Aviation: The Hardest Number in Transport
Aviation is a small share of world energy - a few percent - and an outsized share of the energy used by the individuals who fly frequently. It is also the mode where the physics is least forgiving, because unlike a car or a train, an aircraft pays a compounding penalty for every kilogram it carries.
The Ratio That Decides Everything
Kerosene stores roughly 12,000 watt-hours of chemical energy per kilogram. A current lithium-ion cell stores around 250, and a pack with its casing, cooling and management less than that. Even allowing for the fact that an electric motor converts its energy far more efficiently than a turbine, the usable gap remains around a factor of fifteen to twenty.
For a car this is inconvenient. For an aircraft it is decisive, because an aircraft must lift its energy store and keep it aloft for the whole journey. Heavier storage means more lift, which means more wing and more thrust, which means more energy - a loop that closes badly.
There is a second effect with no equivalent on the ground. An airliner burns a large fraction of its take-off mass as fuel during a long flight and lands significantly lighter, which means the later hours cost less energy per kilometre than the early ones. A battery weighs the same when empty, so the aircraft carries full mass for the entire flight and gains nothing back.
Together these explain the observed pattern precisely. Electric aircraft are entering service for short routes with few passengers, where the battery mass is tolerable and the distances are small. The same technology does not extend to a transatlantic airliner by improving the cells a little, because the penalty compounds with range.
How Efficient Flying Actually Is
The intuitive assumption is that flying is catastrophically inefficient per kilometre. It is not - a full modern narrow-body airliner is in the same range per passenger-kilometre as a car carrying one person, and better than a car on some routes.
Aircraft achieved this through sustained engineering. Fuel burn per seat-kilometre has improved by more than half since the 1970s, through higher-bypass engines, lighter composite structures, better aerodynamics and higher seating density. The last of those is an efficiency gain that passengers experience as a loss, which is worth noting because it shows up in the statistics as progress.
The reason aviation is nonetheless a large share of an individual's energy footprint is distance, not inefficiency. A single long-haul return flight covers more kilometres than a year of typical car commuting, so a comparable per-kilometre figure produces a much larger total. Nobody drives to another continent for a week.
This is why per-kilometre comparisons mislead here in a specific way. The relevant comparison for a personal decision is not flying versus driving the same route, but flying versus not making the journey or making a shorter one - which is a different question from the one mode comparisons answer.
The Part That Is Not Carbon Dioxide
Aircraft emit carbon dioxide like any fuel burner, and they also do something specific to altitude. Water vapour from exhaust can condense and freeze into contrails, and where those persist and spread into cirrus, they trap outgoing heat.
Estimates place this contrail effect at roughly the same order as the carbon dioxide from the same flight, possibly larger, with substantially more uncertainty. Nitrogen oxides at altitude add further effects on ozone and methane that partly offset each other.
The practical significance is that a calculation counting only the carbon dioxide understates aviation's climate effect by a large factor, and most consumer-facing calculators do exactly that. It also means the effect is not proportional to fuel burn alone, since contrail formation depends on atmospheric conditions on the specific route and altitude.
That dependence opens an option unavailable elsewhere. A small fraction of flights produces most of the persistent contrails, and rerouting those flights to avoid ice-supersaturated regions costs a little extra fuel and avoids a much larger warming effect. Trials suggest this is among the cheapest interventions available in aviation, and it is a forecasting and air-traffic-control problem rather than an aircraft problem.
What the Alternatives Actually Offer
Battery-electric aircraft are real and limited. Commuter aircraft carrying single-digit to low double-digit passengers over a few hundred kilometres are entering service, and they serve a genuine market. Extending this to the routes that dominate aviation energy requires cell energy density several times current levels, which is a research question rather than a deployment one.
Hydrogen is the more discussed long-range option and has a different constraint. Hydrogen stores about three times more energy per kilogram than kerosene and roughly four times less per litre even as a cryogenic liquid, so the problem is volume rather than mass. That demands a fundamentally different airframe, and a fuel infrastructure at every airport served.
Sustainable aviation fuel is the option that works with existing aircraft, which is its decisive advantage. It is a drop-in liquid made from waste oils, biomass or captured carbon dioxide plus hydrogen. The constraint is supply: feedstock for the cheap routes is limited, and the synthetic route is expensive because it needs large quantities of clean electricity, placing it in direct competition with every other use of that electricity.
Which leaves efficiency and demand. Aircraft continue to improve by one to two percent a year, and traffic has grown faster than that for most of aviation's history - the addition rather than substitution pattern again. The honest statement is that aviation has no complete technical answer available this decade, and that the sector's emissions trajectory therefore depends more on how much flying happens than on what the aircraft are made of. That is an uncomfortable conclusion and it is what the numbers support.
Frequently asked questions
Why can't aircraft run on batteries?
Because jet fuel stores roughly 12,000 watt-hours per kilogram and a lithium cell around 250. An aircraft must lift its own energy store and keep it aloft, so heavier storage means more lift, more wing and more thrust. A fuelled aircraft also gets lighter as it flies, while a battery does not, which compounds the penalty with distance.
Is flying less efficient than driving?
Not per kilometre. A full modern narrow-body airliner is in the same range per passenger-kilometre as a car carrying one person. Aviation dominates a frequent flyer's energy footprint because of distance: one long-haul return flight covers more kilometres than a year of typical commuting.
What are contrails and why do they matter?
Water vapour from exhaust that condenses and freezes at altitude. Where contrails persist and spread into cirrus cloud they trap outgoing heat, with an effect estimated at roughly the same order as the carbon dioxide from the same flight, and with more uncertainty. Calculators that count only carbon dioxide understate aviation's effect substantially.
Could rerouting flights reduce the climate effect?
Yes, and it appears to be among the cheapest options in aviation. A small fraction of flights produces most persistent contrails, since formation depends on atmospheric conditions at that route and altitude. Avoiding ice-supersaturated regions costs slightly more fuel and avoids a much larger warming effect.
Will hydrogen or synthetic fuel solve aviation?
Neither completely this decade. Hydrogen stores well by mass but poorly by volume even as a cryogenic liquid, requiring a different airframe and airport infrastructure. Sustainable aviation fuel works in existing aircraft but is supply-constrained: cheap feedstocks are limited and the synthetic route needs large amounts of clean electricity.
