I was going to comment exactly the same thing. We definitely CAN turn CO2 into fuel - but the energy cost would be staggering (especially considering that we use fuel, in the first place, FOR it's energy density).

Plants would do that much more effectively (without mentioning that plants are clean, which may not be the case with fuel production from carbon oxides) AND that plants actually give us back the oxygen in such compounds.

Though personally (DISCLAIMER: I'm not a materials, chemical or any kind of engineer/expert) I think the most energy-efficient way to deal with carbon-based greenhouse gases (or really, any carbon-based pollution) is to crack the molecules in order to turn carbon into graphite/graphene (at ~805kj/mol^-1 for CO2, ~1050kj/mol^-1 for CO and ~1200kj/mol^-1 for methane, it sounds relatively energy inexpensive - considering that CO2 and CO are calculated in PPM and methane in PPB, assuming you can cheaply capture them, you could clear a huge volume of atmosphere - then again, I don't know how much energy/money is involved in capturing these gasses...)

Of course, something even more energy-efficient, cheap and effective to reduce carbon dioxide in the air (along with it's less-spoken-of brother, nitrogen oxides) is to actually reduce our consumption of fossil fuels. Do plants even filter out N2O?