The fertility rate - the average number of children a woman gives birth to - is falling. 183 out of 195 countries have a fertility rate below the replacement level. As a result, the researchers expect the number of people on the planet to peak at 9.7 billion around 2064, before falling down to 8.8 billion by the end of the century.
Falling fertility rates mean nearly every country could have shrinking populations by the end of the century. And 23 nations - including Spain and Japan - are expected to see their populations halve by 2100. Countries will also age dramatically, with as many people turning 80 as there are being born.
the global fertility rate nearly halved to 2.4 in 2017 - and the study,published in the Lancet, projects it will fall below 1.7 by 2100.
If the number falls below approximately 2.1, then the size of the population starts to fall. Even with the best healthcare, not all children survive to adulthood. Also, babies are ever so slightly more likely to be male. It means the replacement figure is 2.1 in developed countries. Nations with higher childhood mortality also need a higher fertility rate. In 1950, women were having an average of 4.7 children in their lifetime. Falling fertility rates are a success story, being driven by more women in education and work, as well as greater access to contraception, leading to women choosing to have fewer children.
The study projects:
The number of under-fives will fall from 681 million in 2017 to 401 million in 2100.
The number of over 80-year-olds will soar from 141 million in 2017 to 866 million in 2100.
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. Spain, Portugal, Thailand and South Korea also expected to see their population fall by more than halve. China, currently the most populous nation in the world, is expected to peak at 1.4 billion in four years time before nearly halving to 732 million by 2100. The UK is predicted to peak at 75 million in 2063, and fall to 71 million by 2100. Countries, including the UK, have used migration to boost their population and compensate for falling fertility rates.
A smaller population would reduce carbon emissions as well as deforestation for farmland.
"That would be true except for the inverted age structure (more old people than young people) and all the uniformly negative consequences of an inverted age structure," says researcher Prof Christopher Murray. "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? We will go from the period where it's a choice to open borders, or not, to frank competition for migrants, as there won't be enough," argues Prof Murray.
Prof Stein Emil Vollset said: "Responding to population decline is likely to become an overriding policy concern in many nations, but must not compromise efforts to enhance women's reproductive health or progress on women's rights."
Prof Ibrahim Abubakar, University College London (UCL), said: "If these predictions are even half accurate, migration will become a necessity for all nations and not an option. To be successful we need a fundamental rethink of global politics. The distribution of working-age populations will be crucial to whether humanity prospers or withers."
However, the population of sub-Saharan Africa is expected to treble in size to more than three billion people by 2100. Nigeria will become the world's second biggest country, with a population of 791 million.
Prof Murray says: "We will have many more people of African descent in many more countries as we go through this. Global recognition of the challenges around racism are going to be all the more critical if there are large numbers of people of African descent in many countries."