But this can be the causal root of the recent changes. Populations aren't declining because of famine, but because people aren't having more children.

Your argument need some middle step or mediator like "people already realizing the comming problems and react with getting less childeren". But is this really the case? I mean, we observe the decline even in places like north korea or by groups who denied clima change or simply don't care.

Maybe, the emerging problems of clima change are part of the creation of a pessimistic worldview or something.