Why is there a human population roof?
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Think of humans-on-Earth like goldfish in a fishbowl.
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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.
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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.
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