The story of the Great Famine in Ireland is a starkly clear example of a "Seneca Collapse", that is of a case where decline is much more rapid than growth. Is something in store for us at the global level? (Image source Rannpháirtí anaithnid (old) at English Wikipedia)
In a previous post, I argued that many current global population projections are mistakenly based on the idea that the "demographic transition" will work backward. That is, it is often assumed that impoverished people will tend to make more children and that, therefore, the world's population will keep growing even in the midst of the profound economic decline that could accompany a resource and climate crisis. (this is, for instance, the assumption of the original "The Limits to Growth study in 1972)
I proposed, instead, that the start of a major economic/climate crisis will cause an immediate reduction of the birth rates in part as the result of the declining health of fertile women and, in part, by a rational response by families who would understand that they can only care for a limited number of children in a condition of increasing poverty.
That is, there won't be a population increase in the midst of a major crisis and the decline of birth rates would immediately bring the start of a worldwide population decline that could even take the shape of a true "Seneca Collapse." To support my argument, I brought the example of the Soviet Union, whose population started declining even before the political collapse of the Union. I also mentioned several examples of other Western Countries (e.g. Italy) where birth rates have been going down in parallel with the worsening of the economic conditions, to the point that we are starting to see an overall population decline.
This interpretation was criticized in the comments by some who argued that, yes, my interpretation may be correct for relatively modern and "Westernized" countries, but not for the poorest areas such Africa or Asia. These commentators argued that people in these areas will continue to make as many children as they can, no matter what happens around them; apparently as a result of the Imams telling them to do so (or by evil dictator Erdogan, or the like).
I don't think this criticism is correct and I can counter it with an example. We all know the story of the Irish famine that took place between 1845 and 1852, and that killed a large fraction of the Irish population. We know something about the number of deaths, about how many Irish emigrated, but we know relatively little of how the famine affected birth rates. Did Irish women try to compensate for the higher mortality by having more children?
On this point, I found a comprehensive study produced by Phelim P. Boyle and Cormac O Grada on Demography, Vol. 23, No. 4 (Nov., 1986), pp. 543-562 (here is the link). It takes some assumptions and extrapolations to determine the Irish birth rates before and after the famine. They also do not report graphs, but only tables. However, their conclusion is clear: birth rates declined with the famine in Ireland. In other words, the Irish families didn't try to compensate for the higher mortality by having more children; not at all.
This is confirmed by what we see of the Irish population in the decades after the famine. Even though food supply ceased to be a problem, still the population continued to decline or remain stable well into the 20th century. The Irish of those times didn't have good contraceptives, but they seem to have coped mainly by retarding the marriage age and by adopting a lifestyle that discouraged sexual activity among young people.
This is relevant for the case I am discussing here. In the 19th century, the Irish peasants were Catholic (or, if you prefer, "Papists"). The catholic view of marriage was supposed to be at that time (as it is still today in some circles) that a married couple should have as many children as the Lord sends them - that is as many as possible. After the famine, the Irish remained Catholic, but they totally disregarded the advice that the may perhaps have received from their priests. It was a perfectly rational choice - the Irish were not stupid.
We are clearly discussing something difficult to quantify, but I tend to think that most people on this planet are not so stupid as PR specialists seem to believe. So, from these historical examples (Russia and Ireland) I would say that a future economic/climate crisis will be immediately accompanied by a decline in birth rates and, hence, of population. If that happens, it will be a good thing as the pressure on the ecosystem will be reduced. That doesn't mean that all the problems we face will be solved, but at least we don't have to worry that people will worsen the situation by breeding like rabbits.