Editing IPCC:AR6/WGIII/Chapter-10 (section)

== 10.5 Decarbonisation of Aviation ==

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This section addresses the potential for reducing GHG emissions from aviation. The overriding constraint on developments in technology and energy efficiency for this sector is safety. Governance is complex in that international aviation comes under the International Civil Aviation Organization (ICAO), a specialised UN agency. The measures to reduce GHG emissions that are considered include both in-sector (technology, operations, fuels) and out of sector (market-based measures, high-speed rail modal shift/substitution). Demand management is not explicitly considered in this section, as it was discussed in 10.2. A limited range of scenarios to 2050 and beyond are available and assessed at the end of the section.

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=== 10.5.1 Historical and Current Emissions from Aviation ===

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Aviation is widely recognised as a ‘hard-to-decarbonise’ sector ( [[#Gota--2019|Gota et al. 2019]] ) having a strong dependency on liquid fossil fuels and an infrastructure that has long ‘lock-in’ timescales, resulting in slow fleet turnover times. The principal GHG emitted is CO 2 from the combustion of fossil fuel aviation kerosene (‘Jet-A’), although its non-CO 2 emissions can also affect climate ( [[#10.5.2|Section 10.5.2]] ). International emissions of CO 2 are about 65% of the total emissions from aviation ( [[#Fleming--2019|Fleming and de Lépinay 2019]] ), which totalled approximately 1 Gt of CO 2 in 2018. Emissions from this segment of the transport sector have been steadily increasing at rates of around 2.5% per year over the last two decades (Figure 10.10), although for the period 2010 to 2018 the rate increased to roughly 4% per year. The latest available data (2018) indicate that aviation is responsible for approximately 2.4% of total anthropogenic emissions of CO 2 (including land-use change) on an annual basis (using IEA data, IATA data and global emissions data of [[#Le%20Quéré--2018b|Le Quéré et al. (2018b)]] ).

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[[File:0e3afeef6549b264bebaddfc9c55a1f1 IPCC_AR6_WGIII_Figure_10_10.png]]

'''Figure 10.10 | Historical global emissions of CO''' 2 '''from aviation, along with capacity and transport work (given in available seat kilometres, ASK; revenue passenger-kilometres, RPK).''' Source: adapted from [[#Lee--2021|Lee et al. (2021)]] using IEA and other data.

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=== 10.5.2 Short-lived Climate Forcers and Aviation ===

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Aviation’s net warming effect results from its historical and current emissions of CO 2 , and non-CO 2 emissions of water vapour, soot, sulphur dioxide (from sulphur in the fuel), and nitrogen oxides (NO x , = NO + NO 2 ) ( [[#IPCC--1999|IPCC 1999]] ; [[#Lee--2021|Lee et al. 2021]] ; [[#Szopa--2021|Szopa et al. 2021]] ). Although the effective radiative forcing (ERF) of CO 2 from historic aviation emissions is not currently the largest forcing term, it is difficult to address because of the sector’s current dependency on fossil-based hydrocarbon fuels and the longevity of CO 2 . A residual of emissions of CO 2 today will still have a warming effect in many thousands of years ( [[#Archer--2009|Archer et al. 2009]] ; [[#Canadell--2021|Canadell et al. 2021]] ) whereas water vapour, soot, and NO x emissions will have long ceased to contribute to warming after some decades. As a result, CO 2 mitigation of aviation to net zero levels, as required in 1.5°C scenarios, requires fundamental shifts in technology, fuel types, or changes of behaviour or demand.

