Climate Change Mitigation in the Aviation Sector: A Critical Overview of National and International Initiatives

Abstract Climate change mitigation calls for the limitation and reduction of greenhouse gas (GHG) emissions across all sectors. However, limiting GHG emissions from aviation has proven to be problematic for technical reasons (e.g., lack of low-carbon alternatives) as well as legal reasons (e.g., international aviation does not readily fall within any one state's jurisdiction). Relevant initiatives have followed two streams. At the international level, the International Civil Aviation Organization (ICAO) has adopted technical standards and, more recently, a market-based mechanism to limit emissions from international civil aviation. In parallel, states have adopted their own policies and measures to regulate emissions from both domestic and international aviation, ranging from tax and technical standards to traffic management and infrastructural development. While much of the literature on climate change mitigation in the aviation sector has focused on international efforts, this article reveals the importance of understanding the tensions and complementarities of the two streams.


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States have agreed that anthropogenic emissions of greenhouse gases (GHG) are altering our climate system in dangerous ways. 1 Accordingly, they have agreed on the need Protocol) 10 leave it essentially for each party to decide whether its national policies and measures on climate change mitigation are to address the GHG emissions from international transportation. 11 Some national policies and measures do regulate GHG emissions from international civil aviation or apply indiscriminately to domestic and international aviation. However, even when they apply only to domestic aviation, national actions may interact with international civil aviation, for instance, by achieving economies of scale on the deployment of more efficient aircraft or by causing carbon leakage when less efficient aeroplanes are redeployed. As such, devising effective efforts to reduce the climate impact of aviation requires close coordination between national and international efforts.
This article has two complementary objectives. Firstly, it documents the international and national streams of mitigation action. It shows that the focus on ICAO initiatives is misplaced: thus far, international initiatives have failed to deliver tangible benefits, while national initiatives have been (partially) successful. Secondly, the article highlights the interactions between the two streams. In so doing, it illustrates the need to think about climate law beyond the traditional divide between national and international law, through what could be called a 'transnational' analysis. 12 The decarbonization of the aviation sector does not depend entirely on international or national action, but on the way in which the two interact.
The article is organized as follows. Section 2 flags the unique technological and legal challenges to climate change mitigation in the aviation sector. Sections 3 and 4 identify, respectively, international and national initiatives adopted to date. Section 5 analyzes the relations between these two streams.

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This section exposes the stakes and challenges of climate change mitigation in the aviation sector. It documents the sector's increasing climate impact before identifying the technological and regulatory challenges faced by any initiatives aimed at reducing this impact.

GHG Emissions from Aviation
Aviation is of particular concern for climate change mitigation for three reasons. Firstly, its impact on the climate system is significant. Data compiled by the Organisation for Economic Co-operation and Development (OECD) suggests that civil aviation resulted in 883 megatonnes (Mt) of carbon dioxide (CO 2 ) emissions in 2018, two-thirds of which from international flights. 13 Aviation CO 2 emissions represent about 2.4% of global anthropogenic CO 2 emissions. 14 However, these numbers do not account for all of aviation's climate impact: beside CO 2 , aircraft produce short-term radiative forcers (such as nitrous oxide and soot particles) and contrails, the climate impact of which is significant, albeit relatively poorly understood. 15 Recent studies suggest that the non-CO 2 climate impact of aviation may exceed its CO 2 impact, 16 especially on a short-to medium-term horizon. 17 When all factors are taken into account, in one estimate aviation contributes to 'roughly 5% of the total anthropogenic warming'. 18 Secondly, the climate impact of aviation is increasing more rapidly than that of almost any other sector. OECD statistics show that aviation's global CO 2 emissions increased by 28% from 2013 to 2018. 19 According to data from the International Energy Agency (IEA), emissions from international aviation increased 2.9 times from 1980 to 2017, compared with 1.9 times for overall CO 2 emissions from fuel combustion. 20 Aviation emissions have increased markedly more quickly than emissions from other modes of transportation. 21 Measures taken in response to the COVID-19 pandemic have led to emissions reductions, which, however, are not expected to alter longterm trends. 22 Air traffic is predicted to increase several fold during the first half of the 21 st century, which, despite market-driven fuel-efficiency gains, will significantly increase the sector's climate impact. 23 Thirdly, states are yet to find ways to reduce significantly the climate impact of aviation, especially with regard to international flights. While much of the increase in 14 Our calculation based on 2018 global GHG emissions (including land use, land-use change and forestry activities) from CAIT Climate Watch, available at: https://www.climatewatchdata.org. 15 D.S. Lee  Despite contrary suggestions from industry associations, 33 societies have yet to come up with realistic, long-term plans for climate-neutral modes of rapid, long-distance transportation. 34 Other modes of transportation (such as high-speed rail) provide realistic alternatives only to short-haul flights, whereas long-haul flights are responsible for most of the sector's climate impact. 35 Reliance on biofuel or hydrogen, or attempts to offset aviation emissions, would face challenges of their own. 36 On the one hand, the production of biofuel is likely to require large spans of arable land, 37 thus competing with food production or otherwise hindering the large-scale deployment of bioenergy with carbon capture and storage as a negative emissions technology. 38 On the other hand, producing clean hydrogen fuel would require substantial amounts of energy, which is unlikely to be economically available. 39 Overall, sustainable aviation fuels would only slightly reduce the non-CO 2 climate impacts of aviation. 40 As such, the only known way to substantially reduce aviation's climate impact in the long term appears to involve a limitation, and possibly a reduction, in aviation activities, an option that faces considerable political challenges.

