flows among countries.
In addition, greater demand exists for various policy scenarios for raising, maintaining or lowering future fertility than mortality where reducing mortality is a universal goal. Changes in fertility are also more likely to have sizable demographic effects on a given population. Consequently, population projection variants are largely a function of expected future fertility levels, which is precisely where the greatest uncertainty is encountered.
Moreover, if women on average were to have about two children — the replacement level — and mortality and migration rates remained relatively low, a population eventually stabilizes. However, if fertility is above or below the replacement rate, then over the long term, populations increase or decline, respectively. Replacement fertility is essentially the tipping point for population growth.
According to the United Nations medium projection variant, which is most often used and cited, world fertility declines slowly from its current 2.5 births per woman and settles around two births per woman during the second half of the century. World population would therefore reach nearly 9.6 billion by midcentury and almost 11 billion by the close of the 21st century.
Under the UN low variant projection, world fertility declines to a level of 1.5 births per woman by century’s end. In that scenario, world population initially increases to a high of 8.3 billion in 2050 and plummets to fewer than 7 billion by 2100.
In contrast, world fertility in the high variant projection increases after a decade to nearly 3 births per woman and gradually declines to 2.5 births per woman by century’s close. Thus world population would grow rapidly, approaching nearly 11 billion by 2050 and 16.6 billion by 2100.
Recently, the UN projections were augmented with statistical probabilities assessing future demographic trends for individual countries. The revised projections found an 80 percent probability that world population will increase to 9.6 to 12.3 billion in 2100, with a central figure of 10.9 billion. Those projections also concluded that there is little prospect of seeing an end to world population growth during the 21st century without unprecedented fertility declines in most parts of sub-Saharan Africa, which is still experiencing rapid population growth.
All the scenarios offer several generalizations about future world population. First, they point to continuing world population growth over the next several decades.
Second, world population will most likely continue growing throughout the 21st century. By midcentury, world population falls within a range of 8.3 billion to 10.9 billion. After midcentury, the possible range for future world population widens greatly, with differing views on the likely demographic outcome, especially the impact of women’s education on the high fertility countries in Africa and Asia. The most possible scenario, the UN says, is a world population of nearly 11 billion by the end of the 21st century.
Third, barring possible catastrophic mortality disasters, future world population is largely determined by expected trends in fertility. While the course of fertility in the future remains uncertain, powerful forces are operating to reduce fertility worldwide, including lower mortality rates, urbanization, education, improvements in the status of women, widespread use of contraceptives and delayed marriage and childbearing.
Additional forces keeping fertility low, often under the replacement level, include the decline of marriage, divorce and separation, co-habitation, employment and economic independence of women, costs of childrearing, childless lifestyles and the need to save more money for longer years of retirement and elder care.
Fourth, past population projections have repeatedly showed that there may be unexpected demographic shocks and surprises in the future. For example, few demographers anticipated the 1918 influenza pandemic, the post-war baby boom, the one-child policy, the HIV/AIDS epidemic, sustained below-replacement fertility or millions of men and women living beyond 100 years.
While future demographic uncertainties clearly cannot be eliminated, they can be reduced considerably with updated statistical information, avoidance of preconceived notions and self-serving assumptions and a sound understanding of the dynamics and complexities of population trends.
As new demographic data for countries emerge and more insight is gained on recent trends and population structure, assumptions based on expected future levels of fertility, mortality, immigration and emigration are reassessed and adjusted accordingly. These important periodic revisions of demographic projections for all countries result in a changing future world population.