The non-CO 2 effects of aviation on climate fall into the category of short-lived climate forcers (SLCFs). Emissions of NO x currently result in net positive warming from the formation of short-term ozone (warming) and the destruction of ambient methane (cooling). If the conditions are suitable, emissions of soot and water vapour can trigger the formation of contrails ( [[#Kärcher--2018|Kärcher 2018]] ), which can spread to form extensive contrail-cirrus cloud coverage. Such cloud coverage is estimated to have a combined ERF that is about 57% of the current net ERF of global aviation ( [[#Lee--2021|Lee et al. 2021]] ), although a comparison of cirrus cloud observations under pre- and post-COVID-19 pandemic conditions suggest that this forcing could be smaller ( [[#Digby--2021|Digby et al. 2021]] ). Additional effects from aviation from aerosol-cloud interactions on high-level ice clouds through soot ( [[#Chen--2013|Chen and Gettelman 2013]] ; [[#Zhou--2014|Zhou and Penner 2014]] ; [[#Penner--2018|Penner et al. 2018]] ), and lower-level warm clouds through sulphur ( [[#Righi--2013|Righi et al. 2013]] ; [[#Kapadia--2016|Kapadia et al. 2016]] ) are highly uncertain, with no best estimates available ( [[#Lee--2021|Lee et al. 2021]] ). In total, the net ERF from aviation’s non-CO 2 SLCFs is estimated to be approximately 66% of aviation’s current total forcing. It is important to note that the fraction of non-CO 2 forcing to total forcing is not a fixed quantity and is dependent on the recent history of growth (or otherwise) of CO 2 emissions ( [[#Klöwer--2021|Klöwer et al. 2021]] ). The non-CO 2 effects from aviation are the subject of discussion for mitigation options ( [[#Arrowsmith--2020|Arrowsmith et al. 2020]] ). However, the issues are complex, potentially involving technological and operational trade-offs with CO 2 .

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=== 10.5.3 Mitigation Potential of Fuels, Operations, Energy Efficiency, and Market-based Measures ===

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'''Technology options for engine and airframe.''' For every kilogram of jet fuel combusted, 3.16 kg CO 2 is emitted. Engine and airframe manufacturers’ primary objective, after safety issues, is to reduce direct operating costs, which are highly dependent on fuel burn. Large investments have gone into engine technology and aircraft aerodynamics to improve fuel burn per kilometre ( [[#Cumpsty--2019|Cumpsty et al. 2019]] ). There have been major step changes in engine technology over time, from early turbojet engines to larger turbofan engines. However, the basic configuration of an aircraft has remained more or less the same for decades and will likely remain at least to 2037 ( [[#Cumpsty--2019|Cumpsty et al. 2019]] ). Airframes performance has improved over the years with better wing design, but large incremental gains have become much harder as the technology has matured. For twin-aisle aircraft, generally used for long ranges, fuel-burn is a pressing concern and there have been several all-new aircraft designs with improvements in their lift-to-drag ratio ( [[#Cumpsty--2019|Cumpsty et al. 2019]] ). The principal opportunities for fuel reduction come from improvements in aerodynamic efficiency, aircraft mass reduction, and propulsion system improvements. In the future, [[#Cumpsty--2019|Cumpsty et al. (2019)]] suggest that the highest rate of fuel burn reduction achievable for new aircraft is likely to be no more than about 1.3% per year, which is well short of ICAO’s aspirational goal of 2% global annual average fuel efficiency improvement. Radically different aircraft shapes, like the blended wing body (where the wings are not distinct from the fuselage), are likely to use about 10% less fuel than future advanced aircraft of conventional form ( [[#Cumpsty--2019|Cumpsty et al. 2019]] ). Such improvements would be ‘one-off’ gains, do not compensate for growth in emissions of CO 2 expected to be in excess of 2% per annum, and would take a decade or more to penetrate the fleet completely. Thus, the literature does not support the idea that there are large improvements to be made in the energy efficiency of aviation that keep pace with the projected growth in air transport.

'''Operational improvements for navigation''' ''.'' From a global perspective, aircraft navigation is relatively efficient, with many long-haul routes travelling close to great circle trajectories, and avoiding headwinds that increase fuel consumption. The ICAO estimates that flight inefficiencies on a global basis are currently of the order 2% to 6% ( [[#ICAO--2019|ICAO 2019]] ), while [[#Fleming--2019|Fleming and de Lépinay (2019)]] project operational improvements (air traffic management) of up to 13% on a regional basis by 2050. ‘Intermediate stop operations’ have been suggested, whereby longer-distance travel is broken into flight legs, obviating the need to carry fuel for the whole mission. [[#Linke--2017|Linke et al. (2017)]] modelled this operational behaviour on a global basis and calculated a fuel saving of 4.8% over a base case in which normal fuel loads were carried. However, this approach increases the number of landing/take-off cycles at airports. ‘Formation flying’, which has the potential to reduce fuel burn on feasible routes, has also been proposed ( [[#Xu--2014|Xu et al. 2014]] ; [[#Marks--2021|Marks et al. 2021]] ).

