For the first time in modern history, the global population is projected to decline within the next century, bringing with it a “revolution in the story of our human civilization” and profound shifts in the way people live their lives. 

The world’s population currently stands at around 7.8 billion people. That number is forecasted to grow over the next few decades and peak in 2064 at around 9.7 billion people, before falling to 8.8 billion by 2100, according to a new study published in The Lancet. 

“The last time that global population declined was in the mid 14th century, due to the Black Plague. If our forecast is correct, it will be the first time population decline is driven by fertility decline, as opposed to events such as a pandemic or famine,” Stein Emil Vollset, lead study author and Professor of Global Health at the Institute for Health Metrics and Evaluation (IHME), told IFLScience.

Up to 23 countries could see their populations shrink by more than 50 percent, including Japan, Thailand, Italy, Spain, Portugal, South Korea, and other countries marked by a low birth rate and aging populations. Even China, a country often associated with unfettered population growth, is set to fall from 1.4 billion people in 2017 to 732 million in 2100.

While the global trend is one of declining world population, some parts of the world are forecasted to see a rise in population numbers. This includes North Africa, the Middle East, and – most prominently – Sub-Saharan Africa, which is set to triple over the course of the century from 1.03 billion in 2017 to 3.07 billion in 2100. 

The new study by researchers from the IHME at the University of Washington’s School of Medicine looked at how mortality, fertility, and migration will affect the global population over the coming 80 years using data from the Global Burden of Disease Study 2017. It also accounts for how war, natural disasters, and climate change might affect the number of deaths in different parts of the planet. 

The reasons behind the global population decline are complex and fiddly, although it's underpinned by a general trend towards lower birth rates, driven by female empowerment and access to contraception. 

“There are two key factors: improvements in access to modern contraception and the education of girls and women,” explained Vollset. “These factors drive the fertility rate – the average number of children a woman delivers over her lifetime which is the largest determinant of population. . The global total fertility rate is predicted to steadily decline, from 2.37 in 2017 to 1.66 in 2100, well below the minimum rate (2.1 live births per woman) considered necessary to maintain population numbers.“

Along with these changing tides, we will see many radical shifts in geopolitical power and the way billions of people live their lives across the world. One of the main changes will be brought by certain countries experiencing a dramatic decline in the numbers of working-age adults, which in turn could put a strain on their economy and sway the geopolitical balance of power in the world.  

Who will rule the roost in this world of multiple superpowers remains to be seen, but China was forecasted to overtake the US and become the largest economy by 2035 based on the largest total gross domestic product (GDP). However, if their estimations are correct, the US will reclaim the top spot by 2098. 

"The 21st century will see a revolution in the story of our human civilization," Dr Richard Horton, editor-in-chief of the Lancet, said in a statement.

"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," he added.

"This will truly be a new world, one we should be preparing for today."

The new research also argues that the world will have to change the way it sees migration. Although the past decades have seen a resurgence of nationalist rulers and a growing hostility towards migrants, the report suggests that many countries will have to opt for more liberal immigration policies simply to maintain their population size and support economic growth.

“Ultimately, if [the new] predictions are even half accurate, migration will become a necessity for all nations and not an option,” Professor Ibrahim Abubakar, University College London (UCL), who was not involved in the study, wrote in an accompanying comment article.

“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,” they added. 

“The distribution of working-age populations will be crucial to whether humanity prospers or withers.”

Updated 15/07/2020: This article has been updated with comments from the study author.