[AlotOfReading]

It's worth noting that world3 model is fairly sensitive to initial conditions and small perturbations can yield population predictions exceeding 20 billion or under 1 billion [0]. It's pretty unclear whether the patterns are actually useful or actually "natural" rather than artificial. For instance, virtually all scenarios in world3 end up in a peak->collapse, but it strains credulity to imagine that similarly applies in the real world.

[0] https://doi.org/10.1287/mnsc.44.6.820

[actually_a_dog]

> For instance, virtually all scenarios in world3 end up in a peak->collapse, but it strains credulity to imagine that similarly applies in the real world.

Why? Many civilizations have come and gone through the collapse cycle already. Why do you suspect we're any different? What makes you think the ecosystem can even tolerate 9-10 billion of us?

[AlotOfReading]

If you pick any particular definition of "collapse", very few will actually meet it (let alone on a timescale of mere years or decades). There's quite a lot of literature on this subject already (e.g. Questioning Collapse), but there's also tens of thousands of years of human history apparently lacking records of anything that could be called a collapse.

As for a specific carrying capacity for the Earth, it should be obvious how impossible such a number is to give without a lot more detail in the question. But if we were to assume 10B global population, it could be done with a population density roughly equivalent to precolumbian California. This is not to suggest that indigenous californians lived in perfect natural harmony, but rather to illustrate how low the numbers actually are. I suspect there's probably many reasonable (though utterly alien) ways of life where that density could be "sustainable". Equally, I suspect there are many ways of life where those numbers are not "sustainable".

[jacquesm]

The eocsystem we were born into as humanity would not sustain even a small fraction of the humans alive today. We modify the ecosystem until it will sustain us (or 'just us', for that matter).

[actually_a_dog]

What makes you think we can continue to do that? Even if we can, what makes you think we can do it sustainably? And if we can do it sustainably, what makes you think we will? History doesn't really provide any encouragement here.

[jacquesm]

I don't think we will be able to do that, sustainably or otherwise. We're locusts, pure and simple.

But we got away with it long enough to a large number of people now believe that this is normal. It isn't. The wake up call will be a very harsh one.

[esarbe]

> this is normal. It isn't.

Agreed. Nothing about this is normal. Not the way we live, not the way we work, not the way we take a hot shower every morning. It's highly un-normal. But because we're relative creatures and define the 'normal' by what we experience and not by what has been the normal for hundreds of millennia, we tend to misunderstand the reality of our situation.

The wake up call will be a very harsh one.

[actually_a_dog]

Just to be clear, if by "normal," you're referring to the way humans existed for 95% of their existence, which was as small bands of hunter gatherers, that very clearly implies an advanced state of collapse. Even if some of the worst case scenarios for collapse come true, that's a long ways off. For one thing, it would probably imply a significantly reduced population (probably no more than 10-20% of current population levels) before nomadic hunter gatherer groups becomes an efficient way to live.