Some thoughts on what might need to be done in order to sustain a population of 7.5 billion or more:
- Much of the world's population will have to live in skyscrapers or arcologies, which are the only sort of buildings capable of housing extremely high population densities in small areas. Cities won't be spread out, but reach into the sky instead, and a smaller percentage of the population will live in regional and rural areas.
- Recycling and reuse of materials will become far more widespread as there is less land available
- We'll have to grow food both above and below ground. We'll need lots of cheap, emission-free power to provide artificial light, and I don't see nuclear power being part of that solution.
- Low and no-emission technologies will be vital in order for an increased and more affluent population not to increase pollution over today's current total emissions.
- We'll be mining asteroids in order for future generations to be able to live a lifestyle similar to ours, since the general population cannot conceive living a simpler and less asset-rich society as the one which we have now.
- I don't really see a large off-earth population solving the problems that we face here - the technological hurdles are too great and I don't see how we would have the capacity to transport and house large swathes of the population on another planet without the resources that would be required.
It would be quite difficult for most governments to enact forced population control measures, since I don't see any way for this to be a measure that would be popular with the voting population that these measures would be forced upon.