"The greatest remaining challenge... will be developing a method for making a synthetic liquid fuel from just CO2 and water using renewable energy sources."

Evolution has already taken care of this - this is exactly what plants do.. So all we need is to genetically engineer a plant that can be { composted / refined / fermented / extruded / ... } into { methane / oil / methanol / diesel / ... }. And if it can thrive in an enriched CO2 atmosphere, we have a place to empty the CO2 tank.

@ nickf

I actually prefer this solution too :-) Except perhaps for cleaning the stables and streets.... In Slovakia, I sometimes see a farmer-driven horse & cart in the counrty, it's a nice feeling that the changes in the 'civilized' world haven't affected all...