The global population is likely to shrink after the middle of this century, triggering shifts in economic power, a new report suggests.
There will be around 9.7 billion people on the planet by 2064, but that number will decrease to 8.8 billion by the year 2100, according to a new report in medical journal The Lancet.
The analysis says that improvements to modern contraceptive methods and the increasingly widespread education of women could be a catalyst for a decline in global fertility rates.
That, according to the report, means that populations will not be sustained at current levels without a more liberal immigration approach.
Populations in 23 countries, including Japan, Spain and Italy, are forecast to decline by more than half, according to the research, with another 34 countries, including China, seeing a drop of more than 25%.
However, sub-Saharan Africa will buck the trend, and is set to see growth of more than three times its current population, thanks to falling death rates in the region and the rising number of women reaching child-bearing age.
Across the world, over-80s are set to outnumber under-fives by a factor of two-to-one by 2100, marking a shift in the working age population.
Countries such as China, Spain, the UK and Germany are all expected to see a dramatic drop in the size of their workforce, resulting in a slowing of economic growth that will open the way for African and Arab countries to take the lead economically.
Dr Richard Horton, editor in chief of The Lancet, said: "This important research charts a future we need to be planning for urgently.
"It offers a vision for radical shifts in geopolitical power, challenges myths about immigration, and underlines the importance of protecting and strengthening the sexual and reproductive rights of women. The 21st century will see a revolution in the story of our human civilisation.
"Africa and the Arab World will shape our future, while Europe and Asia will recede in their influence. By the end of the century, the world will be multipolar, with India, Nigeria, China, and the US the dominant powers.
"This will truly be a new world, one we should be preparing for today."
African nations will lead the way in terms of total population growth.
Niger's population is forecast to grow by 765% by 2100, Chad's by 710%, South Sudan's by 594% and Mali's by 321%.
Meanwhile, Latvia is predicted to see the biggest fall in population, by 78%, with El Salvador's population forecast to drop by 77%.
As a result, it means that south and southeast Asia will concede their positions as the most populated regions of the world to Sub-Saharan Africa, which is projected to see an explosion in growth in the middle of the century.
Central Europe will move to the bottom of the table, with North America overtaking the region.
India, which currently has the second-highest population in the world, will rise to the number one spot by the year 2100, despite a fall in population of around 300 million people.
China will fall from top to third place in the table, with its population set to fall by more than 25%.
Nigeria, which currently has the seventh-highest population in the world, will rocket up to second place by 2100, while the Japanese population will plummet from the 10th to 38th.
The Lancet study also predicts a major shift in the way age is distributed throughout the global population.
By and large, age is currently structured as a pyramid with more young people than older people, with people in their mid-20s being an outlier.
However, the journal predicts that the population will become more middle-aged by 2100.
The Lancet report suggests that immigration could be a way to offset population decline.
Western countries that will have a lower birth rate by 2100, such as the US, Australia and Canada, will likely be able to maintain a working age population by liberalising their stance on immigration.
The report warns, however, that population decline should not compromise global progress made on women's rights and reproductive health.
Professor Ibrahim Abubakar, from University College London and the chair of Lancet Migration, said that if the predictions made in the Lancet "are even half accurate", then "migration will become a necessity for all nations and not an option".
He added: "The positive impacts of migration on health and economies are known globally. The choice that we face is whether we improve health and wealth by allowing planned population movement or if we end up with an underclass of imported labour and unstable societies."
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