'''Alternative biofuels, synthetic fuels, and liquid hydrogen.''' As noted above, the scope for reducing CO 2 emissions from aviation through improved airplane technology or operations is limited and unable to keep up with the projected growth, let al.ne reduce beyond the present emission rate at projected levels of demand (assuming post-pandemic recovery of traffic). Thus, the literature outlined here suggests that the only way for demand for aviation to continue to grow without increasing CO 2 emissions is to employ alternative lower-carbon bio- or synthetic aviation fuels ( [[#Klöwer--2021|Klöwer et al. 2021]] ). For shorter ranges, flights of light planes carrying up to 50 passengers may be able to use electric power ( [[#Sahoo--2020|Sahoo et al. 2020]] ) but these planes are a small proportion of the global aviation fleet ( [[#Epstein--2019|Epstein and O’Flarity 2019]] ; [[#Langford--2020|Langford and Hall 2020]] ) and account for less than 12% of current aviation CO 2 emissions. Alternative lower-carbon footprint fuels have been certified for use over recent years, principally from bio-feedstocks, but are not yet widely available at economic prices ( [[#Kandaramath%20Hari--2015|Kandaramath Hari et al. 2015]] ; [[#Capaz--2021a|Capaz et al. 2021a]] ). In addition, alternative fuels from bio-feedstocks have variable carbon footprints because of different lifecycle emissions associated with various production methods and associated land-use change ( [[#de%20Jong--2017|de Jong et al. 2017]] ; [[#Staples--2018|Staples et al. 2018]] ; [[#Capaz--2021b|Capaz et al. 2021b]] ; [[#Zhao--2021|Zhao et al. 2021]] ).

The development of ‘sustainable aviation fuels’ (referred to as ‘SAFs’) that can reduce aviation’s carbon footprint is a growing area of interest and research. Alternative aviation fuels to replace fossil-based kerosene have to be certified to an equivalent standard as Jet-A for a variety of parameters associated with safety issues. Currently, the organisation responsible for aviation fuel standards, ASTM International, has certified seven different types of sustainable aviation fuels with maximum blends ranging from 10% to 50% ( [[#Chiaramonti--2019|Chiaramonti 2019]] ). Effectively, these blend requirements limit the amount of non-hydrocarbon fuel (e.g., methanol) that can be added at present. While there currently is a minimum level of aromatic hydrocarbon contained in jet fuel to prevent ‘O-ring’ shrinkage in the fuel seals ( [[#Khandelwal--2018|Khandelwal et al. 2018]] ), this minimum level can likely be lower in the medium to long term, with the added benefits of reduced soot formation and reduced contrail cirrus formation ( [[#Bier--2017|Bier et al. 2017]] ; [[#Bier--2019|Bier and Burkhardt 2019]] ).

Bio-based fuels can be produced using a variety of feedstocks including cultivated feedstock crops, crop residues, municipal solid waste, waste fats, oils and greases, wood products and forestry residues ( [[#Staples--2018|Staples et al. 2018]] ). Each of these different sources can have different associated lifecycle emissions, such that they are not net zero CO 2 emissions but have associated emissions of CO 2 or other GHGs from their production and distribution ( [[#10.3|Section 10.3]] , Box 10.2). In addition, associated land-use change emissions of CO 2 represent a constraint in climate change mitigation potential with biofuel ( [[#Staples--2017|Staples et al. 2017]] ) and have inherent large uncertainties ( [[#Plevin--2010|Plevin et al. 2010]] ). Other sustainability issues include food vs fuel arguments, water resource use, and impacts on biodiversity. Cost-effective production, feedstock availability, and certification costs are also relevant ( [[#Kandaramath%20Hari--2015|Kandaramath Hari et al. 2015]] ). Nonetheless, bio-based SAFs have been estimated to achieve lifecycle emissions reductions ranging between approximately 2% and 70% under a wide range of scenarios ( [[#Staples--2018|Staples et al. 2018]] ). For a set of European aviation demand scenarios, [[#Kousoulidou--2016|Kousoulidou and Lonza (2016)]] estimated that the fuel demand in 2030 would be about 100 million tonnes of oil equivalent and biokerosene (HEFA/HVO) penetration would provide around 2% of the total fuel demand at that date. Several issues limit the expansion of biokerosene for aviation, the primary one being the current cost of fossil fuel compared to the costs of SAF production ( [[#Capaz--2021a|Capaz et al. 2021a]] ). Other hybrid pathways, for example the hydrogenation of biofuels (the hydrogen assumed to be generated with low-carbon energy), could increase the output and improve the economic feasibility of bio-based SAF ( [[#Hannula--2016|Hannula 2016]] ; [[#Albrecht--2017|Albrecht et al. 2017]] ).