The Regulatory Challenge
States have not been able to agree on a formula to allocate responsibility for the climate impacts of international civil aviation. Responsibilities could be attributed, for instance, on the basis of the state of departure, arrival or overflight, the nationality of the passengers or of the consumers of the cargo, the country of registration of the aircraft, or the nationality of the airline or that of the aircraft-leasing company. 41 Achieving a political consensus is shown to be impossible because any of these criteria would have important implications for the interests of some states, for instance, because they host international aviation hubs, register large airlines, or rely disproportionately on international tourism or trade. 42 Climate treaties have never formally defined, as a matter of principle, the geographical scope of states' general substantive obligations on climate change mitigation. Confusion often arises between the scope of two distinct obligations under the UNFCCC: the procedural obligation to communicate national GHG emissions inventories following methodologies agreed by the Conference of the Parties (COP) 43 and the substantive obligation to implement 'programmes containing measures to mitigate climate change'. 44 With regard to the former, the COP has agreed to the use of inventory guidelines adopted by the Intergovernmental Panel on Climate Change (IPCC), 45 which build on the assumption that each party should generally report the emissions that are generated within its territory, or, when this would not be practical, from the combustion of fuel sold within its territory. 46 As an exception, these guidelines suggest that international aviation and shipping emissions 'are to be excluded … from national totals' and 'should be reported separately'. 47 On the other hand, nothing in the UNFCCC implies that the latter, substantive obligation on climate change mitigation would be limited to the emissions that the party must inventory and report.
The Kyoto Protocol imposed quantified emissions limitation and reduction commitments on Annex I parties for two commitment periods from 2008 to 2020. 48 For the purpose of accounting for this quantified commitment, the Protocol relied on the IPCC inventory guidelines, 49 excluding emissions from international bunker fuels. The Protocol sought to address this gap by requiring each Annex I party to pursue regulation of international civil aviation through the ICAO. 50 The exclusion of international civil aviation from the scope of quantified commitments under the Kyoto Protocol does not necessarily imply that these emissions are excluded from the scope of the general mitigation obligation under the UNFCCC, nor does the commitment of Annex I parties to pursue an arrangement at the ICAO exclude international civil aviation from the scope of UNFCCC negotiations. Nothing in the UNFCCC or the Kyoto Protocol creates or implies any obligation for states not to regulate emissions from international aviation. 51 In fact, the COP continued to express concern for international aviation emissions after the adoption of the Protocol. When the COP observed that the GHG emissions of Annex I parties had generally decreased during the 1990s, it also expressed concern that their international aviation emissions had increased by more than 40%. 52 In 2011, the COP agreed 'to continue its consideration of issues related to addressing emissions from international aviation'. 53 No conclusion can be drawn from the absence of any mention of aviation in the Paris Agreement, as this treaty does not contain sectorspecific provisions. 54 The Agreement requires states to communicate and pursue nationally determined contributions (NDCs) 55 involving 'economy-wide' action (at least for developed country parties), 56 but it does not specify whether this includes international aviation. Consistently, some parties have expressly included international civil aviation within the scope of their successive commitments, pledges and strategies communicated under the UNFCCC and the Paris Agreement. 57

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This section explores the international initiatives negotiated under the aegis of the ICAO and assesses their limitations.

