Why does every discussion about overpopulation immediately devolve to food supply? I get that seems like the obvious thing to talk about, but the real world of supply chains and materials is far more complex than "food". It guaranteed that we can industrially produce far above our current population levels and we'll totally screw up the planet. How do I know?
Because of global warming and mass extinctions.
What overpopulation discussions miss and devolve to also is that it isn't important on the number of people, it is the RESOURCE RATE PER PERSON combined with POPULATION that produces the "are we sustainable".
What doesn't scare me is China and India's population. It's China and India's rapid climb to US-level lifestyles and per capita resource consumption rates.
Here's the most important thing we are not conserving/shows we are overextended:
1) natural habitat destruction
2) everything else (water, metals, food, clean air)
I'm not just talking about the Amazon. The amount of space needed for HUMANS isn't important. The amount of space needed by NATURE very very very much is.
We drain all the swamps. We clear all the forest. We farm all the plains. We trawl the oceans.
This is the most stinging indictment of modern economics: what is the economic cost of destroying a natural habitat? It's not zero, in fact every single economic study will likely point out "well we could exploit this forest for human purposes and gain tax and GDP out of it".
That single failure, a TOTAL inability to quantify the economic value of a natural habitat in terms of sustaining the ecosystems, food webs, species proliferation/redundancy, production of oxygen/removal of CO2 isn't just going to kill us as a species, that's just garden variety extinction. It might kill the entire ecosystem and everything but single celled organisms.
Our developmental economics should STRONGLY prefer cities. A high rise apartment building should be far cheaper, so a 3000 sq ft apartment should be 1/3 the cost of a suburban 3000 sq ft house.