Do you really think the the large part of human population around the world living in extreme poverty (multiple billion people) are the problem? As one poster said, if they do transition to "middle class" (whatever that means to them) then they will become part of the climate problem. But right now, the problem lies with us, our hyper-capitalist and super corrupt western societies. Don't distract from that with whataboutism to Africa. Currently, we (meaning western countries, not just geographically, I would count China for example as a western country as well, since to me "western" is a financial/economic definition, not a geographical one) are the problem, not the people living in mud huts, drinking water that Nestlé seized and now sells to them.

I am thinking out loud here but I am imagining a simple tax. If someone wants to have a child they pay a massive offset fee. Put another way people have to give up spending on other things or create more value for society (earn more income) to have that right. Perhaps this cutoff can be decided on a per country basis to get every country on board. It’s either that or we rely on a very complex series of technological and policy changes that are potentially not going to work out and will cause everyone to live more limited lives of restraint rather than live freely.

> Already, most people live in countries with less than replacement fertility (i.e. with a total fertility rate (TFR) below 2.1 births per woman).

This is a textbook example of what unsustainable means. In the long run most people will live in the countries with the highest fertility rates.

> Even if the people advocating for that aren't motivated by racism, colonialism, or eugenicism, it might be difficult to convince people of their good intentions.

It's enough to let them starve.

In the long run yes, however, (a) realistic population controls would likely mean a stable population, not a much smaller one, so that doesn't help much for deceasing consumption; and (b) even with totally unrealistic population controls (e.g. zero births worldwide) the population decrease would be slower than the time scale in which we need to reduce emissions - zero births would mean a population decrease of less than 1%/year (currently 60m deaths/year over 7.8 billion, the world skews young-ish so the death rates don't match what you'd expect from life expenctancy) but we need an emissions decrease of 1.5%-2%/year to keep the global warming in reasonable limits.

A smaller population would consume less goods and power, but since we need much less carbon emissions, achieving that through a population decrease alone would require a much smaller population i.e. so much extra death that IMHO that would be more horrible than the consequences of any climate change.