We have already overpopulated the earth by about a factor of 3. It can only provide a decent, sustainable living for 1 to 2 billion people and our population is headed toward 8 billion. Around 3 billion are living in poverty.
See no. 1 above. At an average of 1 baby per two people we could reduce world population to approximately 2 billion by 2100. Some people could be good parents and could have 2 children when 2 other people agree to be sterilized.
Our environment is what sustains the people. It cannot endlessly absorb the assaults and demands we are putting on it. We are extinguishing species that we need to sustain us.