ICAO Initiatives
Negotiations on mitigation in the aviation sector have been convened by the ICAO, an international organization created by the Chicago Convention on International Civil Aviation, 58 which subsequently became a UN specialized agency. 59 The two main 51  organs of the ICAO involved in the negotiations are the Assembly of all 193 member states and the Council of 36 elected member states. 60 The Council may adopt and amend 'international standards and recommended practices and procedures' (SARPs) dealing with various aspects of international civil aviation, 61 which are designated as Annexes to the Chicago Convention 'for convenience'. 62 While SARPs are not directly legally binding, a member state is obligated to notify the ICAO if it will not comply with a SARP. 63 Despite the absence of an express mandate on environmental protection in the Chicago Convention, the Assembly has long sought to 'maintain the initiative in developing policy guidance' on environmental matters in the aviation sector so as 'not [to] leave such initiative to other organizations', 64 and the Council has adopted SARPs that address noise and local air pollution. 65 Consistently, since climate change has been identified as a global concern, the Assembly has sought to keep sectorial mitigation initiatives under the organization's purview. 66 However, it was only in the late 1990s that the Assembly started to envisage the adoption of 'policy options to limit or reduce' GHG emissions from civil aviation 67 (including 'technical solutions', 'market-based measures', and 'operational measures'). 68 Not before 2007 did it call upon the Council to 'examine the potential for carbon offset mechanisms'. 69 Since 2010, ICAO negotiations have led to the definition of aspirational goals, the adoption of technical standards, and the creation of a global market-based instrument.
Firstly, the Assembly defined two aspirational goals in 2010. One goal is to achieve 2% global fuel-efficiency improvement per year; 70 the other is to 'keep … the global net carbon emissions from international aviation from 2020 at the same level'. 71 However, as the Assembly emphasized, these goals do not 'attribute specific obligations to individual states'. 72 Secondly, following a 2013 Assembly Resolution, 73 the Council adopted fuel-efficiency standards in 2017. 74 Similar to Council standards on noise and local air pollution, these SARPs apply to new types of aircraft (from 2020) and to new aeroplanes (from 2028), but not to aircraft the airworthiness of which has previously been approved. The 2019 Assembly asked the Council to consider updating these standards. 75 Thirdly, a 2016 Assembly Resolution led the Council to create a market-based mechanism in 2018. 76 This mechanismthe Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA)seeks to make up for any increase in international aviation emissions from 2020 77 onwards by requiring airlines to acquire and cancel emission units from eligible emissions reduction projects. CORSIA starts with a 'pilot phase' (2021-23) and a 'first phase' (2024-26) to which states are invited to participate on a voluntary basis. It is only from its 'second phase' (2027-35) that CORSIA will be applicable to all member states other than least-developed countries, small-island developing states, and landlocked developing countries. 78 In each phase, offsetting requirements apply only to international flights between participating states. 79 The offsetting requirements are calculated initially on the basis of the evolution of global aviation emissions and allocated to each airline in proportion to its emissions. From 2030 onwards, some weight will gradually be given to the evolution of the airline's emissions, creating a stronger marginal incentive for each airline to limit its emissions. 80 The Council will review the implementation of CORSIA every three years from 2022. 81 The Assembly has taken note ofbut not endorsedthe goal, promoted by some industrial associations, of reducing the CO 2 emissions of international air transport by 50% by 2050, compared with 2005 levels. 82 Instead, the Assembly has agreed only on a 'global aspirational goal of keeping the global net CO 2 emissions from international aviation from 2020 at the same level', 83 an objective that CORSIA seeks to achieve.

Limitations
The ICAO has played a limited role in addressing climate change. Throughout the 1990s member states raised doubts over 'the extent to which civil aviation contributes to' climate change; despite the growing recognition of the need for a precautionary 75 ICAO Assembly Resolution A40-18 (2019) approach in international environmental law forums, they contended that the need for policymaking on climate change mitigation within the aviation regime had 'to be based on information which is as complete and accurate as possible'. 84 To date, the ICAO continues to ignore the non-CO 2 impacts of aviation, presumably on the ground of lack of 'complete' information. 85 While recognizing the importance of regulating aviation emissions in 2007, the Assembly also highlighted the need not to lose 'sight of their proper context in assessing overall GHG emissions' from other sectors. 86 Six years later, the Assembly was still expressing concerns that international aviation could be unfairly 'targeted' as a source of climate finance. 87 After the Copenhagen Accord and the Cancun Agreements defined the objective of holding global warming 'below' 2°C above pre-industrial temperatures, and suggested the need to consider the subsequent adoption of a 1.5°C target, 88 the ICAO Assembly only noted a somewhat watered-down objective that global warming 'ought not to exceed 2°C'. 89 From the early 1990s to the mid-2010s, the ICAO pre-empted sector-specific negotiations under the climate regime and opposed 'unilateral' measures, 90 while defining no concrete commitments. 91 On the contrary, the Assembly reaffirmed a policy recommendation for 'the reciprocal exemption from all taxes levied on fuel taken on board by aircraft in connection with international air service' and for the reduction or elimination of 'taxes related to the sale or use of international air transport'. 92 This recommendation was at odds with the growing expectation that states would phase out such fossil-fuel subsidies throughout the economy. 93 The measures that the ICAO did eventually take were immediately viewed as rather ineffective. The EU Member States and the European Civil Aviation Conference criticized the aspirational goals adopted by the Assembly in 2013 for failing to account for the non-CO 2 impacts of aviation. 94 In the same vein, the US and Canadian agencies concluded that the fuel-efficiency standards adopted by the Council were 'technology following standards' which would 'not result in reductions in fuel burn and GHG emissions'. 95 CORSIA is the only ICAO initiative so far that could possibly achieve tangible mitigation outcomes. Yet, it has major limitations. Firstly, it is not intended to (and will not) lead to a deep decarbonization of the aviation sector. Secondly, it ignores aviation's non-CO 2 climate impacts. Thirdly, it does not aim to reduce or limit international aviation CO 2 emissions but only to offset any increase. Fourthly, as at late 2021, several major players had not indicated their intention to participate in the pilot and first phases. 96 Fifthly, state compliance remains uncertain, all the more given the last-minute adoption of essential modalities of implementation and the relatively convoluted, opaque, and altogether rather 'ambiguous' legal status of SARPs. 97 Sixthly, the incremental incentive that CORSIA would ideally create for emissions reduction isat least in some analysesunlikely to induce technological innovation. 98 Seventhly, experience with international transfers of mitigation outcomes, in particular as offset credits, suggests that the complete environmental integrity of emissions reduction projects is unachievable in practice as a result of difficulties in ensuring the additionality of such projects, 99 avoiding double-counting, 100   mitigation in order to present 'additional' projects subsequently as income-generating offsetting. 103 In conclusion, the ICAO has patently failed to exercise leadership in mitigation action in international civil aviation over the last three decades. This lack of leadership is perhaps unsurprising given the ICAO objective of ensuring 'the safe and orderly growth of international civil aviation', 104 which has limited its focus to 'policy options that will reduce aircraft emissions without negatively impacting the growth of air transport'. 105 Beyond some incremental cost-saving efficiency improvements (which need no or minimal policy incentives and, at any rate, could be counter-productive), 106 effective action would inevitably impose some constraint on the sector. One may even argue that achieving substantial mitigation outcomes in international civil aviation implies the need to reconsider the objective of the sector's continued growth. 107

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Without waiting for the ICAO, many states have taken the initiative of adopting and implementing their own policies and measures. This section documents the six main types of initiative that they have taken.

