Human birth rates will continue to drop drastically over the coming century, and within just 25 years, over two-thirds of countries' populations will be in decline.

That's the finding of a new study published in The Lancet. The extensive team of international scientists behind the paper warns that governments must prepare for the massive changes we will face in the coming decades, as a result of these changes to global population patterns.

"These future trends in fertility rates and live births will completely reconfigure the global economy and the international balance of power and will necessitate reorganizing societies," says Natalia Bhattacharjee, a population statistician with the Institute for Health Metrics and Evaluation (IHME) in the US.

Back in 2018, previous studies found fertility rates were falling in half the world's populations, and the plummet continues.

By 2050, the new study forecasts, people living in 155 out of the 204 countries and territories included in the study will be having fewer babies than it would take to maintain a stable population.

The forecasts show that by 2100, that will increase to 198 countries and territories with birth rates lower than their death rates. In Bhutan, Bangladesh, Nepal and Saudi Arabia, rates will fall below one child per female. By that time, the only countries expected to have birth rates that exceed the level needed for population maintenance (at least two births per female) are Samoa, Somalia, Tonga, Niger, Chad and Tajikistan.

And if all that's the case, the authors write, then without strategic migration policies, the human population in places with low birth rates will inevitably shrink.

But is that so bad? In many countries, reduced birth rates are the result of better conditions for women and families in general.

"In many ways, tumbling fertility rates are a success story, reflecting not only better, easily available contraception but also many women choosing to delay or have fewer children, as well as more opportunities for education and employment," says senior author and biostatistician Stein Emil Vollset, also a member of the Future Health Scenarios forecasting team.

In the 1950s, there were just under five children born to each female, compared to just over two children to each female in 2021.

But the authors point out that the impact of such a drastic shift in the human population will also depend on how it is managed, and where people are located. At the moment, they say, we as a global community are not prepared for what's coming.

The study finds that the majority of children – three in four – will be born into low- and lower-middle income countries, over the coming decades.

In 2050, Chad and Niger are predicted to have the highest fertility rates worldwide, and by 2100, Samoa and Tonga are predicted to have the highest fertility rates.

By 2100, one in every two children born will come from Africa, which already contributed one third of the world's babies in 2021.

"The world will be simultaneously tackling a 'baby boom' in some countries and a 'baby bust' in others," Vollset says.

"As most of the world contends with the serious challenges to the economic growth of a shrinking workforce and how to care for and pay for aging populations, many of the most resource-limited countries in sub-Saharan Africa will be grappling with how to support the youngest, fastest-growing population on the planet in some of the most politically and economically unstable, heat-stressed, and health system-strained places on earth."

The authors say "ethical and effective immigration policies with global co-operation" will be crucial in managing the population crashes many countries are due to face.

"Once nearly every country's population is shrinking, reliance on open immigration will become necessary to sustain economic growth," Bhattacharjee says. "Sub-Saharan African countries have a vital resource that ageing societies are losinga youthful population."

But the authors point out it will be important to ensure migration is not one-directional.

"Continued skilled worker migration to high-income, low-fertility economies – a concept referred to as brain drain – can also have devastating effects on the economies these workers leave behind," they write.

While there are some important limitations to the study – like the fact some of the more recent data is from weird COVID-19 times – the forecasts are based on key drivers of fertility – like education levels, contraception availability, child mortality, and living in urban areas – along with data going back to the 1950s, and had relatively high levels of certainty for all countries.

The research has come out of the Global Burden of Disease, Injuries, and Risk Factors Study 2021, and was funded by the Bill & Melinda Gates Foundation.

This study is published in The Lancet.