Costs remain a major barrier for bio-SAF, which cost around three times the price of kerosene ( [[#Kandaramath%20Hari--2015|Kandaramath Hari et al. 2015]] ). Clearly, for SAFs to be economically competitive, large adjustments in prices of fossil fuels or the introduction of policies is required. [[#Staples--2018|Staples et al. (2018)]] estimated that in order to introduce bio-SAFs that reduce lifecycle GHG emissions by at least 50% by 2050, prices and policies were necessary for incentivisation. They estimate the need for 268 new biorefineries per year and capital investments of approximately 22 to 88 billion USD2015 per year between 2020 and 2050. [[#Wise--2017|Wise et al. (2017)]] suggest that carbon prices would help leverage production and availability.

Various pathways have been discussed for the production of non-bio SAFs such as power-to-liquid pathways ( [[#Schmidt--2018|Schmidt et al. 2018]] ), sometimes termed ‘electro-fuels’ ( [[#Goldmann--2018|Goldmann et al. 2018]] ), or more generalised ‘Power-to-X’ pathways ( [[#Kober--2019|Kober and Bauer 2019]] ). This process would involve the use of low-carbon electricity, CO 2 , and water to synthesise jet fuel through the Fischer-Tropsch process or methanol synthesis. Hydrogen would be produced via an electrochemical process, powered by low-carbon energy and combined with CO 2 captured directly from the atmosphere or through BECCS. The energy requirement from photovoltaics has been estimated to be of the order 14 to 20 EJ to phase out aviation fossil fuel by 2050 ( [[#Gössling--2021a|Gössling et al. 2021a]] ). These synthetic fuels have potential for large lifecycle emissions reductions ( [[#Schmidt--2016|Schmidt et al. 2016]] ). In comparison to bio-SAF production, the implementation of the processes is in its infancy. However, assuming availability of low-carbon energy electricity, these fuels have much smaller land and water requirements than bio-SAF. Low carbon-energy supply, scalable technology, and therefore costs, represent barriers. [[#Scheelhaase--2019|Scheelhaase et al. (2019)]] review current estimates of costs, which are estimated to be approximately four to six times the price of fossil kerosene.

Liquid hydrogen (LH 2 ) as a fuel has been discussed for aeronautical applications since the 1950s ( [[#Brewer--1991|Brewer 1991]] ) and a few experimental aircraft have flown using such a fuel. Experimental, small aircraft have also flown using hydrogen fuel cells. Although the fuel has an energy density per unit mass about three times greater than kerosene, it has a much lower energy density per unit volume (approximately factor 4 ( [[#McKinsey--2020|McKinsey 2020]] )). The increased volume requirement makes the fuel less attractive for aviation since it would require the wings to be thickened or fuel to take up space in the fuselage. [[#Bicer--2017|Bicer and Dincer (2017)]] found that LH 2 -powered aircraft compared favourably to conventional kerosene-powered aircraft on a lifecycle basis, providing that the LH 2 was generated from low-carbon energy sources (0.014 kgCO 2 per tkm compared with 1.03 kgCO 2 per tkm for an unspecified passenger aircraft). However, Ramos [[#Pereira--2014|Pereira et al. (2014)]] also made a lifecycle comparison and found much smaller benefits of LH 2 -powered aircraft (manufactured from low-carbon energy) compared with conventional fossil kerosene. The two studies expose the sensitivities of boundaries and assumptions in the analyses. Shreyas [[#Harsha--2014|Harsha (2014)]] and [[#Rondinelli--2017|Rondinelli et al. (2017)]] conclude that there are many infrastructural barriers but that the environmental benefits of low-carbon-based LH 2 could be considerable. [[#Khandelwal--2013|Khandelwal et al. (2013)]] take a more optimistic view of the prospect of LH 2 -powered aircraft but envisage them within a hydrogen-oriented energy economy. A recently commissioned study by the European Union (EU)’s Clean Sky undertaking, ( [[#McKinsey--2020|McKinsey 2020]] ) addresses many of the aspects of the opportunities and obstacles in developing LH 2 -powered aircraft. The report provides an optimistic view of the feasibility of developing such aircraft for short to medium haul but makes clear that new aircraft designs (such as blended-wing body aircraft) would be needed for longer distances.

