For climate change, the problem is not the number of people on Earth. The US produces 14% of emissions with 3.8% of people. Four times the CO2 the 1.2 billion p

For climate change, the problem is not the number of people on Earth. The US produces 14% of emissions with 3.8% of people. Four times the CO2 the 1.2 billion people in Africa. The 1% globally (80 million people) produce as much CO2 as the poorer half of the population. Also, we don't know if it is possible to run civilization with 50 million people. It is possible that static society that depends on leftovers is possible. If it is possible to run society with that few people, relying on robots and AI perhaps, then it is possible to have robust, prosperous civilization with 5 billion or 50 billion.