How Did We Arrive at 7 Billion – and Where Do We Go From Here? [Part One]October 26, 2011 By Elizabeth Leahy MadsenThe United Nations Population Division has estimated world population will reach seven billion on Monday. Which changes in demographic trends led us to this milestone? What do the past and present tell us about how human numbers will change in the future?
The “Day of Seven Billion” was announced this spring following the release of the latest revision of UN population projections. Although the seven billionth person will not be precisely identified, this estimate is based on careful demographic modeling. Every two years, the UN revises its projections to incorporate the latest trend data and modify its assumptions, as seemingly small changes can make a huge difference demographically.The UN estimates that the seven billionth person alive today will be born on October 31. Demographer Elizabeth Leahy Madsen explains how we got to that number, its significance, and where our demographic path might take us from here. Read part two here.
The United Nations Population Division has estimated world population will reach seven billion on Monday. Which changes in demographic trends led us to this milestone? What do the past and present tell us about how human numbers will change in the future?
The “Day of Seven Billion” was announced this spring following the release of the latest revision of UN population projections. Although the seven billionth person will not be precisely identified, this estimate is based on careful demographic modeling. Every two years, the UN revises its projections to incorporate the latest trend data and modify its assumptions, as seemingly small changes can make a huge difference demographically.
Demography Is Driven by Fertility and Population Momentum
Since world population reached three billion in 1959, the rate of growth has increased, peaked, and begun to slow. Each succeeding milestone was reached more quickly than the last: It took 15 years to reach four billion, 13 years to hit five billion, and only 11 years to get to six billion at the end of 1998. The interval leading to seven billion was slightly longer, at 13 years, as the global rate of population growth has slowed.
Although mortality and migration also affect population trends, the factor with the greatest influence by far is fertility – the average number of children born to each woman. The decline in the global fertility rate from an average of nearly 5 children per woman in the early 1960s to 2.5 children today has in turn slowed the pace of world population growth. However, demographic momentum from previous generations of high fertility can drive population growth for decades to come. Even if Nigeria reached replacement-level fertility today, its population would still grow by one-third by 2050 as the number of births continued to exceed the number of deaths.
Population projections consider: 1) current data about fertility and 2) assumptions about the ways fertility will change in the future. These assumptions vary depending on the source, so how much of a difference do they make? As it turns out, quite a lot.
world population in 2050 range from 8.1 billion (if fertility rates fall to a global average of 1.7 children per woman) to 10.9 billion (if they remain unchanged). The gap of nearly three billion between those possibilities is greater than the combined populations of China and India today.
Estimates vary even more widely for the end of the century, with the UN projecting that by 2100 world population could total anywhere between 6 billion (if total fertility falls to an average of 1.55 children per woman) and 27 billion (if every country’s fertility rates remain constant at today’s levels).
While demographers parse the details of the projections, policymakers would like to know which of these scenarios is more likely. After all, the economic, environmental, and political consequences of a population of 8 or 11 billion two generations hence are not the same, and a world of 27 billion is difficult for anyone to fathom.
If we simply projected past trends into the future at a steady rate, the population estimates on the low end of the fertility spectrum seem more likely. The global fertility rate has fallen from 4.5 children per woman in the early 1970s to 2.5 today, a decline of 43 percent, so the 14 percent decline projected in the medium-fertility variant between now and 2050 seems reasonable at first glance, perhaps even conservative. The medium-fertility variant assumes that all countries’ fertility rates will begin moving towards replacement level, around 2.1 children per woman, regardless of whether they are currently above or below that number.
However, even a 14 percent decline in fertility assumes that areas where fertility rates remain stalled at high levels will soon begin rapid declines, paralleling the past experience of other regions. As Population Reference Bureau demographer Carl Haub writes, “the assumption that the developing world will necessarily follow the path of the industrialized world…is far from a sure bet.”
In the last 40 years, fertility rates in the Caribbean, northern and southern Africa, Latin America, and all of Asia declined by 50 percent or more. The pace of decline in sub-Saharan Africa, while still notable, was much slower, at 23 percent. In order to meet the UN medium-variant projections, the region’s fertility rate would need to fall by nearly 40 percent by mid-century.
Some of the largest, fastest-growing populations in the developing world would need to experience a major acceleration from recent trends. In Nigeria, fertility edged down by 15 percent between 1970 and 2010, but the medium variant projection depends on a decline of 37 percent over the next four decades; Ethiopia’s fertility rate will need to fall by half.
Gender Matters, Too
The great irony of fertility trends is that gender inequities play an important role at both ends of the scale. In countries with the highest fertility rates, women tend to have less education than men and less autonomy. Their fertility choices may be greatly affected by the preferences of their husbands or other family members. In Niger, which has the highest fertility rate in the world, married men would, on average, like three more children than married women. In Uganda, where women average more than six children each, 60 percent of men report that domestic violence is justified.
By contrast, in countries with the lowest fertility rates, women have achieved equal access to education and the labor market, with more autonomy about how to earn income and what to do with it. Yet cultural expectations that place the burden for child and elder care and housework almost entirely on women can make marriage an unappealing option. In Japan, which is among the 10 lowest fertility countries in the world, more women are choosing to stay single: The marriage rate has fallen by almost half since the 1970s. Japanese women who do marry are waiting until their late 20s and tend not to give birth until they are 30, both of which result in lower average family size.
Even at this end of the demographic spectrum, the assumptions embedded within population projections seem optimistic. Japan’s fertility rate was last above replacement level in the early 1970s; it has fallen steadily to 1.3 children per woman today. The UN projections assume that fertility will immediately reverse track and begin rising to over 1.8 children per woman in 2050, rebounding above two children per woman before the end of the century.
The stalled high fertility rates in much of sub-Saharan Africa and parts of the Middle East, together with unprecedented low fertility in Eastern Europe and parts of East Asia, indicate that we are currently in an era of remarkable demographic diversity, despite the UN’s projection of future convergence.
Continue reading part two here.
Elizabeth Leahy Madsen is a consultant on political demography for the Wilson Center’s Environmental Change and Security Program and former senior research associate at Population Action International.
Sources: Boling (2008), Haub (2011), Japan Statistics Bureau, Measure DHS, UN Population Division, UN Population Fund, Washington Post.
Image Credit: Chart data from UN Population Division, arranged by Elizabeth Leahy Madsen.
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