The non-CO 2 impacts of LH 2 -powered aircrafts remain poorly understood. The emission index of water vapour would be much larger (estimated to be 2.6 times greater by [[#Ström--2002|Ström and Gierens (2002)]] ) than for conventional fuels), and the occurrence of contrails may increase but have lower ERF because of the lower optical depth ( [[#Marquart--2005|Marquart et al. 2005]] ). Moreover, contrails primarily form on soot particles from kerosene-powered aircraft, which would be absent from LH 2 exhaust ( [[#Kärcher--2018|Kärcher 2018]] ). The overall effect is currently unknown as there are no measurements. Potentially, NO x emissions could be lower with combustor redesign ( [[#Khandelwal--2013|Khandelwal et al. 2013]] ).

In conclusion, there are favourable arguments for LH 2 -powered aircraft, both on an efficiency basis ( [[#Verstraete--2013|Verstraete 2013]] ) and an overall reduction in GHG emissions, even on an lifecycle basis. However, LH 2 requires redesign of the aircraft, particularly for long-haul operations. Similarly, there would be a need for expanded infrastructure for fuel manufacture, storage, and distribution at airports, which is likely to be more easily overcome if there is a more general move towards a hydrogen-based energy economy.

'''Technological and operational trade-offs between CO''' 2 '''and non-CO''' 2 '''effects.''' Since aviation has additional non-CO 2 warming effects, there has been some discussion as to whether these can be addressed by either technological or operational means. For example, improved fuel efficiency has resulted from high overall pressure ratio engines with large bypass ratios. This improvement has increased pressure and temperature at the combustor inlet, with a resultant tendency to increase thermal NO x formation in the combustor. Combustor technology aims to reduce this increase, but it represents a potential technology trade-off whereby NO x control may be at the expense of extra fuel efficiency. Estimating the benefits or disbenefits of CO 2 (proportional to fuel burned) vs NO x in terms of climate is complex ( [[#Freeman--2018|Freeman et al. 2018]] ).

Any global warming potential/temperature change potential type emissions equivalency calculation always involves the user selection of a time horizon over which the calculation is made, which is a ''subjective'' choice ( [[#Fuglestvedt--2010|Fuglestvedt et al. 2010]] ). In general, the longer the time horizon, the more important CO 2 becomes in comparison with a short-lived climate forcing agent. So, for example, a net (overall) aviation GWP for a 20-year time horizon is 4.0 times that of CO 2 alone, but only 1.7 over a 100-year time horizon. Correspondingly, a GTP for a 20-year time horizon is 1.3, but it is 1.1 for 100 years ( [[#Lee--2021|Lee et al. 2021]] ).

