Predicting how much the population on Earth will increase by 2100 is not that simple

According to the United Nations, the population of the Earth will reach 11 billion people at the end of the century, but other studies point downwards. Here's what we know about global demographics

(Image: Unsplash) Understanding how and how much the population of our planet will increase (or decrease), as you can imagine, is a fundamental challenge from an economic, scientific, social and sanitary. The declared objective, to borrow a term from astrobiology, is to keep the population in the so-called Goldilocks zone, an optimal number in which we are neither too many (with all the associated risks: resource depletion, greater pollution, food problems and so on) nor too few.

This week Nature has dedicated a long special to it this week, in which both current scenarios and forecasts for the future are told. Which are actually quite nebulous. For decades, the most reliable (indeed almost the only) forecasts for demography have been those made by the United Nations, and in particular by a relatively small task force of researchers with expertise in demographic modeling within the agency. br>
According to the latest estimates of these scientists, the world population will reach a plateau of 10.9 billion people by the end of the century. In recent years, however, other forecasts have been published, by other groups of scientists, who have revised down the population growth hypothesized by the United Nations. The point is that estimating how, when, where and how quickly the world population will change is anything but simple, especially in the long term.

The parameters that matter

But let's go in order. The crucial fact - one of the crucial data - to estimate the demographic evolution of the planet is of course the knowledge of the present, that is, of the number of people living in each nation. Having this data in hand with the greatest possible precision is particularly important not only because it allows us to develop a solid baseline from which to start p revisions for the distant future, but also to refine today's economic, social and health policies ( let's think, for example, of the allocation of vaccines against Covid19).

As if all the ordinary difficulties were not enough, the pandemic has further complicated the situation, delaying for example the censuses and sometimes significantly changing the expectation of life (a very recent study, for example, conducted by scientists at Oxford University and published in the International Journal of Epidemiology, showed that the pandemic reduced life expectancy by a greater amount of time since the Second World War: that of US men, in particular, has dropped by about two years) and birth rates, at least in the short term e.

In early 2021, China and the United States published the results of the censuses conducted in 2020. Other nations should have done so too, but the pandemic caused several delays. "China and the United States were the exceptions," explained Patrick Gerland, who conducts demographic research for the Un Population Division in New York. The results speak for themselves: both China and the United States are experiencing a time of extraordinarily low population growth, which, Garland says, is in line with forecasts, also because both nations track and produce regular and reliable data on births and deaths. , which allows demographic researchers to monitor trends in real time. And again: among the parameters to be taken into consideration there is of course also the migration rate, which indicates how many people enter and leave each country and is in turn linked to social, economic and historical factors that are not easy to predict in advance. and also affected by the pandemic.

The most important data

There is one thing that almost all demographers agree on. The most significant data, at this time, to estimate the evolution of the population, is the so-called fertility rate, that is, the average number of children delivered by each woman. This is because death and migration rates, net of punctual and (hopefully) unique variations due to the exceptional circumstances we have experienced, appear to be fairly stable, at least compared to those of the fertility rate.

A notable example is Singapore, whose fertility rate hovered at 3.04 in 1972 - when the government encouraged families not to have more than two children - and then dropped to 1.43 in 1986. when there was a U-turn in demographic policies, up to the current (and dramatic) 1.23. And in Italy things are no different. In the 1960s, in the midst of the economic boom, each woman gave birth to an average of 2.5 children. Today our country's fertility rate stands at around 1.3. In order for the population to remain stable (without considering the contribution of migration), the fertility rate should be around 2.1.

How much and how does this parameter change? It depends: in high-income countries, the change in behavior of people (couples) is usually driven by economic factors: as job opportunities increase, women choose to prioritize careers, and couples tend to postpone planning a pregnancy during recessionary times.

In low-income nations, other factors dominate: as female education levels rise, women tend to have fewer children and have them later : More people have access to contraceptive methods, so, in some ways, the decrease in the fertility rate reflects economic development. According to demographers, the pandemic will cause a short-term decline in fertility, at least in wealthier nations, due to rising economic uncertainty. Conversely, poorer nations could experience a moment of growth due to less contraceptive availability.

According to a recent study, currently still in pre-print, data from 17 nations in Europe, Asia and the United States show that the number of births decreased by 5.1% in November 2020, by 6.5% in December 2020 and by 8.9% in January 2021 compared to the same months of the previous year. In Spain there was the sharpest decrease, which reached 20% in the months of December 2020 and January 2021.

But there are those who argue that soon as early as October 2021 a rebound effect could be observed: During the pandemic, for example, Molly Stout's team of the University of Michigan analyzed electronic records to model the number of pregnancies in the surrounding community and predict the expected number of births. The analysis showed that between November 2020 and March 2021 there was a decrease in births of 14%, and suggested that births could increase by the same factor in the last quarter of 2021.

What will happen tomorrow (and the day after tomorrow)

In any case, if on the current scenario (and on that of the near future) the forecasts of the experts are fortunately quite in agreement, the same cannot be said of the long-term ones. As we said at the beginning, the United Nations, in 2019, predicted that population growth will continue, bringing the population from the current 7.7 billion people to about 11 billion by 2100. In 2014 another forecast was published, conducted by scientists from Iiasa (International Institute for Applied System Analysis), according to which the world population will reach a peak of 9.4 billion people around 2070 and will drop to 9 billion by the end of the century.

A subsequent report, released by the same group in 2018, corrected the projections to a peak of 9.8 billion around 2080, and a further update estimated a population of 9.7 billion in 2070. And again: another article, published last year by experts from the Institute for Health Metrics and Evaluation (IHME), estimated the peak at 9.7 billion people in 2064, followed by a decline to about 8.8 billion by the end of. 2100. According to this work, several countries, including Italy, Japan, Thailand and Spain, will see their population halve by the end of the century.

Why are the estimates so different?

Since everyone the scientists started from the same initial conditions, the difference in forecasts must be clearly in assessing how demographic variables will change over time, in particular the fertility rate. UN experts divide the ways in which fertility tends to slow, decline and eventually recover in various stages, and have used time series of each nation's fertility rates in recent years to rank the same nations in one of these stages. , thus producing over 100 thousand possible "evolutions" of future fertility. At this point, they calculated the median of these forecasts and presented it as the most likely scenario.

The Iiasa, instead of looking at the data, brought together a pool of 200 researchers, including economists, demographers and sociologists , making them work together to predict each nation's fertility rates in 2030 and 2050, based on what they thought would happen to social, health and economic factors. Some of these estimates showed great variability: forecasts of fertility rates in India, for example, ranged from 1.5 to 2.5 in 2030 and from 1.1 to 2.5 in 2050. Iiasa, calculated in this way, are much lower than those of the United Nations and.

The Ihme, finally, has done something still different. Instead of basing her model on fertility rates, she looked at another variable, the so-called complete fertility of the 50-year cohort (CC50), which counts the number of children each woman had before she turned 50. This is a slightly different variable than the fertility rate because it is less sensitive to the age at which each woman gives birth, and does not suffer from the same "rebound effect" of the fertility rate.

The main drivers of the CC50 they are the level of education and satisfaction in the demand for contraceptives: in fact, these are the factors that scientists have taken into account in making their estimates. Which caused, among other things, several eyebrows raised. "There are several problematic issues in these forecasts," commented for example Stuart Gietel-Basen, demographer at Hong Kong University of Science and Technology and co-author of a study, currently in preprint, which highlights alleged "internal inconsistencies, discrepancies and trends illogical and implausible ”in the estimates of the IHME. There is no peace among demographers.


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