Divergent population projections portend very different futures for our planet

The Cooperative Society Newsletter
September 2022, Issue 37
by E.G. Nadeau
, Ph.D.

Congratulations (or commiserations), fellow earthlings! The world’s population is about to hit 8 billion.

There were approximately 1 billion of us in 1800. But don’t be too alarmed by the eightfold increase since then. The growth rate has declined dramatically in recent decades and is expected to continue to slow – and possibly begin to decline – during the 21st century.

The United Nations Population Division recently estimated that the number of humans in the world will increase to about 10.4 billion in 2100, level off, and then start to decrease gradually around that time. In contrast, the Institute for Health Metrics and Evaluation (IHME) projects a very different population trend for the remainder of the century, with the number of people peaking at about 9.7 billion in 2064 and then shrinking to 8.8 billion by the end of the century. IHME even has a lower growth scenario that projects a decline to 6.3 billion by 2100.

So, why the divergent projections? Different assumptions – primarily related to fertility rates.[1] To oversimplify a bit, the UN analysts tend to look at past and current data on fertility rates to project future trends. In contrast, the IHME report states: “Our findings suggest that continued trends in female educational attainment and access to contraception will hasten declines in fertility and slow population growth.”

National and regional differences in population trends

As one would expect, the UN and IHME data sets also diverge in their population projections for the number of people within specific countries and regions, sometimes in dramatic and surprising ways. Following are a few examples of these differences that have potentially enormous long-term social, economic, political, and environmental implications.

1. India and China
India will probably surpass China as the world’s most populous country in the next couple of years. The difference in population size between these two countries is expected to increase significantly by the end of the century. The UN estimates that India will have about 1.5 billion inhabitants by 2100, and China, fewer than 1.1 billion. The IHME projections are for 1.1 billion Indians and a little more than .7 billion Chinese by that year. This shift in population dominance between the world’s two largest countries will have an as-yet-unknown impact on the world polity and economy.

2. Sub-Saharan Africa
By 2100, the UN projects that the number of inhabitants of Sub-Saharan Africa will reach 3.8 billion, while IHME projects 3.1 billion. With a current count of about 1 billion, these two sources are predicting in the range of a tripling or quadrupling of the region’s population. However, in IHME’s model assuming accelerated women’s education and increased access to contraceptives, the projected 2100 population drops to about 1.6 billion.

3. South Korea, Japan, and other shrinking countries
On the other end of the scale, South Korea, Japan, and dozens of other countries are experiencing significant decreases in their fertility rates, and some are already experiencing steep declines in their populace or are about to do so. South Korea, in particular, has a very low fertility rate of .8. This is well below the replacement rate of 2.1 children for women of childbearing age. Without a major increase in immigration, South Korea will face a rapid decline in, and aging of, its society in the coming decades.

4. The United States and Western Europe
The IHME data project that the countries of Western Europe will decline from about 430 million to 374 million inhabitants in 2100. In the scenario with the accelerated expansion of women’s education and contraception, the estimated population in 2100 will drop to 330 million.

The same data sets indicate that the United States will grow from about 325 million in 2020 to about 336 million in 2100. The accelerated assumptions about women’s education and contraception would reduce the 2100 population in the United States to about 286 million.

The importance of accurate data and assumptions

There is a saying among statisticians: “Garbage in, garbage out” which essentially means, if your input data and assumptions are flawed, your results will be, too. The issue is even more problematic in projecting future change, such as population trends.

There are two major problems in predicting such change. The divergence of projections becomes more dramatic the further out in time one is projecting. Secondly, humans change their behavior over time and thus affect actual trends. In the case of population projections, fertility rates, the major predictor of the number of people on the planet, may be sharply affected by increases in women’s education and the use of contraceptives. The IHME projections are based on much more “activist” assumptions than the UN projections related to these two variables, and thus project slower population growth and earlier peaks in regional and world population. One could also argue that there is an element of “self-fulfilling prophecy” in projecting future population change. Predicting lower population growth can actually help bring about this lower growth.

The importance of taking action

Whether we are looking at changes in China and India, growth in sub-Saharan Africa, or the shrinking populations of South Korea and other countries, the sooner we know what is likely to happen, the sooner we can take actions to reduce international conflict, maintain and improve the quality of life, and safeguard the environment.

For example, migration is one issue that will affect all of the world’s countries and regions in the remainder of the century. All of the projection data indicate a severely imbalanced world in terms of where people will be born, where the best opportunities for improving their quality of life are likely to be, and where the greatest climate change threats and opportunities will occur. If our information on population trends is fairly accurate, we can take actions to improve this balance in large part through far more enlightened migration policies than we currently have.


As I have reported previously, the UN Population Division’s projections appear to be methodologically flawed because it estimates too-rapid a pace of growth in the remainder of the 21st-century. (A previous example of this pattern is that the UN currently anticipates 10.4 billion people by 2100 whereas in 2019 it projected a much higher level of 10.9 billion.) The Population Division is also strategically flawed because it fails to take into account the ability of women, couples, communities, countries, and international bodies like the UN to “bend the population curve” downward in the coming decades.

In contrast, the IHME demographers provide an approach to population projection that factors in the potential impact of improvements in women’s education and reproductive health that significantly reduce the rate of population growth. Their data give us a better opportunity to prepare for the upcoming changes in population trends and the impacts the large differences these trends would have in different countries and regions of the world.

[1] “The total fertility rate in a specific year is defined as the total number of children that would be born to each woman if she were to live to the end of her child-bearing years and give birth to children in alignment with the prevailing age-specific fertility rates.” A total fertility rate of 2.1 is considered to be the replacement rate. That is, the population is expected to stay at the same level in a country or other region when the fertility rate is at this level.

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