National Targets
States have adopted various quantified pledges and commitments on climate change mitigation under climate treaties. These targets, especially those communicated by developed country parties, are increasingly expected or required to be 'economywide'. 108 While it is largely understood that domestic aviation ought to be included in such targets, some parties have extended them to international aviation. The EU 2020 pledge under the Cancun Agreements 109 and its 2030 NDC under the Paris Agreement 110 encompass emissions from the entire journey of any departing international flights.
In addition to NDCs, the Paris Agreement invites parties to communicate long-term low-GHG emissions-development strategies (LTS). 111 Parties whose NDC does not address international civil aviation have sometimes considered taking a different approach when devising their LTS. Switzerland decided that its 2050 net-zero emissions target would encompass emissions from departing international flights. 112 New Zealand established a statutory process to consider, by 2024, the inclusion of international aviation in its LTS. 113 After the United Kingdom (UK) communicated its LTS, 114 the government announced its intention to review it to include international aviation. A formal commitment is yet to be made, 115 but Scotland's own net-zero carbon target already covers international aviation emissions. 116 On the other hand, among the states whose LTS does not include international aviation, some have pledged to 'keep emissions from [international aviation] in mind when assessing the achievement of climate targets' 117 and affirmed a commitment to 'international regulation' in the sector. 118 Regrettably, no LTS has outlined a realistic plan for the decarbonization of aviation; instead, those that discuss the evolution of aviation emissions tend to speculate on unproven technological fixes (such as biofuel, 119 hydrogen, 120 and electricity), 121 or 'other measures which will drastically reduce emissions'. 122

Technical Standards
Various states have adopted technical standards on aircraft fuel efficiency and renewable-fuel content. National standards on fuel efficiency generally reflect the SARPs adopted by the ICAO Council in 2017. 123 Whereas the Chicago Convention applies only to international civil aviation, national measures typically extend the same standards to domestic civil aviation, thus ensuring the interoperability of aircraft.
The adoption of national fuel efficiency standards has not always been a simple topdown process of implementing ICAO decisions. Several states were already considering the adoption of national standards before the Council 2017 decision. 124 A US Court of Appeal held in 2011 that the Environmental Protection Agency (EPA) had to make 'endangerment findings' under the Clean Air Act with regard to GHG emissions from aircraft engines. 125 As such, the EPA was bound to adopt national fuel-efficiency standards notwithstanding the ICAO's 'initiative' (presumably an attempt of the ICAO to pre-empt national measures). While the ICAO standards imposed no additional efforts on airlines, 126 one cannot exclude the possibility that home-grown standards could have been stringent.
By contrast, standards on renewable-fuel content are entirely home-grown, apply only to some countries, and vary significantly. For instance, Norway introduced in 2020 a 0.5% biofuel-content requirement for aviation fuels. 127 To date, the EU has left it for each Member State to decide in which economic sector to promote the use of renewable energy (for instance, sustainable fuels), acknowledging 'the current technological and regulatory constraints that prevent the commercial use of biofuels in aviation'. 128 Several Member States are considering the adoption of a national biofuel-content standard or supporting the adoption of a pan-EU standard. 129 Some biofuel-producing countries have also been actively supporting research and development on sustainable aviation fuels. 130 In July 2021, the EU Commission proposed a Regulation which would impose a minimum sustainable-fuel content of 2% by 2025, increasing progressively to 63% by 2050. 131 With this Regulation the EU is seeking to exercise global leadership by driving economies of scale on the production of 'sustainable aviation fuel'. The Commission's Explanatory Memorandum highlights both the likelihood of '[s]pill-over effects … whereby third countries may consider adopting similar measures' 132 and the need for 'intensified efforts of the EU and its Member States' to establish a global mandate for sustainable aviation fuel at the ICAO. 133 However, many questions remain about the sustainability of such large-scale production of biofuels and synthetic fuel 134 and, more generally, about the non-CO 2 climate impacts that these fuels would not avoid. 135

