The UN says the world's population could be 5 times bigger by 2300

REUTERS/Rupak De Chowdhuri

Your great-great-great-great-great-great-great-great-children had better be OK with roommates.

According to a 2004 report from the United Nations titled simply "World Population to 2300," the international organization predicts the world's population could reach 36.4 billion within three centuries.

That's nearly five times the 2016 population of 7.4 billion — or, you know, a lot of people.

On the low end of the UN's forecast is a population of 2.3 billion people — about 5 billion less than we have now. The vast difference between the two estimates hinges on projected fertility rates around the world.

To understand how populations explode or decrease, those who study population trends rely on a given country's fertility rate. When that country's population stays mostly the same, that's because its fertility rate hovers around what demographers call "replacement fertility," or roughly 2.1 children per women.

"Replacement fertility" refers to how many children a woman needs to have to literally replace the portion of the population who will die.

At present, countries like Japan and South Korea have fertility rates well below replacement levels: Japan at 1.41, South Korea at an even more dire 1.19. Countries like these will lose population if those trends continue long enough. Many African countries, meanwhile, have some of the highest fertility rates in the world — Niger's is 6.89 children per woman — which will lead to massive growth in the coming decades.

You might think it takes a sharp increase or decrease in the fertility rate to produce large-scale demographic change, but in a section of the UN report, Paul Demeny of the Population Council of New York writes that the difference between 2.3 billion and 36.4 billion is actually "the result of remarkably modest differences in the assumed fertility parameters."

In the medium projection — essentially the UN's least exciting forecast — the world more or less settles into replacement fertility. But the lowest and highest population projections were calculated by changing global fertility rates by only 0.25 children.

That's why Demeny calls the differences "remarkably modest." Think about it: A quarter-child shift in fertility rates, depending on the direction, could lead to alternate realities that differ by 34.1 billion people.

Whether we end up with 2.3 billion, 36.4 billion, or some number in between depends on a range of other factors as well: growth in developing regions like Asia and Africa, medicine's ability to extend people's lifespans, and innovations in how people grow and consume food.

And with more people moving into cities, those great-great-great-great-great-great-great-great-children will most likely be forced into living arrangements that we can't even imagine.

Or perhaps we'll finally escape to the moon.

NOW WATCH: The fitting room of the future is totally interactive