Carbon zero would probably mean not putting any new carbon into the cycle. Not all carbon emissions are positive contributions to net carbon, if they’re just reusing carbon that was already being cycled through.

Consider as a thought experiment a world where we suddenly have limitless but “clean” power (ie. fusion comes sooner and better than even the most optimistic expectations) and we start doing carbon capture that also produces burnable hydrocarbons as a byproduct. Running an ICE off that fuel doesn’t put any new carbon into the air, it just recycles what was already there. Obviously this isn’t likely, but it illustrates the premise in an exaggerated way.

More practically, in theory wood from farmed, new growth trees fits this mold, but in practice it’s not a terribly efficient or clean fuel once you take all the externalities out. And obviously you’re not gonna power a car on it.

But fundamentally, the main thing we need to do to get to zero is stop breaking the planet’s own sequestration of all the carbon that was in the atmosphere before it was hospitable to modern life. At this point doing that is very much a political problem, we probably have the tech to replace the energy we’d lose from that, we just aren’t willing to build it out fast enough.