People did not fear having kids in the middle of world wars, famines, genocides etc. My guess it is not fear of climate crisis but fear of responsibility + desire to spend money on themselves + ability to choose due to contraceptives.

Is it not interesting that we are witnessing the biggest selection filter/pressure in human population since smallpox and black death? It is not purely genetic this time though, there is a huge cultural component, societies that do not breed do not survive.