Continued from #11 Synopsis of the world population projections
Because fertility rates are falling everywhere, as the decades continue, more and more countries on that map will get coloured in.
And gradually, the world will get older. We can't shy away from old age
Humans like to focus on the start of life and not the end. We're not good at talking about ageing and death.
But the change in the world's demographics will transform our society, and we can't hide from it.
"If you want to think about the future demographic challenges, I think population ageing is probably number one in terms of people needing to change the way they do things, their expectations, governments needing to change public systems that support the older population," Mr Willmoth says.
Some of the consequences are relatively obvious, such as greater demand for health and aged care services.
The IHME's Amanda Smith says as part of that we should expect "a shift toward a larger burden of disease coming from non-communicable diseases," and that in many countries the COVID-19 pandemic has exposed weaknesses that will need to be addressed.
Dr Charles-Edwards says our tax base will have to support more and more people.
Countries with government-funded aged pension schemes will see welfare costs go up, and then there's the issue of having enough workers.
"We're going to have more and more countries where there are more older people than young children," she says.
"This is going to really shape the way we make decisions about how we govern, how we spend our money. It's a massive cultural change."
Families are getting smaller, and downsizing houses for the elderly is already on the agenda in many ageing countries, Australia included.
But what about our transport systems, which are great at getting people to and from offices in capital cities but not so good at getting us to shops, parks, doctors’ clinics and across suburbs?
What happens when fewer of us go to work every day, and more of us need help to get around?
It's not just the makeup of our societies that will be reshaped, but the texture of our cities and the way we move around them.
"We do need to be designing cities, for example, in somewhere like Australia, with a view that there are going to be many more older Australians here, so thinking about walkability and accessibility is all really, really important," Dr Charles-Edwards says.
It will change workplaces, too.
With fewer workers available and more caring jobs required to look after the ageing cohort, companies may have to look to automation, artificial intelligence and robotics to help fill the gaps.
Entrepreneur and artificial intelligence expert Vaibhav Namburi says workplaces may look very different with the increased use of automation.
"I think in the next 10, 20 or 30 years we're going to see a lot of 'mundane' or repetitive jobs be phased out, and those people [will] be retrained to be focused on more upper-level skills," he says.
So maybe we'll all be working smarter, but will we also be working harder? After all, during the pandemic, remote work has led to many of us working longer hours.
"[That] was actually counter to what the entire remote flexible work lifestyle was meant to bring," Mr Namburi says.
"I've fingers crossed that the next hundred years is more focused towards the mental health aspect of our work-life balance."
The countries growing fast
Some parts of the world are facing a different problem.
In the map we saw earlier, you might have noticed there were no nations in Africa on the population decline.
In fact, Africa is one of the fastest-growing places on Earth right now.
Just eight countries are projected to be responsible for more than half the world's population increase by 2050.
One of them is India, which is set to overtake China as the most populous country in the world next year.
Pakistan and the Philippines are also on the list, and the remaining five are all in Africa: Nigeria, Tanzania, Ethiopia, the Democratic Republic of the Congo and Egypt.
[These eight countries will contribute more than half the world's population increase to 2050]
Sub-Saharan Africa, in particular, is growing fast.
It is projected to contain about a third of the world’s population at the end of the century, although there is a lot of uncertainty about projections that far out.
[Africa is projected to have 38 per cent of the world's population in 2100]
That's a lot of extra people in a region of the world with some of the highest levels of extreme poverty.
Ensuring food production keeps up with the twin pressures of a growing population and the effects of climate change is the focus of a lot of development attention.
World Vision's CEO Daniel Wordsworth says it's a significant challenge in places such as Somalia, which is currently facing its worst drought in decades.
"I think about it a little bit like a whole bunch of fishing lines that are all tangled up," he says.
"There's no root-cause driver, but there are all of these interconnecting forces that are reinforcing one another and creating these gnarly knots that are making life incredibly difficult.
"You have climate change in a place like Somalia … in a place that's had five years of drought.
"You also have a country that's gone through, like all of us, two years of economic lockdown and shutdowns … and this country is in conflict."
Dr Rachel Carey, a sustainable food systems expert from the University of Melbourne, says growing enough food isn't really the biggest problem.
"To date the world has managed to keep pace with food production, the world has produced enough food," she says.
"Certainly we see local shortages of food at different times, and that could be due to issues around climate change, also conflict and war, but there has been enough food produced around the world.
"The key issue has been about how that food is actually distributed and the inequality in the way that is done currently."
The world is also being shaped by rapid urbanisation, which accelerated in the 20th century.
More people have lived in cities than rural areas since 2017.
The share of the population in our cities will continue to grow from the 55 per cent it currently is.
[More people have lived in cities than rural areas since 2007]
Dr Carey says that's likely to impact how much prime agricultural land is available, and could put pressure on food production in future.
"Cities are often built in fertile plains and close to rivers, close to good sources of food," she says.
"As cities grow, many are growing out as well as growing up, and often growing into highly fertile areas of farmland."
Researchers are working on technology that could help, including at the University of Sydney which is working on robotic farming technology.
There's a lot of work being done, but ultimately, Dr Rachel Carey says we're not yet doing enough.
"We need to do much more, and I think we need to do it much faster," she says. Can we meet the challenges?
So what kind of life will 9 billion people on Earth live?
Fewer babies will lead to fewer workers, and will that lead to a smaller economy? How will we solve the challenge of providing a comfortable retirement for all? Do we need to rethink how we design cities for more and more older people who aren't commuting to offices? Will the robots and artificial intelligence of the fourth industrial revolution finally deliver the promise of less work and more leisure?
You might think this sounds like the start of a sci-fi dystopia, but the experts say it's not all doom and gloom.
"As a demographer, we're really optimistic people because we've seen massive change over the past 100 years," Dr Charles-Edwards says.
"Everyone's living longer, fewer babies are dying, fewer women are dying.
"Across a whole range of metrics, we're doing better than we did. We've seen lots of small actions creating massive change for people."
But, like most global problems, the impacts won't be shared equally and richer countries face the linked challenge of what they're prepared to sacrifice to help others.
"We know that some of the most vulnerable communities are going to be the most impacted," Dr Elin Charles-Edwards says.
"If you're rich, you are able to adapt in a way that you just can't do if there's not the resources there.
"Climate change is the big elephant in the room here. We know that some of the places that are going to face the most impact are the least resilient, and so that is something that we need to be mindful of as well."
The challenge is set. It's up to humanity to work out what to do next.
Join Casey Briggs at 8pm on Tuesday for an ABC News Channel special on world population.