system
Given the following post, respond as if you're commenting on Reddit:
human
Is there an estimated maximum possible population of the Earth? If so, what is the limiting factor?

It seems to me like there could always be enough room for more people by building up, etc.  Would there not be enough food or water to support the growing population, or is it something else?
gpt
No. And there fundamentally cannot be.

Humans are unusual creatures because our behavior, our "ecological niche", and our use of resources are not determined purely genetically as it tends to be for other animals. We are fundamentally a technological species. That doesn't just mean we use tools, as some people have come to take the meaning, it means that we are capable of adapting, learning, and teaching. Which means that we are capable of passing on how to build and use a specific tool, of course, but it's much more than that. It also means we can pass on non-corporeal tools, tools of the mind and of behavior, tools such as language, culture, ethics, art, etc.

What this means for the human species is that the coupling between "natural environmental limits" and the limits of population is effectively broken. If you take, say, a group of deer you can draw strong connections between their environment and the degree to which it can support a population of a given size, if you get too many deer there just won't be enough food, etc.

Humans, however, do not have fixed behavior. More so, we are capable not just of consuming resources but of producing, or causing the production of, resources. Humans have the ability to change their diet, change how they live, where they live, etc. And we produce food, produce energy, produce raw materials, etc. For a given level of technological / industrial capability and a given set of human behaviors you could make a claim about the carrying capacity of the Earth, however none of those things is constant.

Take food, for example. The amount and type of meat that people eat greatly impacts the amount of resources it takes to supply food for that person. Also, developments such as more efficient means of farming, new strains of crops, and artificial fertilizer have enabled massive increases in the efficiency and effectiveness of food production. If there were some particular major limiting factor on some aspect of human living then there would be pressure, due to economics etc, to change behaviors and avoid that limit. For example, if we could no longer supply much meat to people then folks would switch to more vegetarian diets. If we ran into a limit in production of copper then people would turn to other materials like aluminum or iron or plastic for many of the uses of copper. And if we ran into a problem with producing fresh drinking water people would start changing their water usage patterns, we'd stop washing our cars, watering our lawns, and flushing our toilets with drinking water, for example. And so forth.

And that's the fundamental problem of estimating the human carrying capacity of the Earth. You're dealing with two major moving targets (behavior/lifestyle and industrial/agricultural capability) each of which breaks down into innumerable smaller areas that are all of enormous complexity. Any attempt at a holistic analysis is going to be a gross oversimplification one way or another.