Carbon Pricing
Many jurisdictions have adopted multi-sectoral carbon-pricing instruments ranging from simple taxes to more complex cap-and-trade mechanisms. 136 Some jurisdictions have extended the application of these instruments to domestic and, more rarely, international aviation. For instance, British Colombia's carbon tax applies to the purchase of aviation fuel, 137 while Canada imposes a federal carbon charge on the use of aviation fuels in provinces that lack their own carbon-pricing policy. 138 Multi-sectoral cap-and-trade mechanisms have been applied to aviation emissions in the EU, 139 South Korea, 140 New Zealand, 141 Switzerland, 142 and the UK. 143 Half of China's pilot emissions trading schemes (implemented by local governments) included some aviation emissions, 144 and the national government has considered including domestic civil aviation in the scope of a national scheme, 145 although not in its initial phase. 146 By contrast, California (US) and Quebec (Canada) do not include aviation emissions within their cap-and-trade mechanisms. 147 Applicable carbon-pricing instruments are typically limited to intra-jurisdictional flights, for instance, within New Zealand or within South Korea. 148 Similarly, the carbon tax of the Canadian province of British Colombia applies exclusively to flights within the province, 149 whereas Canada's federal charge applies only to flights that take place within or between listed provinces subject to the Pan-Canadian carbon-pricing regulation (for instance, Ontario). 150 As such, flights between two jurisdictions that are individually subject to their own carbon-pricing instruments (such as from New Zealand to South Korea, or from British Columbia to Ontario) are not covered by any national carbon-pricing instrument. Given the prevalence of emissions from medium-to long-range flights, this observation highlights the limited effectiveness of national carbon-pricing instruments, especially those applicable to smaller jurisdictions.
The EU Emissions Trading Scheme (ETS), by contrast, applies directly to 30 countries 151 and is integrated with two other national ETSs: 152 it 'prices' emissions from international flights between any of the 32 participating countries. In addition, the EU initially decided to extend its ETS to all international flights which either depart from, or arrive at, an airport within the territory of a participating country (including flights originating from or destined for third countries). 153 This measure proved to be politically contentious, and questions were raised about its compatibility with international aviation law. 154 The EU eventually agreed to suspend the application of its ETS to flights to and from third countries in the light of the progress made, largely as a reaction to the EU initiatives, at the ICAO towards a global market-based mechanism. 155 Following the adoption of CORSIA, the EU is likely to continue to apply the ETS to intra-regional flights, while applying CORSIA only to other international flights. 156 Notwithstanding their level of regulatory sophistication, carbon-pricing mechanisms are only as effective as the price they impose on GHG emissions. Taxes imposed on aviation fuel are relatively low, 157 while cap-and-trade mechanisms often rely largely on free allocation of allowances ('grandfathering'), which further reduces average prices. 158 Like CORSIA, these national carbon-pricing mechanisms do not take into consideration the non-CO 2 impacts of aviation. The resulting price signal is a small fraction of fuel prices, creating little additional incentive for airlines to reduce emissions, or for passengers to fly less. For instance, the EU Commission estimates that the EU ETS imposes an increase in flight ticket prices of only 0.4 to 0.9%. 159

Ticket Taxes
Various jurisdictions have imposed taxes on flight tickets. These taxes may aim to cover the cost of running airports and civil aviation services, or they may be devised as carbon-pricing mechanisms, but in practice the distinction is not always obvious. France, 160 Germany, 161 the Netherlands, 162 Sweden, 163 and the UK 164 levy fixed-rate taxes on passengers of any domestic or international flight, with varying rates applicable to groups of countries for different distances and, sometimes, according to the flight classthus reflecting, albeit approximately, the greater climate impact of longhaul flights and premium classes. For instance, a long-haul international passenger leaving the UK would need to pay £26 in economy, £176 in business or first class, or £528 on a private aircraft. 165 Ticket taxes do not always apply in the same way to domestic and international flights. For instance, the US imposes a small, fixed-rate tax on all domestic or international flights and an additional tax, proportional to the price of the ticket, on domestic flights. 166 By contrast, Australia 167 and South Africa 168 impose only one, fixed-rate tax on outbound international flights perhaps because, unlike the US and many other jurisdictions, these two countries subject domestic flights to consumer taxes.
Ticket taxes could incentivize consumers to consider alternative modes of transportation or to travel less. On the other hand, unlike direct carbon pricing, ticket taxes do not incentivize efficiency gains, as the amount to be paid by a passenger does not depend on the flight's actual per passenger emissions. The effectiveness of ticket taxes is also limited by the frequent exemption of transiting and transferring passengers (aimed at avoiding trade distortions), 169 which often applies to high-emitting long-haul flights. Lastly, fewer jurisdictions levy equivalent taxes on cargo flights. 170

