Taking your questions in turn: 1) The conversion will always be energy-negative due to the laws of thermodynamics, as a sibling comment mentioned. This is tru

Taking your questions in turn: 1) The conversion will always be energy-negative due to the laws of thermodynamics, as a sibling comment mentioned. This is true even of existing fossil fuels, though we tend not to think of it that way. 2) The process of converting CO2 to hydrocarbon fuels leads to a decrease in atmospheric carbon. Burning these fuels or fossil fuels (what I assume you mean by "the existing process") produces an increase in atmospheric carbon. This means it's possible to create a net-positive, net-neutral, or net-negative outcome with respect to atmospheric CO2 concentration depending on how you balance these two processes. As you point out, "zero emission" doesn't mean the same thing as "no net CO2 release"; rather, it more often means "zero emissions of CO2 at the point of energy usage," with CO2 emissions usually occurring elsewhere. That said, with a system of hydrocarbon fuel production from air-captured CO2 it is genuinely possible to have "zero emission" mean "zero net emission of CO2 over the entire energy transport process." EDIT: The key enabling technology is, of course, carbon-free renewable energy, of which solar is likely to represent the lions' share as time passes.