Fertility changes are exponential. They can be approximated (assuming a population with a common fertility rate among generations) as having a fertility_rate/2

Fertility changes are exponential. They can be approximated (assuming a population with a common fertility rate among generations) as having a fertility_rate/2 scalar impact on population per 20 years. So imagine a fertility rate of 1. The scalar would be 1/2 so the population would declining by about 50% every 20 years. And this doesn't stop until you go extinct or start having more children. And you'd have to start having more children in the midst of economic collapse. So think about a period of just 80 years, a single human lifetime - that would be a decline of 1-(1/2)^4 = 15/16ths of your previous population. The US would go from 345 million to 22million, about the size of Sri Lanka or Chile. And you're right religious groups do retain healthy fertility rates, so we're bee-lining to a world where secular educated individuals are simply removing themselves from the gene pool, while highly religious, less educated individuals are thriving. Guess what the world would thus look like in the future and how concerned it might be with your issues of choice, like climate change. Children are essentially your voice in the future - have no children, have no voice.