Why is aviation so hard to decarbonise?
Because the problem is energy density, and no alternative currently matches jet fuel closely enough for long flights — which is why aviation is one of the genuinely hard sectors rather than a matter of insufficient will.
The physical constraint. Kerosene stores roughly 12,000 watt-hours per kilogram. The best current lithium batteries store around 250 — nearly fifty times less. Worse, an aircraft burning fuel gets lighter as it flies, while a battery-powered one carries the same mass throughout. Electric flight is therefore plausible for short regional routes and small aircraft, and not for long-haul on any foreseeable battery chemistry.
Hydrogen has excellent energy per kilogram — better than kerosene — and poor energy per litre, even liquefied at −253°C. It requires large insulated tanks that do not fit in wings, so the airframe itself must be redesigned, alongside entirely new airport fuelling infrastructure.
Sustainable aviation fuel is the nearest-term route: chemically similar enough to be dropped into existing aircraft and airports, made from waste oils, agricultural residues, or synthesised from captured CO₂ and green hydrogen. The constraints are supply — waste feedstocks are limited and contested — and cost, currently several times conventional fuel.
The non-CO₂ effects are the underappreciated half. Contrails, nitrogen oxides and soot at altitude produce warming estimated to be comparable to or larger than aviation's CO₂, which means fuel efficiency alone does not describe the impact, and also that rerouting a small number of flights to avoid contrail-forming air could deliver a large, fast reduction at modest cost.
Efficiency continues to improve by roughly 1–2% a year through engines, aerodynamics and operations — consistently outpaced by traffic growth.
Demand is extremely concentrated. A small minority of the world's population flies at all, and frequent flyers account for a large share of emissions, which is why frequent flyer levies are discussed.
Offsetting has a poor record here and is not a substitute.