Phasing Out Tax Exemptions
States have long supported aviation, sometimes through direct subsidies to airlines or airports and other infrastructures, 171 but more often by exempting airlines and their passengers from various taxes to which they would otherwise be subject. Most jurisdictions exempt civil aviation from fuel excise duties 172 or apply a discounted rate. 173 In addition, airlines engaging in international transportation are often exempted from value-added tax (VAT) on the goods and services they purchase (including fuel), 174 and passengers are not required to pay VAT on the purchase of tickets. 175 A 2019 OECD survey of 44 OECD and G20 members concluded that fuels used in domestic aviation are 'sometimes taxed but rarely reflect a low-end carbon benchmark', while fuels used in international aviation are not taxed 'at all'. 176 Most states also provide for income tax exemptions, 177 following here also ICAO policy recommendations. 178 These tax exemptions go against a growing trend towards the phasing out of fossilfuel subsidies. Following a series of declarations of the G20, 179 states agreed at COP26 that they should 'accelerat[e] efforts towards the … phase-out of inefficient fossil fuel subsidies'. 180 Both the EU and the US have engaged in comprehensive reviews of these subsidies, 181 and the EU is considering a directive that would require phasing out the tax exemption on aviation fuels for intra-EU flights while allowing states to do the same for extra-EU flights '[w]ithout prejudice to international obligations'. 182 The support that tax exemptions provide to civil aviation far outbalances the effect of even the most stringent carbon-pricing policies. For instance, an average airline flying within the EU in 2019 had to pay a marginal price of €0.062 per litre of kerosene for emissions allowances; this resulted (taking into account the free allocation of 85% allowances) in an average cost of €0.009 per litre. 183 By contrast, were it not for their exemption of excise duties, airlines would need to pay at least the pan-EU minimal rate of €0.33 per litre of kerosene. 184 Phasing out the exemption of EU excise duties would roughly double airlines' fuel acquisition costs in this market, 185 creating a clear incentive for airlines to reduce emissions or for passengers to fly less. A study commissioned by the EU Commission suggests that phasing out the exemption of fuel excise tax on airlines would increase ticket prices by 10% and reduce aviation CO 2 emissions by 11%; 186 which would be an order of magnitude more effective than the EU ETS. 187 Likewise, the benefits of VAT exemption on ticket sales often outbalances the costs imposed by ticket taxes. 188 By contrast to VAT, ticket taxes rarely increase the overall price of the ticket by more than a single-digit percentage point. 189 In one estimate, imposing a 19% VAT rate on all flights departing from the EU could achieve an 18% reduction in CO 2 emissions from such flights. 190

Sectorial Governance
Beyond subsidies, states are often active partakers in the aviation sector: they influence the sector's development, for instance, by approving airport construction and by regulating routes and traffic. Sectorial policies have relatively complex implications for the climate impacts of aviation. For instance, limited airport capacity, if ill-managed, can cause traffic congestion, resulting in additional CO 2 emissions, 191 but a scarcity of landing and take-off slots could also incentivize airlines to maximize seat occupancy, hence improving carbon efficiency. 192 In many jurisdictions the climate impacts of relevant projects are subject to environmental assessment procedures, although national authorities generally maintain considerable discretion in making the final decision. 193 Improving routing and traffic management could reduce aviation's climate impact while also improving its economic efficiency. Accordingly, several jurisdictions have adopted or are considering measures to promote, for instance, more direct routes and continuous descent approaches. 194 In general, these measures apply indiscriminately to domestic and international aviation but, like other efficiency-driven measures, they run the risk of a rebound effect. 195

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This last section examines the interactions between national and international mitigation actions in the aviation sector. It shows that these two streams of mitigation action are generally compatible from a legal perspective and complementary from a policy perspective. In fact, the rapid decarbonization of the aviation sector is unlikely without a combination of national and international initiatives.

Compatibility
One could question whether national mitigation action in the aviation sector is compatible with states' international law obligations. As far as international climate law is concerned, the question is relatively straightforward: notwithstanding whether international climate law requires states to implement mitigation action in international aviation, it certainly does not prevent such measures. 196 A more complex question relates to the compatibility of national mitigation initiatives with international aviation law. It bears repeating that reducing air transport may be the only way to reduce its climate impact substantially: 197 as such, there is an obvious tension, even possibly a contradiction, between the climate regime's objective of carbon neutrality 198 and the ICAO objective of a 'safe and orderly growth of international civil aviation'. 199 If a direct normative conflict is avoided on the international front between climate and aviation law, it is in large part as a result of the open-ended and bottom-up nature of commitments on climate change mitigation. 200 However, the tension between these two policy objectives re-emerges when states seek to implement their obligations. Aviation pundits have interpreted the Chicago Convention as 'placing strong legal constraints on unilateral action'. 201 The ICAO's attempt at keeping mitigation action under its purview 202 reflects a distrust not only of initiatives adopted under the UNFCCC regime, but also of any national ('unilateral') measures 'which would adversely affect the orderly development of international civil aviation'. 203 There remains undoubtedly a regulatory space for national mitigation action in the aviation sector. Few, if any, observers have questioned the right of states to decide how to develop national infrastructure and manage air traffic within their territory, to adopt non-discriminatory technical standards on planes or fuel, or to define national mitigation targets applicable to all aviation activities under their control. Rather, legal controversies have centred on the legality of two types of national measure: the phasing out of tax exemptions and the imposition of a carbon price. 204 As the following shows, some bilateral air service agreements require states to maintain some tax exemptions, but many other national mitigation measures in the aviation sector, including carbonpricing mechanisms, appear to be consistent with international aviation law.

