[spodek]

Because the data keeps matching their Business as Usual model. I think there's more recent research, but here's a paper from 2014, showing 40 years of reasonable fit: http://sustainable.unimelb.edu.au/sites/default/files/docs/M.... If we keep matching their model, we'll soon see dramatic population declines.

Also, the patterns revealed in their models are useful for understanding patterns in nature and human interaction with it.

[geoalchimista]

Just looking at Figure 1 on page 8 of the paper you mentioned, it can go either way because we haven't seen a reversal in any of the trends. That is hardly a prediction. For example, if one looks at the "Services per capita", is there any sign that it will meet an inflection point in the next few years?

I know it is always appealing to tell a story with a grand unifying narrative. But sound research must prop it up with empirical evidence. Could there be limits to growth? Probably. But a highly simplistic model not informed by appropriate data or economic understanding is not the way to tell such limits.

[baq]

> Could there be limits to growth? Probably.

that's what an economist would answer. ask a physicist.

for best results, get an economist and a physicist in the same room and ask them both.

[api]

There have to be limits to growth, but we won’t know where they are except in retrospect. We also may never hit them due to fertility decline which occurs in all wealthy societies. Fertility decline plus efficiency increase could cause consumption of some resources to fall.

Space migration doesn’t change the equation much on Earth unless you start mining and manufacturing off world and importing product, and that is pretty far off.

[teabee89]

"Could there be limits to growth?" Yes 100% (you cannot extract indefinitely more and more every year from finite resources), the debate is simply when we'll peak for a given ressource (20 years, 100 years, 1 million years) and the size of such an impact on the economy.

[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.

[perfect_kiss]

https://dash.harvard.edu/bitstream/handle/1/37364868/BRANDER...

Just in case, here is another empirical data comparison for world3 predictions, this time from 2020.

[actually_a_dog]

We're already starting to see evidence for the beginnings of global population collapse: https://www.weforum.org/agenda/2021/06/birthrates-declining-...

[nl]

Slowing birthrates is very far from a population collapse. It's also (to generalise vastly) desirable - slower population growth is desirable environmentally.

[actually_a_dog]

This is why I said "starting to." You did notice those two very important words, didn't you? Because this is how it starts.

I agree that decreasing population is desirable from an environmental POV, to the extent that population is more or less directly proportional to consumption. But, capitalism as we know it can't survive it. Here's a good explanation: https://www.axios.com/the-new-threat-to-capitalism-73ff54bd-...

[nl]

> This is why I said "starting to." You did notice those two very important words, didn't you? Because this is how it starts.

"starting to" is a prerequisite, but a reduction in birth rate doesn't necessarily lead to a population collapse, and you don't seem to present any evidence that this is one of the cases where it does.

[actually_a_dog]

What evidence would you consider sufficient, besides globally falling birthrates?

Again, I will remind you, the comment says we are "starting to see evidence," of population collapse, not that population collapse is happening or is inevitable. Globally declining birth rates is certainly evidence that it may be happening.

[nl]

Well population dynamics are pretty predictable for the next ~40 years or so.

The inputs are the number of people of childbearing age over that time period and the number of births per adult. We know the maximum number of people (since new people don't get born at ages above zero). The number of births per adult tends to change very very slowly and pretty predictably.

A population collapse would be caused by one or both of these things changing dramatically.

There's no evidence of this. Instead there is evidence of a slow decline in population as the birthrate (especially in Africa) slowly decreases.

> What evidence would you consider sufficient, besides globally falling birthrates?

Something that indicates that the current models showing a slow decline are wrong.