The world is not prepared for the global crash of birthrate, which is set to have a "jaw-dropping" impact on countries, say the researchers at the Institute for Health Metrics and Evaluation at the University of Washington.
The falling fertility rates could mean that almost all the countries worldwide could have a shrinking population by the end of the century. According to the study, countries such as Spain and Japan and 23 others are expected to see their populations halve by the end of 2100.
Many countries will age dramatically with as many people turning 80 as there are being born.
What is happening?
The fertility rate is the number of live births every 1,000 women of reproductive age per year. It is gradually decreasing, and if it falls below approximately 2.1, the size of the population could start to fall.
Seventy years ago, women were having an average of 4.7 children in their reproductive life. The researchers of the study showed that the global fertility rate nearly halved in 2017. Their study, published in the Lancet, estimated that it could fall below 1.7 by 2100.
They expect that the number of people on the planet to rise at 9.7 billion around 2064 before it decreases to 8.8 billion by the end of the century.
Researcher professor Christopher Murray told BBC, "that's a pretty big thing; most of the world is transitioning into natural population decline."
"I think it's incredibly hard to think this through and recognise how big a thing this is; it's extraordinary, we'll have to reorganise societies," he added.
The falling fertility has nothing to do with sperm count or the usual things that come into mind when the topic is brought up. Instead, more women are in education and work, as well as greater access to contraceptives, which leads women to choose to have fewer children.
That means falling fertility rates are considered to be a success story.
Which countries will be most affected?
One of the countries that are most likely affected by this is Japan. The country is projected to fall from 128 million in 2017 to less than 53 million by 2100.
Another country is Italy, which is expected to see a dramatic crash on its population from 61 million to 28 million by the end of the century.
Furthermore, 23 other countries are also expected to follow the trend, including Spain, Portugal, Thailand, and South Korea. These countries are also likely to see their population to decrease more than half.
Indeed, the decrease in populations is jaw-dropping, says Professor Christopher Murray.
China, who is currently leading in terms of population, will peak in four years before nearly halving to 732 million by the end of the century, and then India will take its place.
Why is it a problem, and are there any solutions to it?
At face value, a smaller population may be better. However, with more old people than young people, this could be a problem.
The study projects that the number of children aged below five will fall from 681 million I 2017 to 401 million by 2100. Moreover, they also predicted that the number of people aged 80 years old would increase from 141 million in 2017 to 866 million in 2100.
"It will create enormous social change. It makes me worried because I have an eight-year-old daughter and I wonder what the world will be like," says Professor Murray.
Questions such as who will pay the tax and healthcare system in a massively old world, and most importantly, who will care for the elderly, or will people ever retire from work?
According to some experts, if these predictions are accurate, migration will become necessary for all nations rather than an option.
