Blog 1 discussed the nature of pollution from the aviation industry. As with many other sectors of concern, though, demand continues to grow, waiting for a solution to miraculously appear. For many, this solution is identified in the knowledge that aircraft are becoming more and more efficient, in terms of fuel use and harmful emissions. What can we make of these claims?

Engineering improvements have been gradually improving fuel efficiency since jet-powered aviation was widely adopted in the 1950s and 60s. These improvements have been linked to incremental gains in various areas, such as aerodynamics, weight reductions, and engine efficiency. Though these changes may be primarily driven by financial incentives for airlines, as jet fuel is of course costly, some have come to celebrate the industry as increasingly ‘green’. Stats provided by OurWorldInData for instance show that emissions per ‘revenue passenger kilometre’ (RPK), or each kilometre travelled by paying passengers, were eleven times lower in 2018 than in 1960. The gradual gains in efficiency over these decades would be expected to continue "at a similar rate in a business as usual scenario" according to a group of researchers writing in Nature Communications.

Source: Hannah Ritchie, OurWorldInData (data source Lee et al, 2020), distributed under a CC-BY 4.0 license.

However, efficiency gains like this can be used to obscure the reality that the absolute emissions of the industry continue to increase at a significant rate, faster than that predicted by UN models. As demand for air travel increases rapidly, the growing number of flights taking place outweighs incremental gains made in efficiency. Such a situation is the case for various products and sectors that claim to be getting more sustainable: they may reduce the carbon produced per unit of output, but since their output continuously grows, there is no actual decline in their contribution to climate change.

Most will recognise that incremental gains in aircraft efficiency are not a long-term solution to the climate impact of the industry. But engineering processes are not the only mechanism being discussed to tackle aircraft emissions without stopping flights. Airbus has committed to deploying a fleet of hydrogen-powered aircraft by 2035, presenting these as the first zero-emission aircraft that will be available on the market. Steps like these are envisioned by many as a solution to aviation’s climate problems, which will not require compromise on access to flight. Glenn Llewellyn, behind the Airbus programme, sees the company as leading the way on climate action in this way, stating in an interview with CBC: "if aviation can decarbonize and eliminate its climate impact, then there is no excuse for any industry."

The present picture – sustainable fuels?

It remains to be seen whether hydrogen power can provide the ultimate redemption for the industry. At present, the picture appears much more mixed on technologies being deployed as ‘revolutionary’. Some in the industry have placed weight behind developments in what is marketed by the IATA as Sustainable Aviation Fuel, or SAD, describing a broad umbrella of fuels with origins other than crude oil. BP describes these as coming from ‘sustainable feedstocks’ such as waste oils from cooking and manufacturing, general household waste, and agricultural byproducts. Optimistic estimates suggest that, while emitting the same amount of greenhouse gas at the point of combustion, SAFs could reduce net CO2 emissions linked to aircraft flight by up to 80%. This would appear to represent a major step toward net-zero commitments for the industry, but the implementation of these fuels doesn’t appear as radical as the numbers suggest. At present, just 0.05% of fuel used by the industry falls under the SAF umbrella, and airlines’ commitments to the fuels are extremely conservative: Ryanair promises 12.5% SAF fuel use by 2030, just above the 10% figure for the International Airlines Group (which owns British Airways, Aer Lingus, and Vueling). EasyJet’s commitment to net-zero by 2050 stresses that SAFs are a minor short-term solution, opting to centre offsetting at the centre of its present sustainability action.

Image: a jet engine in action at a testing facility. Source: Sv1xv, distributed under a CC-BY 2.0 license.

Despite the apparent obstacles to the widespread deployment of SAFs, policymakers have begun to include them in measures to regulate aviation emissions, for better or worse. Indonesia was one of the first countries in the world to establish legislation mandating the use of SAFs: in 2013 it launched a minimum quota of 5% SAF used in its national aircraft. This modest figure precedes much larger quotas that appear on the horizon around the world. A proposal brought to the EU would legislate a quota for up to 63% SAF use by 2050, with legal minimum quotas designed to ensure that EU airlines could not gain an advantage over competitors by continuing to use higher proportions of conventional fuels. The proposal also takes aim at practices such as fuel tankering, when aircraft operators take on more fuel than necessary at a given airport, to avoid refuelling partially or fully at a destination where aviation fuel is more expensive. By taking on greater mass and making planes heavier, fuel tankering leads to higher fuel burn than necessary, and hence higher emissions. In all, these interventions are advertised to have the potential to deliver a major contribution to EU climate targets for 2030 and ultimately net-zero objectives. SAFs are therefore becoming increasingly consolidated as part of the global policy response to the drive to decarbonise the aviation industry.

