Global crash in fertility rates set to have a "jaw-dropping" impact on societies
We are looking at a world with far more old people than young people to look after them.
And this is going to happen sooner than you think.
In fact, some countries, including Spain and Japan - are expected to see their populations halve by 2100. And instead, the population will age, with as many people turning 80 as there are being born.
The problem? The world is ill-prepared for the global crash in children being born which is set to have a "jaw-dropping" impact on societies, say researchers.
Researchers at the University of Washington's Institute for Health Metrics and Evaluation showed the global fertility rate nearly halved to 2.4 in 2017 - and their study, published in the Lancet, projects it will fall below 1.7 by 2100.
"That's a pretty big thing; most of the world is transitioning into natural population decline," researcher Prof Christopher Murray told the BBC.
"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."
The reasons fertility rates are falling are many and complex. And contrary to popular belief, it has little to do with low sperm counts.
Instead, the researchers discovered, the trend in fewer babies being born is being driven by more women in education and work, as well as greater access to contraception, leading to women choosing to have fewer children.
According to the latest predictions and research, Japan's population is projected to fall from a peak of 128 million in 2017 to less than 53 million by the end of the century.
Italy is expected to see an equally dramatic population crash from 61 million to 28 million over the same timeframe.
They are two of 23 countries - which also include Spain, Portugal, Thailand and South Korea - expected to see their population more than halved.
Who pays tax in an ageing world?
Researcher Prof Christopher Murray thinks this chance means we will eventually have to reorganise our entire societies.
"It will create enormous social change. Who pays tax in a massively aged world? Who pays for healthcare for the elderly? Who looks after the elderly? Will people still be able to retire from work?"