Your second sentence is an overreach. The natural death rate in the United States is about 0.9% of the population. [1]
[1]. https://www.macrotrends.net/countries/USA/united-states/deat...
If the natural death rate is 0.9%, the population would decline by about 9% each decade.
The drop doesn’t have to go to 50% - just down to whatever the sustainable limit is.
> how do you stop all people on earth from having children without a world war?
I’m not sure that even a world war would achieve that. And again, it’s not necessary or desirable to go to that extreme. If the goal is sustainability, we just need to rightsize the population for sustainability. The actual population depends on the prevailing per capita emissions from whatever energy technologies we are using at any given time. The more carbon neutral the technology, the higher the population can be sustained, at least with respect to greenhouse gases.
To reduce the birth rate, rather than force everyone to stop, we could just reward those who are willing with tax breaks. X% tax break for zero children. Y% tax break for 1 child. 0% tax break for 2 children. And extra taxes for >2 children.
Humans tend to be on their own barring social pressures carbon neutral. The biggest expenses being food and water, if these aforementioned practices are applied. It gets even better if these young can work to reduce the impact of the warming.
We'll need a lot of manpower to pull off the necessary changes. Just due to the scale of it. Well educated people too. We're on at least one to two generations of lag in education related to physics, chemistry, agriculture, material science, civil engineering, process engineering, general ecology, social engineering... And the old will start to wear down and think in old patterns.
Additionally further down the list is "live together in higher density" and "buy locally made things" bits.
I'd disagree on the facts though. There's already such a wild difference in carbon intensity between two humans, that it would be ridiculous to suggest that the first step would be to remove humans rather than the activities that make that difference (and are sometimes wasteful, inefficient or counterproductive)