India is estimated to have the largest working-age population by 2100, followed by Nigeria, China and the US, despite a huge decline in the number of workers, according to a study published in Lancet recently.

Huge declines in the number of workers are seen in China and India by researchers who projected the number of working-age individuals (aged between 20 and 64 years) for the 10 largest countries by population in 2017.

The researchers estimated the population from 2018 to 2100 for 195 countries and territories with the standard cohort-component method of projection using estimates from the Global Burden of Diseases, Injuries, and Risk Factors Study (GBD) 2017.

Global population will peak in 2064 at 9·73 billion and then decline to 8·79 billion in 2100, considering statistical models of fertility, mortality and migration, the study said.

India, Nigeria, China, the US and Pakistan are likely to be the five largest countries by population in 2100, the study added.

By 2050, 151 countries are projected to have a total fertility rate (TFR)—the average number of children born per woman—lower than the replacement level (TFR <2·1), and 183 nations will have a TFR lower than replacement by 2100, the study said.

“Replacement level fertility" is the total fertility rate at which a population replaces itself from one generation to the next, without migration. This rate is roughly 2.1 children per woman for most countries, although it may vary with mortality rates.

Between 2017 and 2100, the study said, 23 countries, including Japan, Thailand, and Spain, are projected to have population declines greater than 50%. China, whose population is expected to decline by 48%, is projected to become the largest economy by 2035 before the US overtakes it in 2098.

The authors said the findings suggest continued gains in female literacy and access to contraception will hasten declines in fertility and slow down population growth.

A sustained TFR lower than the replacement level in many countries, including China and India, will have economic, social, environmental, and geopolitical consequences. Policy options to adapt to continued low fertility, while sustaining and enhancing female reproductive health, will be crucial in the years to come, the study added.

“In our model, in a population where all females have 16 years of education and 95% of females have access to contraception, the global TFR was projected to converge at 1·41," said the study.

“One important determinant of population growth is the rate of fertility decline in high-fertility countries, particularly those in sub-Saharan Africa. This rate of decline was driven largely by improvements in access to education and modern contraceptives," said the study.

Calling a decline in total world population in the latter half of the century as potentially “good news" for the global environment, the study said, fewer people on the planet in every year between now and 2100 than the number forecasted by the UNPD would mean less carbon emission, less stress on global food systems, and less likelihood of transgressing planetary boundaries,

However, despite lower population, environmental and climate change might still have major and serious consequences in the intervening years unless preventive action and mitigation is vigorously pursued, the study said.

The researchers projected that 118 of 195 countries and territories will have net migration rates between −1 and 1 per 1,000 population in 2100, with an additional 44 countries having net migration rates between −2 and 2 per 1,000.

The countries with the largest immigration forecasts in absolute numbers in 2,100 are the US, India, and China, whereas emigration was projected to be highest in Somalia, the Philippines, and Afghanistan, the study said. Net immigration rates are likely to be the highest in Canada, Turkey, and Sweden, whereas emigration rates will be at the highest in El Salvador, Samoa, and Jamaica, it said.

DR Congo, Pakistan, India, and Indonesia are forecasted to have the lowest life expectancies among the 1large countries, ranging from 76·9 years in DR Congo to 79·5 years in Indonesia.

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