LONDON, Nov 1 (Reuters Breakingviews) - Aviation doesn’t sound like one of global warming’s most pressing headaches. At just over 900 million tonnes of carbon dioxide, flying contributed only 2.5% of the world’s carbon emissions in 2019. Yet getting these down could prove fiendishly difficult. Last month the International Air Transport Association (IATA), the airline industry body, said it wanted aviation’s net carbon footprint to fall to zero by 2050. That’s as it should be. Its own forecasts assume emissions grow 3% every year, meaning roughly 2 billion tonnes by 2050. Because other sectors are decarbonising rapidly as they move to zero-carbon technologies, aviation’s relative proportion of global emissions will increase. The catch is that cutting from 2 billion tonnes requires solutions that right now are expensive, futuristic, or both.Historically, improvements in jet-engine design have delivered efficiency gains of 1.5%-2% a year. Eking out future technological savings will get harder. But swapping old planes for fuel-efficient ones like Boeing’s (BA.N) 737 MAX or Airbus’s (AIR.PA) A320neo will make the global jet fleet up to 25% leaner by 2050, the United Nations’ International Civil Aviation Organization (ICAO) reckons. This cuts projected CO2 output to 1.5 billion tonnes with minimal additional outlay. Thankfully, the world has moved on from the Cold War, when airspace over countries like the Soviet Union was off limits, forcing passenger jets into long detours. That said, there’s still scope for more point-to-point flying, especially in crowded regions like Europe. ICAO reckons such improvements can shave another 10% – or 200 million tonnes – off 2050’s CO2 output. That brings emissions down to 1.3 billion tonnes, again for minimal cost.Still, that’s 1.5 times what Germany belched out in 2019. It’s also where the quick fixes end. The first port of call is biofuel, one of the sustainable aviation fuel (SAF) options. The CO2 benefits derive from feedstocks like reused cooking oil or agricultural waste being part of the atmosphere’s existing carbon cycle, rather than fossil fuels that have trapped carbon underground for millions of years. The good news is biokerosene is a near-perfect substitute, requiring minimal engine tweaks. Biofuel blends are already in limited commercial use. By 2023, Rolls-Royce’s (RR.L) new long-haul engines will be able to run on 100% biokerosene. The bad news is that, overall, they still produce some CO2, reducing emissions by at best 80%. They’re also expensive. At three times the price of normal kerosene, biofuels upend aviation’s economics. In 2019, Europe’s Ryanair (RYA.I) spent 2.4 billion euros on jet fuel, twice its operating profit. Had Chief Executive Michael O’Leary’s fleet flown entirely on biofuel, the bill would have been 7.2 billion euros, roughly the same as the budget carrier’s revenue. Availability is an even bigger challenge. Airline bosses have rightly red-carded feedstocks like palm oil or soybeans that endanger forests or food security. But there’s only so much old cooking oil lying around, and efforts to get SAF off the ground so far haven’t amounted to much – 100 million litres of annual production. IATA’s roadmap points to that soaring to 450 billion litres, a staggering 4,500-fold increase. That said, a 25% biofuel blend would shave a fifth off CO2 emissions, bringing mid-century CO2 output to just over 1 billion tonnes. But it would add 50% to aviation’s fuel bill. HARDER YARDS From a pure carbon perspective, biofuel’s chemical cousin e-kerosene scores higher. Its base ingredients are water and CO2 captured from the air. The water is split into oxygen and hydrogen using electricity from renewable sources; the hydrogen is then combined with the CO2 to create a hydrocarbon. Again, price and availability are problematic. With no existing commercial production, it’s hard to say what e-kerosene will cost. But if it was cheaper than biofuels, it would already be in use. And the energy input is colossal. Brussels lobby group Transport & Environment thinks Europe’s 2050 fleet would require 535 terawatt hours of green electricity, 1.6 times Britain’s entire electricity consumption last year. Assuming complete carbon efficiency, adding a 25% e-kerosene blend to the mix would take aviation’s emissions down to 780 million tonnes, but inflate its fuel bill by another third. CLIPPED WINGS Unless airlines want to offset that CO2 by planting the equivalent weight in trees, batteries and hydrogen will have to step up. Battery planes are already a thing – Rolls-Royce boasts a 300-mile-an-hour one-seater Spitfire lookalike. Yet batteries’ weight remains problematic. Barring an Elon Musk-style technological breakthrough, electric planes will thus be confined to city- or island-hopping for a few dozen passengers, generating negligible CO2 savings. Hydrogen is theoretically promising, but entails a major rejig of the traditional airframe – in liquid form, it is at minus 253 degrees Celsius and takes up three times the space of kerosene with the same amount of energy. Airbus is bullish about a 100-seater hydrogen-powered jet by 2035. U.S. rival Boeing is not. Even if Airbus is right, to make a difference it would have to replace a significant number of the 25,000 passenger jets currently in circulation. That looks unlikely. The outlook, for airlines, is therefore not encouraging. In the absence of a technical hack, flying demand will have to fall. That may come from sharply higher operating costs feeding through to higher ticket prices. But governments may also feel the need to impose punitive taxes or regulation. As during the pandemic, business travellers can probably make do with remote substitutes, especially given the inevitable telecommunications advances. Barring the advent of an airborne deus ex machina, the days of mass-market leisure aviation look numbered.