From memory I believe one of the ideas in the world model is that over time, as existing stocks of non-renewables are consumed, the remaining stocks of non-renewable resources become more expensive for society to access. E.g. EROI [1] for fossil fuel deposits decreasing. Similarly when mining ore, if the richer ore deposits have all being exhausted, then only lower-grade ore remains, so maybe you need to expend more energy blasting and moving 2x or 3x as much rock as a few decades ago to obtain the same output of metal.
So the idea is that at some point it starts to become more and more expensive -- in terms of the energy input required -- to produce resources that society requires to exist in a steady-state, let alone to continue to fuel growth of the physical economy.
Similarly for pollution -- e.g. for global warming you can regard the atmosphere as a stock where you can store some finite amount of CO_2 pollution before it really starts creating a mess. Once there isn't much stock of cheap CO_2 pollution storage capacity remaining in the atmosphere, you have to start figuring out something else to do, which consumes more energy.
The increasing energy costs of extracting enough non-renewables and dealing with pollution to fuel society leaves less energy for everything else. At some point the energy required for industrial output grows to the extent that there is no longer sufficient energy left for services such as healthcare, education, etc. Therefore the quality of healthcare and education falls off. That has consequences for the human population.
[1] https://en.wikipedia.org/wiki/Energy_return_on_investment
[1] The EROI for fossil fuels has decreased as the availability of oil, coal, gas has reduced and the cost of production increased. Fracking for oil or gas (as has been done in Taranaki for years) is expensive and polluting. Extracting oil from the tar sands in northern Canada is a similarly destructive process. So, what was an EROI of 100:1 last century is now sometimes little more than 10:1.
[1] https://www.newsroom.co.nz/why-hydrogen-is-not-a-cure-for-em...
https://sustainable.unimelb.edu.au/__data/assets/pdf_file/00...
1. The "comprehensive technology" scenario in which population stabilizes, food production increases to keep pace, pollution eventually drops to essentially zero, and industrial output actually decreases as a result of increased efficiency.
2. The "stabilized world" scenario, where population, industrial output, and food output all stabilize quickly, and pollution also eventually drops to 0.
The difference between these 2 scenarios is essentially that in the latter, technology does not progress fast enough to save us, but we are able to put the brakes on and save ourself; while, in the former, increases in efficiency due to technology are what allows us to stave off collapse.
It's where I first heard the term "peak child" or "peak baby" to describe this.
It's curious that population growth peaked in most Axis countries before Allied ones in WW2. It would make sense if each country's values were more aligned with others in their faction.
"The world’s fertility rate, which stood at 3.5 births per woman in the mid-1980s, fell to just 2.4 in 2019. Indeed it is possible, given observed declines in rich-world births during the pandemic, that covid-19 may have pushed the world as a whole within sight of a replacement-level fertility rate, if only temporarily."
https://www.economist.com/finance-and-economics/2021/12/11/w...
Interesting package. Neat work