Note well that we do have a say in birth rates. We can't control them (at least not very morally and not very well), but we can influence them, if we want to.
South Korea, which is a constrained mountainous peninsula, decided in the '60s to influence their birthrate, with improved health, education, and then public campaigns. "Two is enough!" in the '70s and then "Two is too many" in the '80s. Their birthrate fell from 6 to 1.05 by 2005. That's a much faster decrease than China accomplished by decree.
They decided, and then influenced it.
When countries in Africa see a beautiful green future for themselves, they could decide and do the same. The World Bank's work on eliminating extreme poverty is what makes this possible. However that's not sufficient--we need to want a healthy population (level) which supports a healthy planet too.
I call this campaign, "Norm of one"--to create a norm of one child families (which would yield probably an average of 1.4) for a few decades until the climate migrations we see now stop. We might even continue with small families until the mass extinctions we're causing stop--assuming we collectively choose to value nature over raw growth.
What global population would you guess that will be?