CO2 is actually useful, but don't forget that mineral carbonation is an option. Magnesium and Calcium Oxides are abundant and they react exothermically with carbon dioxide to produce magnesium and calcium carbonate, obviously we have no problem putting those in long term storage. My own preference if possible is obviously to build up a biochar in the dirt itself, terra preta is damn good soil and the carbon stored in it is literally tons per acre even if fairly thin, and we've got billions of acres of soil that would benefit from that sort of enrichment.