The global population is likely to peak in a few decades' time before declining, new modelling research shows, causing a huge shift in how the world's geopolitical power is distributed that favours Africa and India.
The study, conducted by the Institute for Health Metrics and Evaluation (IHME) and published in scientific journal The Lancet, estimates the world population will peak at around 9.7 billion in 2064 before dropping to about 8.8 billion by the year 2100.
This places the world's population in 2100 at more than 2 billion people fewer than other estimates - including a UN Population Division model that predicts there will be 11 billion people on Earth by then due to continued growth.
The new research estimates that in 2100, the overwhelming majority of countries will have total fertility rates below 2.1, the level needed to ensure replacement of the population, causing population declines in these countries unless they're bolstered by immigration.
But it's the shift in age demographics that are of particular note, as this will have major knock-on effects on the number of people in the workforce, how our health and social support systems will be managed and ultimately on the distribution of the world's power.
By 2100, the IHME's models estimate there are likely to be 2.37 billion people over 65, compared to just 1.7 billion under the age of 20.
Dr Richard Horton, editor-in-chief of The Lancet, says the changes will establish India, Nigeria, China and the US as the world's most dominant powers by 2100.
"The 21st century will see a revolution in the story of our human civilisation," he said.
"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… This will truly be a new world, one we should be preparing for today."
Sub-Saharan Africa will become an increasingly powerful geopolitical force amidst huge population increases, Science Daily reports, with Nigeria not only becoming one of the world's most populated nations but the only one in the top ten to grow its working-age population.
This will set the foundations for massive economic growth, with its GDP ranking to surge from 23rd in 2017 to 9th by the start of the 22nd Century.
Meanwhile India will be the only major Asian nation to retain most of its working-age population. This will see it surpass China's workforce population in the mid-2020s, causing it to rise up the GDP rankings from 7th to 3rd.
While the UK, Germany, and France are expected to remain in the top 10 for largest GDP worldwide at the turn of the century, Italy (from rank 9th in 2017 to 25th in 2100) and Spain (from 13th to 28th) are projected to fall down the rankings, reflecting much greater population decline.
"The societal, economic, and geopolitical power implications of our predictions are substantial," IHME Professor Stein Emil Vollset said.
"In particular, our findings suggest that the decline in the numbers of working-age adults alone will reduce GDP growth rates that could result in major shifts in global economic power by the century's end."
The IHME research shows North Africa and the Middle East are the only regions predicted to have a larger population in 2100 than in 2017. The vast majority of populations will shrink, with Asia and Europe the continents to see the most marked changes.