There’s plans for artificial Islands, (not the Libertarian guys), that could house the whole of the population of planet Earth with four times the energy use that a typical American uses in only fifteen years.

James Bowery, “Exponential Remediation of Civilization’s Footprint”, on creating new territory.

http://jimbowery.blogspot.com/2014/05/introduction-extinction-of-human-race.html

There are even newer forms of making food would be even more efficient than bio-reactors. Using electricity and air to grow food at up to ten times the efficiency of photosynthesis.

https://www.ndtv.com/food/the-new-kind-of-protein-created-from-air-and-electricity-1731757

So this is happening now. I think one of the big steps missing is cheap construction materials. We have that coming. Graphene is one of the strongest materials we have and it keeps getting cheaper and cheaper to make. Graphene made from coal or some other cheap carbon source could be the magic material that lowers the cost of construction materials dramatically while providing very good performance.