Why is there a human population roof?


Why is there a human population roof?

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I’m not sure what you mean by a “roof”. There are various constraints on population. For instance, on average humans need about 2000 calories of food per day. So a population will need to grow or import at least that much to maintain itself.

There is a maximum number of humans that one could have on the planet, because well, the planet is only so big. Each person takes up space. Space to live, grow food, have fun, what have you. Even if we could pack humans into Amazon boxes and didn’t have to feed them, they’d still take up an amount of space. So the cupboard that is our world can only hold so many.

That being said, the earth can hold A LOT of people (even assuming we aren’t shoving them into Amazon boxes). Earth is far bigger than we tend to comprehend in day to day life. It’s so so big. What is A LOT? Arguably 100s of billions. Arguably 10s of billions. People argue on this point and it changes depending on what you think a human “needs” and how much space you believe to be “available”.

The UNs expected population peak at 10 or 11 billion ish. Much lower than the vast majority of theoretical maximums. Why? People are having less children. All over the world we are making less and less new people each year. In many places, we make people so infrequently that our populations are already dropping. Many places import people from else where, just to have enough young people to do all the work that needs doing.

By the time we’ve reached ~10 billion-ish. The lack of new people being born is probably gonna be a really big issue.

It’s a combination of:

– how many resources do we have ?

– how do we use the resources (including efficiency, recycling, etc) ?

– what standard of living do we have ?

– how many of us are there ?

So, you can pick various combinations of those things.

If you wanted to figure out an absolute maximum possible number of humans on Earth, pick most efficient use of resources and lowest standard of living. So maybe pack everyone into tiny living units and feed us IV nutrients derived from growing algae or something. Probably could support many trillions of humans that way.

Think of humans-on-Earth like goldfish in a fishbowl.


Each adult goldfish takes up a certain amount of space and the fishbowl is only so big… so clearly it’d be physically impossible to squeeze 1-billion goldfish into a 2-gallon fishbowl.

You could physically fit maybe 100 or something in there, so you could say that there is a “population cap” of 100-fish imposed by physical size constraints… but even then they’d still be too crammed-in to swim or even breathe so it’s not a very useful limit.

So maybe you could fit 10 in there and they’d still be able to swim and breathe, so you could say that there is a “population cap” of 10-fish imposed by the amount of room they’d require to swim and breathe… but even then there might be so much fishpoo that many get sick and die, so that might bot be a useful limit.

So maybe you can only really fit 2 or so in there and have them survive healthily for long periods of time.


Earth is a really *really* big fishbowl.

Physically, having 196.9-million mi² in surface area means we could fit about 5.5-quadrillion people shoulder-to-shoulder if each person got 1-ft² worth of room to stand… even more if we stack layers and layers of people on top of each other. So, the projected “population ceiling” numbers you might see usually fall way way short of this “How many can we physically fit?” question in the same way that we know you can’t fit 100-goldfish in a tiny tank.

Instead, we have to think about the stuff like “air and water and food and poo”, and when it comes to these sorts of considerations the biggest bottleneck to consider is usually food.

Unlike fish-in-a-fishbowl, who get all their food supplied by some giant benevolent with a shaker of fishflakes, we humans-on-a-wet-rock-hurtling-through-space have to source our food from what we are able to farm and gather right here. And because all that food grows by absorbing energy from the sun, that means that the maximum amount of food we could possibly make per year is determined by how much total energy the Earth gets from the sun per year (plus some bonus energy from the “fossilized sunlight” hydrocarbons we can burn until we eventually run out). This means that there is an “energy cap” that limits the total amount of food we could possibly make per year. And that maximum is further lowered if we consider the energy needed to harvest, transport, process, and transport all that food to the people, not to mention inefficiencies and losses at each step of the process. Add onto that the fact that people probably want a variety of foods rather than just MaximallyEfficientGruel (as seen on TheMatrix brought to you by the makers of SoylentGreen!) and that brings the limit down even further.

So, the worldwide human “population ceiling” or “population cap” or “population roof” you might hear about are all estimates like these that keep factoring in more and more of these important details to estimate what kind of real maximum limit might exist.