We will be, this Tuesday, 8 billion people on Earth. To tell the truth, the course may have already been crossed or will be later. This November 15 is in any case the day marked by the United Nations in its world population projections, which the organization updates every two years. If this mark is symbolic, “we should not conclude that we know little about the state of the world population, reframes the demographer Gilles Pison, adviser to the management at the National Institute for Demographic Studies (INED)*. On the contrary. Over the past 50 years, no country has escaped one or more censuses. And demographic trends are also relatively well known. »
The global trend, precisely, is on the rise, again and again. There were 2.5 billion humans in 2050, 4 in 1974, 6 in 1999, 7 in 2011 and therefore 8 now… That is to say a tripling in seventy years, often set up as the main threat to the planetary balance.
Fertility rates that are falling
Yet, at the same time, the birth rate – the average number of children per woman – continues to decline. It is 2.3. “It was more than double more than fifty years ago,” resumes Gilles Pison. This fall was not immediately perceived in the projections of the United Nations. At least not to this extent. “For a long time, the assumption was that the fertility rate would converge around the world around 2.1, generational replacement threshold”, recalls the demographer. It didn’t happen that way. “In the industrialized countries already, fertility has dropped significantly below 2.1, resumes Gilles Pison. In the EU, the average is around 1.59. And the lowest fertility rate is in South Korea, with 0.9 children. »
Above all, second surprise, many countries of the South have joined those of the North much faster than expected. “This is the case for several Asian states and almost all of Latin America,” explains Gilles Pison. This changeover is still in progress. “In 2019, more than 40% of the world’s population lived in countries whose replacement level was at or below 2.1 children per woman. In 2021, this represents 60%”, wrote on July 11 Michael Hermann, Advisor on Economics and Demography to the United Nations Population Fund (Unfpa). And even now two-thirds, adds the UN in its report “World Population Prospect 2022”.
A population that will grow for several more decades
So why does the world’s population continue to grow? There are already regions on the globe where fertility is still high. “At 2.5 or more,” says Gilles Pison. “This is the case in practically all of Africa, but also in part of the Middle East and a strip in Asia that goes from Kazakhstan to Pakistan via Afghanistan”, he specifies. But this is not the only explanation. “Many densely populated countries have a high proportion of young people, of childbearing age or who will be,” he recalls. This is the case of India, which should become the most populous country in the world in 2023, overtaking China, and where more than half of the population is under 25 years old. Even with a fertility rate now below the fertility threshold, “The fact remains that two children per woman, in this country, that’s a lot of births”, summarizes Gilles Pison.
This demographic inertia makes certain the increase in the world population in the coming decades, “at a rate that has been decreasing year by year for sixty years **”, specifies Gilles Pinson According to the United Nations, we should thus be 8.5 billion people in 2030 and 9.7 in 2050. “More than half of this increase will be concentrated in just eight countries ***”, indicates the UN.
The big blur for 2100?
And by 2100? At this horizon, on the other hand, the trajectories are more uncertain. The UN’s medium scenario, which assumes a decline in fertility in countries where large families still prevail, predicts that we will peak at 10.4 billion people in 2086, before leveling off until 2100. In this average scenario, around 60 countries will see their population drop by at least 1% by this horizon. As for the world fertility rate, it would drop from 2.3 to 2.1 in 2050 and 1.8 in 2100.
Here is a first screening. American researchers from the Institute for Health Metrics and Evaluation (IHME) draw up a completely different one in a study published in the summer of 2020 din the medical journal The Lancet and which caused a stir. Like the UN, the authors expect a peak in population in the second half of the century but reached much earlier, from 2064, at 9.73 billion people. Then, the curve evolves gently to reach 8.7 billion humans at the end of the century.
The differences between these projections are mainly explained by the different fertility scenarios used. In their calculations, the IHME researchers anticipated, in countries with high population growth today, the effect of policy decisions aimed at improving girls’ education and providing better access to contraceptives. Thus, the study published in The Lancet predicts a dramatic drop in fertility rates in sub-Saharan Africa. In Niger, for example, we would go from 7 births per woman in 2017 to 1.8 at the end of the century. This region would then only have 3.07 billion inhabitants in 2100 when the UN is counting on 3.44.
4 billion humans at the end of the century? “A wild hypothesis”
Still, 8.7 billion Earthlings in 2100 may seem almost measured in comparison with the HSBC study published at the end of August. Based on other assumptions of changes in fertility and mortality rates, it went so far as to project 4 billion humans at the end of the century. “A far-fetched hypothesis”, sweeps away Gilles Pison. All the same, the UN keeps open very diverse evolution scenarios by 2100. To the “medium” one, we can add the “low fertility” one, which would bring us to 7 billion, and an opposite scenario which would lead to 15 billion. “But in the opinion of the agency itself, these two trajectories are unlikely, resumes Gilles Pison. To sum up, she estimates that there is a 95% chance that we will be between 8.9 and 12.4 billion inhabitants in 2100.