The world's population will begin to decline in the coming decades, for the first time since the Black Death epidemic in the 14th century, due to a sharp decline in the birth rate, according to a study published in The Lancet, The Telegraph.

To maintain its population size, a country requires a fertility rate of 2.1 children per woman. By 2050, researchers suggest that number will have dropped to 1.83, and to a further 1.59 by 2100.

According to The Lancet, Chad, Nigeria, and Somalia were the leaders in global fertility rates in 2021, with rates of 6.99, 6.97 and 6.54, respectively. In Russia, the UK and the US, the birth rate in 2021 was 1.48, 1.49 and 1.64, respectively.

By 2050, Russia's birth rate is projected to drop to 1.33, and by 2100—to as low as 1.21.

Professor Melinda Mills, director at the University of Oxford’s demographic science unit, told The Telegraph that shrinking and ageing populations demand preparedness and reorganization of societies.

“From impacted food security and migration patterns to the very infrastructures of countries,” she said. “Population composition affects infrastructure such as schools, housing, transport, housing and health care and pensions but also cultural and voting changes.”

Dr. Natalia Bhattacharjee, co-author of the study and lead research scientist at the Institute for Health Metrics and Evaluation (IHME) at the University of Washington, US, said the declining fertility rates “will completely reconfigure the global economy and the international balance of power and will necessitate reorganizing societies.”

“Global recognition of the challenges around migration and global aid networks are going to be all the more critical when there is fierce competition for migrants to sustain economic growth and as sub-Saharan Africa’s baby boom continues apace.”