More than three out of four countries may have a shrinking population by the middle of the century due to "tumbling" fertility rates, research suggests.
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And by 2100, more than 97 per cent of countries - 198 out of 204 - will have fertility rates below what is necessary to sustain population size over time, according to a study published in The Lancet.
The analysis predicts that sub-Saharan Africa will account for one in every two children born on the planet by the end of the century.
Meanwhile in western Europe, the total fertility rate - the average number of children born to women of child-bearing age - is predicted to be 1.44 in 2050, dropping to 1.37 in 2100.
In the UK, the total fertility rate was 1.49 in 2021. By 2050, the rate is predicted to be 1.38, dropping to 1.30 by 2100.
South Korea would have the worst fertility rate by the middle of the century with 0.82 children per female, according to the analysis.
A rate of 2.1 is needed to maintain a steady population without significant immigration.
The researchers said the findings pose "serious challenges" to global economic growth, especially in the midst of a shrinking workforce and ageing populations.
Dr Natalia V Bhattacharjee, co-lead author from the University of Washington's Institute for Health Metrics and Evaluation (IHME), said the implications were immense.
"These future trends in fertility rates and live births will completely reconfigure the global economy and the international balance of power and will necessitate reorganising societies."
Senior author Professor Stein Emil Vollset, from the IHME, said: "We are facing staggering social change through the 21st century.
"The world will be simultaneously tackling a 'baby boom' in some countries and a 'baby bust' in others."
The analysis predicts Niger would hold the top spot in 2050 for the highest fertility rate, with an average of five children for each woman, but this could come down to 2.7 children if universal education targets were met.
The researchers said it will be important for countries with low fertility rates to implement policies "that support those who wish to have children and offer additional benefits to society such as better quality of life and greater participation of women in the workforce, alongside open immigration policies".
"Social policies to improve birth rates such as enhanced parental leave, free childcare, financial incentives, and extra employment rights, may provide a small boost to fertility rates, but most countries will remain below replacement levels", Dr Bhattacharjee said
"And once nearly every country's population is shrinking, reliance on open immigration will become necessary to sustain economic growth.
"Sub-Saharan African countries have a vital resource that ageing societies are losing - a youthful population."
She added: "There is very real concern that, in the face of declining populations and no clear solutions, some countries might justify more draconian measures that limit reproductive rights.
"It is well established that nations with strong women's rights are more likely to have better health outcomes and faster economic growth.
"It is imperative women's rights are promoted and protected and that women are supported in having the number of children they wish and pursuing their careers."
Australian Associated Press
