The vast majority of the world's populations will shrink by 2100, suggests a new study published in the Lancet journal on Tuesday.
This goes against what the U.N. predicts, which foresees around 2 billion more people roaming around Earth than the new study projects.
According to the new research, Earth will have 8.8 billion inhabitants by 2100.
A new world order
"The 21st century will see a revolution in the story of our human civilization," said Dr. Richard Horton, editor in chief of The Lancet. "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 U.S. as the dominant powers. This will truly be a new world, one we should be preparing for today."
According to the study, more than 20 countries will see their populations cut in half — these include Japan, Spain, Italy, Thailand, South Korea, Portugal, and Poland.
China's population is set to drop by almost that much, from today's 1.4 billion down to 730 million by 2100.
Sub-Saharan Africa will jump up, however. Nigeria alone will go up to 800 million.
Lead author of the study, Christopher Murphy, noted that "most countries outside of Africa will see shrinking workforces and inverting population pyramids, which will have profound negative consequences for the economy."
The study urges high-income countries to focus on flexible immigration laws and social support for families if they want to sustain their population levels and economic growth.
There will be sharp declines in working-age populations across the world by then, with over a quarter of the population over 65 years old by 2100. And those over 80 will increase from 150 million today to 866 million then.
"Societies will struggle to grow with fewer workers and taxpayers," noted Stein Emil Vollset, one of the study's authors.
Economically-speaking, shifts will happen too. India's GDP will rise to sit in third place, after the U.S. and China, followed by Nigeria. Japan, Germany, France, and the U.K. will remain within the top 10 largest economies worldwide.
Brazil will drop from eighth to 13th, Russia from 10th to 14th, and Italy and Spain will drop from being in the top 15 down to 25th and 28th respectively.
"By the end of the century, the world will be multipolar, with India, Nigeria, China, and the United States the dominant powers," said Richard Horton, who described the study as outlining "radical shifts in geopolitical power."