Carbon-neutral aviation

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The climatic impact of aviation

At present, virtually all freight and passenger-carrying aircraft operate in one of two ways: burning kerosene to turn a propeller, generating thrust that the wings partially convert to lift, or generating thrust by burning kerosene in a jet engine. Virtually all of that kerosene is produced by refining petroleum. As such, burning it adds to the stock of greenhouse gasses in the atmosphere. These include carbon dioxide (CO2) (a basic product of the combustion) and other greenhouse gasses (GHGs), like nitrous oxide (NO2). It is also possible that aircraft have an effect on cloud formation (both by producing vapour trails and inducing cirrus cloud formation), but my understanding of the science is that scientists aren’t sure whether that has a net warming or a net cooling effect. The latest IPCC report says:

Moreover, the uncertainties on some aviation forcings (notably contrail and cirrus) are still high, such that the overall radiative forcing consequences of changing cruise altitudes need to be considered as a time-integrated scenario, which has not yet been done. (p. 355)

Helpfully, the report does identify that, if contrails prove to be a significant problem, they “can be easily avoided – in principle – by relatively small changes in flight level, due to the shallowness of ice supersaturation layers.” There is also some uncertainty about the relative emissions of short-lived but potent GHGs like nitrous oxide, compared with long-lived but less potent ones like carbon dioxide. All told, the report does conclude that aviation has a “larger impact on radiative forcing than that from its CO2 forcing alone.”

Carbon neutral possibilities

A couple of logical possibilities exist for making air travel carbon-neutral, though they differ in practicality. Electric planes are conceptually possible, and small versions exist. As I understand it, the big problem is storing enough energy in light enough batteries. My sense is that we are nowhere near being able to do this for large commercial aircraft. Similar issues exist for hydrogen aircraft, in term of storage, and there is the added question of where we get the hydrogen. To me, biofuels seem like the most plausible near-term option. That being said, there are technical issues to be overcome within aircraft themselves, such as the gelling of biofuels at the low temperatures found at high altitudes. While some airlines have tested multi-engine planes with a single engine running on a biofuel/kerosene mix, as far as I know nobody has flown such a plane exclusively using biofuels.

Additionally, not all biofuels are carbon neutral. Ethanol derived from corn might actually represent more greenhouse gasses than an equivalent amount of gasoline, once you factor in fertilizer production, emissions from farming and farm equipment, ethanol fermentation, etc. The same might be true of palm oil derived biofuels, given how their production can lead to the destruction of rainforests that are major carbon sinks.

My sense is that the air travel industry has yet to demonstrate that it will be able to exist in a carbon neutral world, regardless of how expensive tickets become. That being said, it does make sense to displace emitting activities in order from lowest cost to highest cost. If we can replace fossil fuelled ground vehicles with electric vehicles running on renewable power, we should do so first before pouring enormous effort into trying to produce a carbon neutral aircraft. That being said, there does seem to be a strong moral imperative to reduce emissions generally, including by limiting the amount of long-distance travel we undertake.

As usual, I expect any mention of aviation to produce a lively discussion.

Author: Milan

In the spring of 2005, I graduated from the University of British Columbia with a degree in International Relations and a general focus in the area of environmental politics. In the fall of 2005, I began reading for an M.Phil in IR at Wadham College, Oxford. Outside school, I am very interested in photography, writing, and the outdoors. I am writing this blog to keep in touch with friends and family around the world, provide a more personal view of graduate student life in Oxford, and pass on some lessons I've learned here.

24 thoughts on “Carbon-neutral aviation”

  1. Another option that is at least logically possible is to compensate for aviation emissions by sucking CO2 out of the air and burying it.

  2. Ya, we could just pump pressurised CO2 down old natural gas wells. Nothing bad could possibly happen.

  3. Even with very deep cuts in emissions – such as a 95% reduction below 1990 levels by 2050 for high emitting states – producing a stable climate may require removing some of the carbon we have added to the atmosphere and burying it.

  4. There was a nuclear powered aircraft tested by America during the age when keeping bombers in the air constantly was considered desirable. Perhaps that could be the future of air travel. Although, it would probably be even more difficult than hydrogen.

  5. An air travel system using uranium and plutonium as fuels might be easier to establish in a carbon neutral way than one relying on hydrogen, but there would be a lot more risks associated: everything from crashes spraying isotopes everywhere to the proliferation risks that would accompany the facilities and technologies.

  6. Wouldn’t it be more straight forward to use nuclear plants on the ground to produce hydrogen?

  7. I don’t know if anyone has seriously investigated the logistics and economics of nuclear powered aviation.

    I am not really qualified to speculate.

  8. I think that the only nuclear aircraft that have so far flown weren’t actually powered by the reactor, but they simply lifted a reactor in a conventional airplane to test the concept. I remember reading the crew got a pretty bad dose of radiation from the poorly shielded assembly.

  10. I’m with Milan on this one when it comes to nuclear planes. Not sure how feasible a lot of these ideas are actually.

    You can still use offsets to fly carbon neutral though. I know belgrave trust offers them for commercial and private flight. I think native energy and standard carbon might offer these offsets too. Not sure though don’t use em.

  13. Pingback: Paths to geoengineering

  15. This flowchart gives a really good at-a-glance understanding of which greenhouse gasses and energy uses are most important.

