India set to overtake China, UN predicts global population over 10 billion by the end of the century

These are among the latest projections published by the UN in its World Population Prospects report, which also highlights the rapid decline in global population growth - now at its slowest rate since 1950 - continuing into the second half of the century.

"The cumulative effect of lower fertility, if maintained over several decades, could be a more substantial deceleration of global population growth in the second half of the century," says UN Department of Economic and Social Affairs population division director John Wilmoth.

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The UN predicts global population could reach a further 8.5 billion by the end of this decade, 9.7 billion by 2050, and peak at 10.4 billion by the end of the century.

That's a reduction of around 300 million people in 2100 from its estimates three years ago.

It's still higher than other projections in recent years, suggesting the world population might peak before the end of the century.

Research from the University of Washington, US, published in 2020 predicted that the world population would peak at about 9.73 billion in 2064, observing that increases in female education and access to contraception would see declines in fertility and population growth.

That followed a 2018 report from the European Commission's Joint Research Centre that predicted a peak of 9.8 billion between 2070 and 2080, but also suggested rapid social development and education reach in line with sustainable development goals could see a peak of 8.9 billion by 2060.

The reason for these different projections comes down to the assumptions researchers make along the way.

At the most basic level, explains Associate Professor Gour Dasvarma, from Flinders University in Adelaide, a population projection considers trends in birth and death rates.

"Projections are done by extrapolating past trends, long term trends in fertility, mortality and migration for a country population," he explains. "For the world population, migration doesn't matter.

"One of the things with the projections is that as and when new data become available, people will revise those.

"The latest predictions for the UN is that the world's population will peak at 10.4 billion by 2100 and then it will start declining.

"By that time, the trends indicate that fertility in most of the countries of the world will have declined to a sufficiently low level, the ageing of the population will take hold, and the so-called momentum of population growth will slow down."

Nations transition through cycles of population growth, stability and decline as their economies develop. From periods of stability with high birth and death rates, populations increase as mortality drops.

Over time, fertility rates begin to decline, causing stabilisation in population numbers. It's only when death rates nudge above births that populations begin to naturally decrease.

For nations like those in Europe, North America, Australia and New Zealand, this demographic transition was completed between the pre-industrial era and the mid-20th century - a period of about 200 years.

"But after 1950, some developing countries like China, other parts of Southeast Asia [...] and also Latin America have done it within 70 years because of the increase of contraceptives and faster decline in fertility," says Dasvarma.

With life expectancy projections increasing, nations in the Global South will continue to see their populations to do likewise.

Although more than half of the world's population lives in East, South-east (29 per cent of global population), Central and Southern Asia (26 per cent), the UN expects these regions along with Latin America, the Caribbean, Europe and Northern America to begin declining before the end of the century.

In contrast, sub-Saharan African nations are likely to keep growing through 2100, while the next quarter century will see over half of the world's population increase come from just eight nations.

They include India - which will overtake China to be the world's most populous nation next year - the Democratic Republic of the Congo, Egypt, Ethiopia, Nigeria, Pakistan, the Philippines and Tanzania.

Two thirds of the world's population now live in areas where lifetime fertility has dropped below 2.1 births. Long term, that equates to zero population growth: one child to replace each parent in nations with low mortality.

COVID-19 has also impacted population data - with a drop in global life expectancy (now 71, down from 72.9 before the pandemic) and short-term decreases in pregnancies and births. But the pandemic's impact was unevenly distributed around the world. In regions hardest hit by deaths, life expectancy at birth dropped by nearly three years. In contrast Australia and New Zealand saw this indicator increase by more than a year, likely thanks to border closures imposed throughout much of 2020.

These decreases in national fertility rates will see populations age further in the coming years. By the century's midpoint, 16 per cent of the global population is expected to be aged over 65 - the same proportion as people under 12 years of age. It's prompted the UN to recommend nations with ageing populations invest in social safety nets to meet the needs of older people.