Tax exemptions phase-out
Exemptions from excise duties and VAT impede climate change mitigation in the aviation sector. These exemptions are often presented as a legal requirement under international aviation law. 205 Yet, such requirement arises not from any general norms reflected in multilateral treaties, but from ad hoc bilateral arrangements, which a state is more likely to be able to renegotiate.
The Chicago Convention does not require its contracting parties to exempt airlines from VAT or excise duties. The ICAO Council interpreted this treaty as making a distinction between 'charges', referring to 'levies to defray the costs of providing facilities and services for civil aviation', and 'taxes', which are 'levies to raise general national and local government revenues that are applied for non-aviation purposes'. 206 In that sense, VAT and excise duties are not 'charges' but 'taxes'. Yet, while the Chicago Convention regulates the imposition of 'charges', it is silent on the issue of 'taxes'. Article 15 is interested only in 'airport and similar charges'. 207 While Article 24 prohibits the imposition of 'customs duty' on any fuel on board an international flight on arrival and retained on board until departure, it does not preclude the imposition of excise duty on the purchase of additional fuel. 208 The ICAO Council has formulated 'policies' advocating an exemption for international civil aviation from excise duties and VAT, 209 though these are not legally binding. The Assembly has recommended compliance with these policies but, since 1992, it has also noted that 'taxes are increasingly being imposed by some states in respect of certain aspects of air transportation'. 210 The latter observation rules out any argument based on the interpretation of the Chicago Convention in the light of subsequent practice of the parties 211 as it establishes that this practice lacks consistency. Since 2016, the Assembly has reflected the evolution of state practice by putting more emphasis on the need to avoid 'discriminatory taxes' (referring to discrimination between aviation and other modes of transport) and 'double taxation', 212 rather than calling for a complete exemption. VAT or excise duties are unlikely to be imposed in ways that discriminate against aviation 213 or result in double taxation. 214 Rather than the Chicago Convention, the main legal obstacle to imposing taxes on international civil aviation lies in the (mainly bilateral) air service agreements through which states allow one another's airlines to operate flights between or within their territories. 215 Some of these treaties provide for excise duty exemptions, on the basis of reciprocity, for the fuel used for these flights. 216 By contrast to multilateral treaties, these bilateral agreements can more easily be modified through bilateral negotiations. The EU has already promoted air service agreements that allow the application of excise duties on fuel, at least, for intra-EU flights operated by foreign airlines. 217 Overall, as far as the authors are aware, no air service agreement prevents VAT and ticket taxes, provided that such measures are applied in a fair and non-discriminatory manner to national and foreign airlines.

Carbon-pricing mechanisms
The unilateral application of carbon-pricing mechanisms to international aviation has also proved to be controversial. 218 Yet, neither carbon taxes nor cap-and-trade mechanisms are among the charges and customs duties prohibited under the Chicago Convention. Even if one were to find that these instruments create 'charges' aimed at defraying 'the costs of providing facilities and services for civil aviation', 219 these charges would not fall within the scope of the prohibition, under Article 15, of charges imposed 'in respect solely of' an aeroplane's transit over, entry into, or exit from the state's territory, 220 as these mechanisms are imposed in respect of (and in proportion to) an aeroplane's climate impacts. Nor do carbon-pricing mechanisms create customs duties levied for the importation of fuel 'retained on board', which would be prohibited under Article 24. 221 Articles 15 and 24 do not prevent a state from requiring airlines to take responsibility for their environmental impact, at least in relation to activities taking place within the state's own territory. 222 A more probable legal obstacle to carbon-pricing mechanisms arises, here again, from bilateral air service agreements that prohibit the imposition of excise duties. For practical reasons, carbon taxes are typically levied on the purchase of fuelin much the same way as excise dutiesrather than directly on the combustion of fuel or on CO 2 emissions. 223 A distinction arguably could be made between carbon taxes and excise duties on the ground that the former apply only to non-sustainable fuels and that revenues are often directed to climate-related funds rather than to the state's general budget. At any rate, provisions on excise duties exemptions in bilateral air services agreements are less likely to affect cap-and-trade mechanisms that impose liabilities on fuel use rather than fuel purchase. 224 The ICAO Assembly and Council have appeared to recognize the lawfulness of national carbon-pricing mechanisms, in particular cap-and-trade mechanisms. This acceptance was implied, in 2010, when the Assembly adopted a set of guiding principles that states should follow when implementing 'market-based mechanisms'. 225 Subsequently, the Assembly noted that CORSIA 'is the only global market-based measure applying to CO 2 emissions from international aviation' and highlighted the need 'to avoid a possible patchwork of duplicative State or regional MBMs', 226 but these observations do not outlaw national or regional market-based mechanisms. As China then observed, the ICAO Assembly 'has no right to prohibit Member States from using other market measures for addressing aviation emission'. 227 The concern of the ICAO Assembly with national carbon-pricing mechanisms relates not to their legality but to their political opportunity, especially in conjunction with the adoption of CORSIA. Since 2010, the Assembly has expressed concern that market-based mechanisms could become 'duplicative', and it has suggested that 'international aviation CO 2 emissions should be accounted for only once'. 228 This concern was initially far-fetched, as no market-based mechanism applied to any international aviation emissions beyond the EU ETS, and it remains unfounded as most of aviation's climate impact remains entirely unaccounted for. As CORSIA aims only to offset any increase in CO 2 emissions from international civil aviation, overlapping national or regional carbon-pricing mechanisms could be justified as addressing other aspects of aviation's climate impactincluding avoiding (rather than offsetting) an increase in CO 2 emissions, addressing pre-existing levels of CO 2 emissions, and limiting non-CO 2 impacts of international civil aviation.

