While there is now a consensus on announcing a global population decline, projections differ considerably. For United Nations (UN) experts, who are still leading the debate, the most likely hypothesis is that the peak will be reached shortly after 2100, with a global population of 10.9 billion compared to nearly 8 billion today. But there is a wide range of possibilities among all the projections, and the peak oscillates between 9.4 and 12.7 billion. This is due to variations in estimates about changes in fertility rates on the one hand, and mortality on the other. Other voices are rising, meanwhile, to announce a major crisis induced by global warming, that will result in the collapse of the population from 2050. Although demographer Henri Leridon considers such a hypothesis implausible, he thinks the rapid fertility drop lends credibility to the UN’s low-variant hypothesis. His colleague Hervé Le Bras follows the same line of thought. Picking out a number of biases in the UN’s main scenario, Le Bras argues that the experts tend to underestimate the decline in fertility and overestimate the rise in life expectancy. He believes the peak will be reached between 2060 and 2075.

2 Le Bras also cautions against the temptation to consider the world’s population as anything other than a “conceptual construct.” This echoes an analysis conducted seventy years earlier by demographer Alfred Sauvy, who protested against the catastrophic prognoses formulated at the time on the risks of a demographic explosion and contested the idea of imposing birth control. Sauvy was already affirming that the main factor behind fertility rates is the development of education and health care. History proved him right.

A wide range of hypotheses

Pakistan floods. Displaced people fleeing Sindh streamed into Balochistan

3 Since the 1960s, the scenarios elaborated by UN experts have been derived from what is deemed to be a proven method: The “cohort-component” method, based on analysis of levels of fertility and mortality, by age and sex. Aggregating the projections made for each country, the UN Population Division publishes its global projections every two years. In Population & Societies, the journal of the Institut national d’études démographiques (INED) (National Institute for Demographic Studies), Henri Leridon breaks down the main hypotheses resulting from these projections and questions the legitimacy of the most pessimistic scenarios rolled out today.

4 The UN presents a central hypothesis called the “medium variant,” branching off above or below into several variants, that are deemed less likely. The medium variant projects an additional gain of 3.2 billion people between 2019 and 2100, with the global population then leveling out at 10.9 billion, before beginning to drop. But “the diversity of past trends” in fertility rates introduces a vast potential margin of error. In 2100, the world population may reach only 9.4 billion, or, by contrast, peak at 12.7 billion. Demographers also engage in purely theoretical calculations: If fertility is arbitrarily lowered by 0.5 children per woman compared to the medium variant, the population in 2100 is reduced to 7.3 billion—that is, less than today; whereas if it is increased by 0.5, the peak reached in 2100 is 15.6 billion.

5 Since the 1950s, notes Henri Leridon, the population curve has stayed close to the “medium-variant” projections. Is it conceivable that this might change in the future? He speculates. The UN assumes that fertility rates will keep dropping, as they have over the past decades, except for a slight increase projected in Europe and North America. For sub-Saharan Africa, the medium variant more than halves current fertility rates: from 4.72 children per woman to 2.16 in 2100. Alternatively, the UN experts engage in further conjecture, theorizing an immediate and general transition to 2.1 children per woman (replacement level fertility). This would imply an increase in countries where the rate is lower, and a decrease elsewhere. The result is close to the medium variant.

6 Henri Leridon points out that the drop in fertility may be greater than anticipated by UN experts and that the reality is now approaching the low variant. He then examines the most pessimistic scenarios, put forward by “collapsologist” authors who conjure up the possibility of a “sharp increase in mortality.” The UN’s medium variant projects a continuing decline in mortality—like fertility—due to rising life expectancy. But it also considers the possibility that the mortality rate will level out, which would limit the world population to 8.92 billion in 2100. Collapsologist theories are based on the idea of the potentially catastrophic impact of climate change on agricultural production and access to water. The model used by the authors of the Club of Rome report, drawn up in 1972, is aired anew here, in light of a projected population collapse starting in 2050. Henri Leridon considers such a prospect unlikely. If the UN’s low-variant projection is likely to come true, it “is due to a rapid fertility decline, and not an increase in mortality.”

Source – https://www.cairn-int.info/dossiers-2023-1-page-1.htm?contenu=article