The book Half-Earth Socialism covers this very extensively as a global resource constraint problem. The sort of "default settings" would be:
- 100% mandatory veganism
- Essentially zero jet travel
- 100% electrification of all personal heating/cooling/transportation needs
- Very little gas-powered public utility transportation/manufacturing, and only from biofuels
- 1500-2000 watts/person averaged primary energy usage; USA is currently at 12,000, see 2000 watt society
The main limiting factor is land usage. If we want to preserve half the earth's landmass for the biosphere to do whatever the fuck it does to keep us all alive, and keep 85% of existing wild species from going extinct, we don't have a lot of land to work with. Land reserved for human use will be split into:
- Agricultural food production. Pretty obvious. Plants are much, much more efficient in terms of calorie/acre than any animal products.
- Solar farms. These use a lot of land, but are needed to produce power for a fully electrified society.
- Biofuel production with BeCCS. This uses a lot of land relative to how much fuel is produced, but it also sequesters carbon.
- Actual human living/working/manufacturing space.
Needless to say the societal changes necessary to reach this state would make the cultural revolution look like child's play.