Several European countries including Spain and Italy could see their populations halve by the end of the century, according to new research.
Poland and Portugal are also among the nations possibly facing at least a 50% drop by 2100, research posted in medical journal the Lancet has suggested.
A total of 23 countries around the world could see their populations drop by more than half of their current number, the report, compiled by the University of Washington, says.
Richard Horton, editor of the Lancet, said: “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.”
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The fall in European countries’ populations would have significant implications for their economies.
“If the assumptions used in our reference scenario were to hold true, Russia and Brazil’s relative ranking of GDP would decline moderately, whereas Spain and Italy would see substantial declines,” the report says.
In Europe, Spain is modelled to see a drop from 46 million people in 2017 to 23 million, while Italy falls from 61 million to 31 million in the same time frame.
Poland is modelled to fall from 38 million to 15 million, while Portugal is said to face a drop from 11 million to 4.6 million.
Andorra, Bosnia, Bulgaria, Croatia, Greece, Romania, Serbia, Slovakia, Slovenia, Latvia, Lithuania, Moldova and Ukraine are the other European nations modelled to face a drop of at least roughly half.
The UK, on the other hand, is due to grow from 67 million to 71 million, with France growing slightly.
The researchers based their forecasts on a falling overall fertility rate – the average number of children a woman gives birth to – and suggest the global population could peak at about 9.7 billion in 2064 before falling to 8.8 billion by 2100.
Other countries outside of Europe that could see a steep decline in population include Japan and Thailand, the study says.
Sub-Saharan Africa could see its population climb from about 1.03 billion in 2017 to 3.07 billion by the end of the century because of the amount of women entering reproductive age there and a decline in the death rate.
A comment appearing with the report says policy solutions could include finding ways to increase fertility rates and that “migration will become a necessity for all nations and not an option”.
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