As soon as we can convert CO2 into hydrocarbons using solar or wind power in a way that's cost-competitive with conventional extraction we'll have turned an important corner in developing as a society.
Growing sugarcane does a pretty good job of converting CO2 into hydrocarbons using solar power.
But to be cost-competitive with fracked methane, I'm pretty sure we're going to need some heavily genetically-engineered strains of algae, growing in artificial forests of pipes.
The irony is that algae can be used to clean up fracking wastewater, so it makes methane production cheaper as it competes.
Sugarcane and switchgrass might be able to close the loop, but these take a lot of mechanical processing and land to do so, which could introduce additional costs.
The promise of syntetic fossil fuels from things like water and CO2 is that you don't need a lot of land, you just need energy, and that energy can be renewable.
If you can discover a way to cost-effectively synthesize hydrocarbons (CH chains) from CO2 and H2O, which contain all of the necessary components, you're pretty much guaranteed a Nobel prize.
So far the process to do this involves a lot of messy intermediate steps, like hundreds of millions of years and enormous pressure in the mantle of the Earth.
We need energy in portable forms (and have other uses for hydrocarbons). Getting that energy from clean sources and storing it with atmospheric CO2 is better than burning fossil sources of energy.
