A 'generation' is equal to a birthing window, so ~20 years. And these are exponential systems, because they keep multiplying against themselves. If you start with a population of 100 and a fertility rate of 1 then your first generation will be 50 people, then 25, then 12, then 6, then 3, then 1, then extinct. By contrast imagine for comparison if this group had a fertility rate of 4. It'd go to 200 people (4x as many), 400 (16 times as many), 800 (64x as many ignoring rounding), and so on. All in one human lifetime! Pretty insane.
It's really counter-intuitive how fast fertility changes will change the face of the Earth! And now consider demographics. If you want to predict fertility it's easy: high religiosity, low income, high conservatism, low education. And vice versa for predicting low fertility. And while people can obviously differ from their parents, it's equally obvious that most people will end up like the parents. So the group having lots of kids is also the group that's generally not too bothered by climate change. And the group not having kids is the group that is concerned about climate change. Guess who's going to win that argument in the future?
This also applies to just about everything. By not having children you're essentially conceding your opportunity to have a 'voice', even if not your own, in the future of humanity. People talk about things like wanting to transfer their consciousness into a computer, but that's nonsense. There is no "transferring" anything - you might create a chatbot in your likening, but you'll be dead all the same, and that chatbot will be along as soon as somebody gets bored of running totally_sentient_chatbot_v2.37.exe. Human immortality is a social thing - and little more than a continuation of the species. And we all get to choose whether we want to play our own little part in that, or not.