Yet even these most ambitious of targets don’t envision SAF as accounting for much over half of jet fuel use, and itself is not emission-free. Can these ‘sustainable’ fuel innovations and policy support for them really be trusted to lead us away from dangerous emission levels? Finlay Asher of Safe Landing, a pro-sustainability group within the aviation industry, has argued they cannot. Speaking to the anti-flight group Flight Free, Asher described SAF as “a distraction from policies that would lead to a reduction in fuel use in the aviation industry,” and asserted that climate response requires “think[ing] beyond our current habits and lifestyles.” In a similar vein, Mira Kapfinger of Stay Grounded has argued that “instead of the proposed new technologies – such as electric aircraft, hydrogen, biofuels or e-fuels – what is needed is bans on airport expansions and the promotion of alternatives such as rail.” There is, then, scepticism amongst activists that SAFs and other proposed technologies may simply enable the continued growth of the industry, and not actually make meaningful cuts to its climate impact. It is true that even if the most ambitious SAF quotas were implemented, the continued growth of the industry would likely lead to continued increases in its absolute emissions. This raises questions about how real limits might be placed on the impact of the industry without falling back on the empty rhetoric of sustainability.

What about offsetting?

It would be incorrect to suggest that actors within the aviation industry had focused solely on SAF and other technological innovations to address its climatic impact. The UN’s ICAO, International Civil Aviation Organisation, has foregrounded offsetting in its immediate strategy for worldwide aviation emissions reductions. Its CORSIA resolution, adopted in 2016, requires aircraft operators to offset ‘excess’ carbon emissions by purchasing carbon credits. ‘Excess’ in this case would refer to increases relative to 2020 levels and would require airlines to monitor emissions on all international routes to set and meet these targets. Assuming that the carbon accreditation scheme could be trusted, this would mean that any growth in the industry’s net emissions would be firmly capped, peaking in 2020. As of 2022, 115 states have accepted the body’s resolution.

A list of critiques of the approach has been made by a group of experts assessing aviation emissions reduction pathways in Nature Communications. Firstly, given CORSIA’s base year of 2020, there is no scope in the scheme to further constrain carbon emissions below these already-excessive levels; nor would it make economic sense for airlines to do so, given that they are only required to spend on carbon offsets if their emissions increase above those of 2020. CORSIA does not take into account pollutants other than CO2, including plane emissions with great short-term significance such as ozone. It also only accounts for international flights, which excludes a large number of flights, including major domestic markets in the US, China and Australia.

Image: several rows of plastic tree guards in a planting area. Source: djim, distributed under a CC-BY 2.0 license.

One can take issue with these limitations in the scope of CORSIA to develop a sustainable future for aviation. But perhaps the greatest shortcoming is the present state of offsetting itself: much attention has been raised around flaws in the current state of offsetting systems, which means carbon reductions are often not as advertised. As explained in a report on CORSIA by the European NGO group Transport and Environment, it is ‘inherently challenging to prove’ that mitigation measures funded by carbon credit purchases were a direct result of that funding, and therefore that greenhouse gases were truly sequestered that otherwise would not have. Furthermore, as also covered in this report, it is difficult to place concrete estimates on the carbon storage capacities of offsetting schemes, given the variable nature of ecological carbon absorption. An EU report on the UN’s Clean Development Mechanism, designed to offset emissions linked to economic growth, suggested that 73% of offsetting projects had a low likelihood of actually meeting their credited carbon storage levels. Where accurate carbon accounting could be the difference between a stabilised climate and sustained warming, these difficulties in true carbon offsetting might mean these mechanisms ultimately prove unsuitable.

It is for this reason that activists have largely favoured an absolute reduction in flights over offsetting mechanisms, seeing this as the only safe way to reduce the industry’s climate impact. At present, it appears that policymakers are fixated on allowing continued growth in flight numbers, focusing on technological and otherwise compensatory measures. This certainly provides grounds for unease when we consider how likely it is that the aviation industry will make the cuts required to meet its climate targets. Offsetting, SAF, and other present regulations by national and international bodies appear insufficient, while the technological promises of hydrogen power are uncertain and unlikely to be viable soon enough to make a meaningful contribution to Paris Agreement targets. A real shift may be required in policymaking as well as public perception, recognising that air travel must decrease, and promoting alternatives wherever possible. As with countless other industries, this focus must replace an inappropriate reliance on uncertain technologies as soon as possible to avoid simply postponing disaster.