If you mean "what externalities would come from using fuel made using an air-to-fuel process, rather than from oil", then perhaps none. If you mean "what exter

If you mean "what externalities would come from using fuel made using an air-to-fuel process, rather than from oil", then perhaps none. If you mean "what externalities would come from turning CO2 into fuel and then burning it again to fly an aeroplane, rather than not flying that aeroplane", then that is a complicated and somewhat open question: https://www.carbonbrief.org/explainer-challenge-tackling-aviations-non-co2-emissions https://www.carbonbrief.org/explainer-challenge-tackling-avi...