Here's an interesting exercise : choose two out of three of the following :
- Keep the environment intact
- Maintain first-world standards of living,
- Have a population of seven billion people.
Personally, I'm in favor of cutting that last one; I'd rather have the population of the 16th century and the technology of the 21st than the other way around.
If we can ever manage to make space colonisation into a reality, we can have all three. And by all means, we should focus on making space colonisation into a reality as soon as possible! But since that's not on the table yet, we really need to figure out which two options we're choosing in the meantime, or we're going to end up with none.
Before anyone makes these suggestions, here are some things that are absolutely not the solution:
1.) Libertarian utopia. Yes, I know, if everyone was a perfectly rational actor, natural socioeconomic forces would guide us to an optimal outcome without any need for active coordination. Unfortunately, almost no one is a perfectly rational actor. We live in a world of homo sapiens, not homo economicus, deal with it.
2.) Communist utopia. Look up the track record that actual communist nations have had on environmental issues. Yes, I know, that wasn't real communism, or it only turned out that way because of capitalist interference, or it actually wasn't that bad and all claims to the contrary are revisionist propaganda. Sorry, I guess I'm just one of those old-fashioned rubes who relies on facts and data and historical documentation over abstract theories of How The World Really Works.
3.) You, personally, choosing not to have children. If you're reading this, then you're probably not in a part of the world where overpopulation is currently a major problem. If there's a public bathroom with three sinks and two of them are overflowing, turning down the faucet on the one functional sink isn't going to stop the flood.
4.) Genocide, forced sterilisation, or similarly coercive measures. For one thing, I'm sceptical it would actually work. For another, I'm extremely sceptical that forced depopulation would be applied in a way that could even remotely be considered fair and unbiased; too many people would see an opportunity to get rid of any racial, ethnic, or cultural groups they happen to find troublesome.