But exponential human population growth is currently unsustainable, and has been for the last 70 years. Global warming is proof that the are using ecosystem services faster than they can be replenished, which is having cascading effects across trophic levels. We're in the middle of a mass extinction. In order to get down to levels of human population that we should have, the nonviolent solution is to drop reproduction below replacement rate for a few generations. This is good.

Why on earth do people equate negative population growth with extinction? Populations of most species rise and fall all the time with availability of resources and amount of predation. This is Ecology 101 stuff. I feel like most of the people who argue this have some sort of deep-seated fear that the world with run out of white people and God forbid non-Caucasians take over the world. But societies reproduce memetically, not genetically, so it doesn't really matter about local reproduction levels -- as long as you have immigration, a society survives.

Agreed that energy removes a lot of barriers, but not all of the limits. Earth is still a constant size, and having an ecosystem is helpful to us. But with enough energy, other planets start looking attractive.

You’re the one making this about race, not me. It is exactly because societies reproduce memetically that immigration isn’t a sustainable solution—if the rest of the world either globalizes or immigrates enough, they too will also reproduce below replacement levels.

It’s also abundantly clear that human population decline has absolutely nothing to do with availability of resources, because human populations with more resources are paradoxically less fertile.

Speaking to global warming in particular—once a single technologically advanced country figures out how to cheaply extract CO2 from the atmosphere, either through a more efficient means of doing that or through a breakthrough in energy production, that country can unilaterally solve the problem forever, even with continued use of hydrocarbons (since they can operate a closed loop of reextracting used hydrocarbons from the atmosphere). The fact that we aren’t there yet means that population growth outstripped technological growth for a long while, but in the long term, it’s a solvable bottleneck, just as the low agricultural output of much of the world was a bottleneck solved by Norman Borlaug and the Green Revolution.

Yes, I'm the one who brought up race first. Got me there.

I feel like you're borrowing trouble, though. Populations with more resources are less fertile, for now. But unless and until global population falls below two billion humans, it's not really our problem to solve. Any number of things could change socially by that time. In the meantime though, dropping below replacement rate increases our odds of survival as a species, as we can avoid the potential Malthusian catastrophe.

Our current falling birthrate has to be socially constructed. Having children used to an economic advantage, but this is no longer the case. Anything that changes that equation can reverse the trend, be it a social safety net, more technology, cheaper education, or more housing availability.

The entire point of futurism is to borrow trouble from the future. Sometimes the trouble of the present is just too boring ;)

My basic point is that, insofar as the more liberalized and secular cultures of the present developed world can't sustainably maintain their current populations, there's a limited amount of runway for these cultures to persist. Eventually, we will either run out of immigrants and turn the entire world into one with a fertility rate below replacement, or some other, more fertile culture will out-reproduce or convert us faster than we can convert them, and that will be the culture of the future.

Perhaps it's possible that improvements in economic circumstances will allow fertility to rebound in secular cultures, but they've had the opposite effect so far. It's also worth pointing out that, by necessity, we're going to need some lead time to get the fertility rate back up. A fertility rate of 1.4 means that each generation is 70% the size of their parents' generation, and half the size of their grandparents' generation, and we're already having a hard time affording pensions as it is.

China, in particular, may have a massive self-inflicted demographic crisis on their hands due to the one-child policy. If Chinese parents during the one-child era had a strong preference for girls, maybe it would have worked out, but as it happened, they had a strong preference for boys. They might be able to handle the problem of caring for larger elder generations with smaller younger generations by offsetting the population decline with strong per capita economic growth, but the demographic issues involved with having large numbers of excess men will persist.

I'm also curious about the math that leads you to the two billion number. A Kardashev I civilization on Earth could sustainably support much more than that, and a civilization that advanced beyond Kardashev I, even more. By the time we run into the long term sustainability issues of our current population, or even of a population of 9-12 billion, it's entirely likely that we will be at such a level of technology that we can solve those issues anyway.

A technologically augmented (neo)liberal society that owns most of the wealth, including most of the neo-feudal labour and resources, would only need to be the size of a small tribe - less than a thousand, more than a hundred people - to be stable and self-sustaining.

If there's an "obviously" here, it's that this is the direction in which current trends are heading.

The peasant class can reproduce all it wants. Managed Malthusian resource limitations will keep it from being a political or - eventually - even an ecological threat.