ICAO shortcomings
There are some obvious reasons to wish that states would reach an agreement on effective and ambitious mitigation action via multilateral negotiations convened by the ICAO, rather than taking separate initiatives. The very existence of the ICAO reflects state understanding that unilateral measures may seek to offer a competitive advantage for national airlines and airports 229 or otherwise create a more complex regulatory environment. From a climate perspective, national initiatives run the risk of merely displacing emissions if, for instance, less efficient aeroplanes are redeployed on unregulated routes; 230 they could even be counter-productive if passengers fly longer routes to evade carbon-pricing instruments.
Yet, the ICAO has a limited political capacity to initiate effective mitigation action. In line with the text of the 1944 Chicago Convention, the ICAO has clearly placed the growth of international civil aviation before any environmental or social concerns, such as climate change mitigation. 231 The ICAO has considered measures aimed at improving aviation's carbon intensity, 232 but not measures intended to limit or reduce aviation activities altogether. It has not advanced a coherent, long-term vision of the sector's decarbonization, 233 nor has it questioned the need for, or the possibility of, continued growth in the sector. Arguably, the need to foster interaction and cooperation among nations through the growth of international civil aviation was far more obvious in the midst of the Second World War than in the age of globalization and virtual reality. Overall, the opacity of ICAO negotiations seems to benefit industrial lobbies far more than environmental advocates. 234 Altogether, the ICAO strategy on climate change mitigation appears to be mainly dilatory: the ICAO took 'initiative' (as it once admitted) only when and in so far as this was necessary to prevent other organizations, or states, from adopting more effective measures. 235 It was only after the US EPA was mandated to define national fuel-efficiency standards that the Council adopted the first SARPs on CO 2 emissions, pre-empting the adoption of national standards that could have been more effective. 236 Likewise, despite decades of negotiations, it was not until the EU applied its ETS to every international flight that the ICAO could agree to set up a global market-based mechanism. 237 The subsequent implementation of CORSIA would lead the EU to reduce its efforts on mitigation in the aviation sector significantly if it does not decide to maintain its ETS for intra-EU international flights (thus applying CORSIA only to extra-EU international flights). 238

The need for national initiative
States have the capacity to implement various policies and measures that contribute to mitigating climate change in the aviation sector, including from international flights. Their NDCs and long-term strategies under the Paris Agreement could seek not only to improve aviation's climate efficiency, but also to limit or reduce societies' reliance on aviation, an approach that cannot easily be pursued under the ICAO. They can do so, for instance, by developing alternative modes of transportation (such as highspeed rail) as well as alternatives to transportation (such as local resorts and teleconferencing). The policy tools available to states in this respect range from fiscal policies, carbon pricing, and technical standards, to infrastructure development, land-use regulation, and subsidies.
The ICAO distrust of 'unilateral' measures 239 is unwarranted: while a state's initiative could always be considered 'unilateral', 240 the national initiatives on climate change mitigation in the aviation sector are not usually promoting national interests at the expense of foreign and global interests. 241 A state is typically not pursuing any uniquely national interest when implementing carbon-pricing measures, phasing out subsidies, or imposing technical standards on aircraft. Indeed, when these measures do distort international competition, it is almost always at the expense of the state's own airlines and airports. Contrary to what the word 'unilateral' may suggest, these measures advance a global objectiveclimate change mitigationwhich every ICAO member state has agreed to pursue. 242 While there are legal constraints on the measures that states may impose, states retain significant regulatory space to implement measures aimed at addressing the climate impact of the aviation sector. The larger jurisdictions, in particular, have considerable market power to set de facto global standards, for instance, on aircraft efficiency, 243 and they could even implement carbon-pricing mechanisms in cooperation with other parties interested in exercising global leadership. They also have diplomatic leverage to facilitate the adoption of a new generation of bilateral air service agreements allowing a systematic phase-out of tax exemptions. States would be able to give a clear signal to the ICAO and the aviation industry that, if no effective long-term strategy is formulated for the decarbonization of aviation, the world will have to find a longterm strategy without air transportation.

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This article has shown the existence of two distinct streams of mitigation initiatives in the aviation sector. While the ICAO has claimed exclusive competence on the regulation of international civil aviation, neither climate nor aviation law prevents national initiatives aimed at reducing the impacts of aviation, including international civil aviation, on the climate system. In fact, while the ICAO is yet to achieve any significant mitigation outcomes, national initiatives have long resulted in the implementation of effective measures, including direct and indirect carbon pricing, infrastructure development, biofuel-content requirements, and air traffic management.
Overall, our analysis has shed critical light on ICAO initiatives on climate change, which too often have been aimed at forestalling the implementation of more effective national measures. CORSIA should be saluted as the first global market-based mechanism, but one must also keep in mind its limited aim (to offset increases in the sector's CO 2 emissions, without addressing pre-existing emission levels and non-CO 2 impacts) and the foreseeable difficulties of ensuring the environmental integrity of offsetting units. There is a risk that, like previous ICAO initiatives, 244 CORSIA could constitute a red herring, distracting the attention of regulators from the need for far more effective actionthe type of action that, so far, has been undertaken only through national initiatives. As the ICAO has proved unable to exercise true leadership over the last three decades, it is in the national stream that lies the best hopes for the implementation of effective mitigation action and the definition of a long-term decarbonization strategy for the aviation sector.