    All of transportation is only about 13.5% of emissions, with all air travel representing 1.6% of the total. By contrast, the cement industry represents 3.8%, deforestation is 18.3%, and unintentional natural gas leakes are 3.4%.

  16. Electric planes
    High voltage
    Transport: As electric cars make steady progress on land, battery- powered aircraft of various kinds are quietly taking to the air

    Jun 10th 2010

    RANDALL FISHMAN, a retired jeweller from New Jersey, is a keen pilot who has spent a lot of time hang-gliding over the Hudson river when he should have been working. Despite some 30 years’ experience as an aviator, he yearned to fly in a new and purer way. So in 2006 he converted a powered hang-glider, removing its small combustion engine and replacing it with a battery and an electric motor. He was not trying to be green. “I just did not like all the vibration and noise from a combustion engine,” he says. “With electric power it was like that dream you had as a kid—of soaring and flying silently.”

    Mr Fishman is among a growing group of aviators in America, Europe and Asia who are flying under electric power. What began with hang-gliders and microlights has moved on to full-sized gliders and now two-seater aircraft. A dozen or so electric aircraft are expected to turn up at the Green Aviation Show, which will be held at Le Bourget, France, this month. And they will also be out in force at the Experimental Aircraft Association’s annual gathering at Oshkosh, Wisconsin, in July.

  17. Not all forms of transport are so easy to electrify. One of the hardest is aviation, where battery power runs up against a serious problem: weight. Kilo-for-kilo, fossil fuels contain roughly 100 times as much energy as a lithium-ion battery. On the road, that is a problem which can be designed around. For a machine that must lift itself into the sky, it is much harder to solve.

    But it is not impossible. Dozens of firms are working on electrically powered planes of all shapes and sizes. Some resemble flying cars, such as those which Larry Page, one of Google’s founders, is backing. Others are hovering, drone-like machines that could operate as autonomous aerial taxis (Uber is keen on these). Pipistrel, a Slovenian company, already makes a two-seater electric training plane. Another two-seater, the E-fan, has been flown by Airbus, a European aviation giant, although it recently abandoned the project.

    The reason for that became clear on November 28th, when Airbus announced something more ambitious. It has teamed up with Rolls-Royce, a British jet-engine producer, and Siemens, a German electricals group, to convert a small airliner into a “flying test bed” to prove the feasibility of hybrid-electric propulsion. “We are entering a new world of aviation,” said Frank Anton, head of Siemens eAircraft. Electric power, he predicted, would prove to be as significant to commercial aviation as the invention of the jet engine.

    …

    Such aircraft will, their designers hope, serve as bridges to fully electric planes in the future. Overcoming the weight problem will be tricky. For big planes flying long-haul routes, full electrification may never happen, although hybrid systems would reduce their fuel consumption. But design changes can help. Airbus, for instance, thinks it can blend its electric motors into the aircraft’s fuselage to reduce drag. And electric power offers some advantages that offset its big drawback. One is that combustion engines are not very efficient at turning the energy in their fuel into motion. Instead, a great deal of it ends up wasted as heat. A jet engine might manage around 55% efficiency during a steady cruise at the ideal altitude. But that number could fall by half or more when taking off, climbing, landing and taxiing on the ground, which is what aircraft that fly short routes spend much of their time doing.

    …

    Combining all these benefits and drawbacks into a single figure is tricky. Paul Eremenko, Airbus’s chief technology officer, says a single-aisle hybrid electric airliner would be “safe, efficient and cost-effective”. Zunum’s Dr Kumar is prepared to go further and talk numbers. For airlines, the important figure is the CASM—cost per available seat mile. This is obtained by dividing operating costs by capacity, measured as the number of seats in an aircraft multiplied by miles flown. Zunum claims that its plane will have a CASM of 8 cents. Oliver Wyman, a firm of aviation analysts, reckons that the average for American airlines in 2016 was 11 cents.

  22. Rolls-Royce carried out the world’s first ground test of an aircraft engine that has been converted to run on green hydrogen created by wind and tidal power. Hydrogen-powered planes can help the aviation industry cut its emissions, although passenger-carrying jets are still years away from entering service. Airbus, which has been aiming to introduce a hydrogen plane by 2035, warned this week that a lack of green hydrogen and associated infrastructure could delay its project.

    https://www.economist.com/the-world-this-week/2022/12/01/business

  24. International airlines will still have to hew to the un-backed scheme, which is known as the Carbon Offsetting and Reduction Scheme for International Aviation (corsia) and applies to the whole industry. Under corsia an airline must offset emissions beyond a baseline by buying credits, with the split based on the airline’s share of total industry emissions. For the pilot stage, which runs until the end of 2023, the baseline is industry emissions in 2019.

    As airlines have not fully recovered from the covid-19 pandemic, there is currently no requirement to offset emissions. From 2024 the baseline will be lowered to 85% of emissions in 2019. Yet even this will not have much impact. Credits are cheap, costing about $3 a tonne compared with around €90 ($96) a tonne in the eu. And airlines may be able to avoid buying them at all by using a bit of sustainable aviation fuel, made from waste cooking oil, which is heavily subsidised in America.

    https://www.economist.com/finance-and-economics/2022/12/15/the-struggle-to-put-a-carbon-price-on-a-flight

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