We have to stop generating more of ourselves. We use more of some resources than the Earth and Sun generate. We use more fossil fuels than life on Earth can sustain. We are working on getting to Peak Humanity, and then backing down some, but not yet quickly enough. And we still have to work on renewables, carbon capture in soils and otherwise, conservation, efficiency, design for recycling, and all the rest that we all know about.
tl;dr
The global average fertility rate was 5 children per woman until the end of the 1960s and has halved since then, to a bit below 2.5. We need to reduce it by 0.5 births per fertile woman in order to peak below 9 billion people by 2050. We know how:
- Renewable energy even in the poorest and most remote villages
- Cell phones and broadband Internet for all
- Education, especially for girls, enabled by the above
- Availability of family planning services and birth control, likewise
- Economic growth, rule of law, and human rights in the poorest countries
(Birth rates in developed countries and many others are well below replacement, and still declining.) That's a decline of a bit over 2.5 in about 60 years, so at that rate it would take 12 years to reach global replacement. It will of course be slower. We got from 3.0 to 2.5 in 20 years.
So we don't have to throw up our hands in despair. We can discuss means, funding, and political will.