Excellent report (in Dutch) by engineering company RoyalHaskoningDHV, which analyzes all the options there currently are for reducing the carbon footprint of flying, and the impact of each option in 2030 and 2050. Electric planes for short distances and electrofuel (fuel created from CO2 and H2O using clean electricity) for long distances are the most significant options. Pity that electrofuels currently cost 5-6 times as much money as conventional fuel, whereby 50% of the electric energy needed in the production process is wasted... The only short-term options is to fly (a lot) less....