In general, sub-Saharan Africa is expected to see significant growth in the coming decades: The UN estimates its population could increase from 1.3 billion to 4.2 billion by the end of the century. Others have made more conservative estimates, but all of the projections agree that Nigeria will be a demographic superpower by 2100 — the third-most populous country in the world, with the second-largest working-age population, according to The Lancet.
India, meanwhile, is undergoing its demographic transition, but because it is so large, the country will still contribute the most births — 273 million — to the global population between 2020 and 2050. That same Lancet study found that India’s working-age population will surpass China’s in the mid-2020s and become the world’s largest by 2100.
Nevertheless, the study predicted that the United States will remain an economic superpower, largely due to immigration flows, with the world’s fourth-largest working-age population and a GDP that could again exceed China’s by 2098. As China ages out, African countries and India will join the United States as leading world economies.
Tourists at the Bund in Shanghai, China, August 4, 2020. Yang Jianzheng/VCG via Getty Images
We are now an urban species. At some point in the late 2000s — the exact timing is difficult to pinpoint — humanity became an urban species, with more than half of the world living in cities. According to a 2018 United Nations report, that share will grow to nearly 70% by 2050.
There are already roughly 30 “megacities” with populations over 10 million, and that number is set to grow, especially in Asia.
As a result, coastal hubs will host an increasing number of inhabitants. One could say that the large coastal city is the future of humankind — a model which of course raises sustainability questions in the event of a significant rise in sea levels.
A men with flowers at a market in downtown Budapest on March 8, 2021. ATTILA KISBENEDEK/AFP via Getty Images
COVID-19 will only make a dent in the evolution of the world’s population. There is wide agreement that the official number of COVID-19 deaths, 4.3 million, is underestimated.
According to a Washington State University study released in May, the real tally is double that, closer to 7 million; The Economist put it even higher, at 10 million. It is a devastating loss, but when compared to previous global pandemics, COVID-19 appears to have had much less impact on population.
The total number of deaths worldwide in 2019 was 58 million, and the Earth’s population grows by around 80 million a year. In contrast, the 1918 flu pandemic, which killed about 50 million, reduced the world’s population by 1% to 5%. Centuries earlier, the Black Death may have killed half of the world’s population at the time.
Still, COVID-19 is having an important impact on the population of certain countries. The most affected ones, in terms of deaths per million inhabitants, are almost all in Eastern Europe: Hungary, the Czech Republic, Bosnia and Herzegovina, Azerbaijan and Bulgaria. Here, the coronavirus pandemic will exacerbate the existing demographic crisis.
Elderly women wait for the bus on Waiheke Island. James Pasley/Business Insider
Older adults are now more numerous than infants. Despite these complex national trends and the concentrated areas of growth, the age structure of the world is changing. Due to advances in medicine that prolong life and a declining overall birth rate, adults over 65 years old now constitute 10 percent of the world’s population and will reach at least 15% of it by 2050. The global median age is now 31, compared to 22 in 1965.
This aging of the world raises significant questions about health care and pensions. It is possible that the world will adjust to a longer working life, on the condition that those aged 60 in 2050 are as healthy as, say, those aged 50 in 2020.
But humanity is so far aging faster than countries are making the needed reforms to address the problem. It’s therefore likely that the transition to an older society will be accompanied by serious social unrest in countries that do not have generous pensions systems already in place.
The good news is that, all other things being equal, an older world could very well be a more peaceful one. One of the most politically useful findings of population studies in recent decades has been that there is a tight correlation between a society’s age structure — or, more precisely, the relative shares of young and older populations — and its propensity to violence.
This trend has been used to explain, for example, why African countries, which all have a median age of 33 years old or lower, witness so much internal strife and civil conflict. Thus, in a few decades, after the demographic transition is complete at the global level, the world should be a better place.
People cross the street with a stroller in downtown Oneonta, Alabama, June 30, 2021. Elijah Nouvelage/AFP/Getty Images
We may be heading for a “big crunch.” In 2019, the United Nations’ Department of Social and Economic Affairs predicted, as its baseline scenario, that the world’s population will hit 10.8 billion by 2100, and that fertility will hover around the 2.1 replacement rate.
This looks almost too good to be true. In fact, other institutions envision that the Earth’s population will sharply decrease by the end of the century.
The Vienna-based International Institute for Applied Systems Analysis believes that this reduction will happen as early as 2070 and that the global population will have dropped from a high of 9.5 billion down to 9 billion by 2100. A more recent study in The Lancet claims that fertility is likely to decline faster than the UN’s projection, particularly in Africa, and that humans will number only 8.8 billion by 2100. No less than 23 countries across Asia and Europe are predicted to lose more than half of their populations by that time.
It is overwhelmingly difficult for countries to recover when their fertility rates fall too far below the level of replacement — generally speaking, below 2.1 children born per woman. The Lancet forecasts an average fertility rate of 1.7 at the global level by the end of the century. Humankind has no experience with sustaining such low fertility almost everywhere in the world. This would truly be unchartered territory.
In the world of cosmology, a once-popular theory posited that the universe would expand and expand until gravitational forces eventually reversed the trend, causing it to contract and collapse. Is global population similarly doomed in the long run, headed for its own “big crunch”?
It’s hard to say, because the world’s population cannot be projected beyond 2100 in a useful way. In 2005, the UN devised heuristic tools to project the human population at 2300 and demonstrated that fertility was the key variable in determining the very long-term future of the planet: Adding or subtracting a few decimal points to the 2.1 replacement rate could lead to an Earth inhabited by 36 billion people three centuries from now — or by just 2 billion. In other words, the difference between a “Nigerian” future and a “Japanese” one.
Still, what we do know about this century is illuminating. Paul Morland, a leading scholar of demography at University College, has said that in the 21st century, the world’s colors will show “less white, more grey, and more green.”
Less white, because African populations will grow more rapidly than Western ones; more gray, because aging will affect almost all countries; and more green, because a richer and stabilized world population is, by and large, good news for the environment. It may also be, for the reasons stated above, a more peaceful one.
Bruno Tertrais is the deputy director of the Paris-based Fondation pour la Recherche Strategique (Foundation for Strategic Research). He was a member of the 2007 and 2012 presidential commissions on the White Paper on Defense and National Security.