A widely discussed opportunity for mitigation of non-CO 2 emissions from aviation is the avoidance of persistent contrails that can form contrail cirrus. Contrails only form in ice-supersaturated air below a critical temperature threshold ( [[#Kärcher--2018|Kärcher 2018]] ). It is therefore feasible to alter flight trajectories to avoid such areas conducive to contrail formation, since ice-supersaturated areas tend to be tens to hundreds of kilometres in the horizontal and only a few 100 metres in the vertical extent ( [[#Gierens--1997|Gierens et al. 1997]] ). Theoretical approaches show that avoidance is possible on a flight-by-flight basis ( [[#Matthes--2017|Matthes et al. 2017]] ; [[#Teoh--2020|Teoh et al. 2020]] ). Case studies have shown that flight planning according to trajectories with minimal climate impact can substantially (up to 50%) reduce the aircraft’s net climate impacts despite small additional CO 2 emissions ( [[#Niklaß--2019|Niklaß et al. 2019]] ). However, any estimate of the net benefit or disbenefit depends firstly on the assumed magnitude of the contrail cirrus ERF effect (itself rather uncertain, assessed with a low confidence level) and upon the choice of metric and time horizon applied. While this is a potentially feasible mitigation option, notwithstanding the CO 2 per contrail trade-off question, meteorological models cannot currently predict the formation of persistent contrails with sufficient accuracy in time and space ( [[#Gierens--2020|Gierens et al. 2020]] ); this mitigation option is speculated to take of the order of up to a decade to mature ( [[#Arrowsmith--2020|Arrowsmith et al. 2020]] ).

'''Market-based offsetting measures''' . The EU introduced aviation into its CO 2 emissions trading scheme (ETS) in 2012. Currently, the EU-ETS for aviation includes all flights within the EU as well as to and from Eastern European and West-Central Asian states. Globally, ICAO agreed in 2016 to commence, in 2020, the ‘Carbon Offsetting and Reduction Scheme for International Aviation’ (CORSIA). The pandemic subsequently resulted in the baseline being changed to 2019.

CORSIA has a phased implementation, with an initial pilot phase (2021–2023) and a first phase (2024–2026) in which states will participate voluntarily. The second phase will then start in 2026–2035, and all states will participate unless exempted. States may be exempted if they have lower aviation activity levels or based on their UN development status. As of September 2021, 109 ICAO Member States will voluntarily be participating in CORSIA starting in 2022. In terms of routes, only those where both States connecting the route are participating are included. There will be a special review of CORSIA by the end of 2032 to determine the termination of the scheme, its extension, or any other changes to the scheme beyond 2035.

By its nature, CORSIA does not lead to a reduction in in-sector emissions from aviation since the programme deals mostly in approved offsets. At its best, CORSIA is a transition arrangement to allow aviation to reduce its impact in a more meaningful way later. From 2021 onwards, operators can reduce their CORSIA offsetting requirements by claiming emissions reductions from ‘CORSIA Eligible Fuels’ that have demonstrably reduced lifecycle emissions. These fuels are currently available at greater costs than the offsets ( [[#Capaz--2021a|Capaz et al. 2021a]] ). As a result, most currently approved CORSIA offsets are avoided emissions, which raises the issue of additionality ( [[#Warnecke--2019|Warnecke et al. 2019]] ). The nature of avoided emissions is to prevent an emission that was otherwise considered to be going to occur, for example, prevented deforestation. Avoided emissions are ‘reductions’ (over a counterfactual) and purchased from other sectors that withhold from an intended emission ( [[#Becken--2017|Becken and Mackey 2017]] ), such that if additionality were established, a maximum of 50% of the intended emissions are avoided. Some researchers suggest that avoided deforestation offsets are not a meaningful reduction, since deforestation continues to be a net source of CO 2 emissions ( [[#Mackey--2013|Mackey et al. 2013]] ; [[#Friedlingstein--2020|Friedlingstein et al. 2020]] ).

'''Modal shift to high-speed rail.''' Due to the limitations of the current suite of aviation mitigation strategies, the potential for high-speed rail (HSR) is of increasing interest ( [[#Givoni--2006|Givoni and Banister 2006]] ; [[#Chen--2017|Chen 2017]] ; [[#Bi--2019|Bi et al. 2019]] ). The IEA’s ''Net Zero by 2050'' roadmap suggests significant behavioural change, with more regional flights shifting to HSR in the Net Zero Emissions by 2050 scenario pathway ( [[#IEA--2021e|IEA 2021e]] ). For HSR services to be highly competitive with air travel, the optimal distance between the departure and arrival points has been found to be in the approximate range of 400 to 800 km ( [[#Bows--2008|Bows et al. 2008]] ; [[#Rothengatter--2010|Rothengatter 2010]] ), although in the case of China’s HSR operations, this range can be extended out to 1000 km, with corresponding air services having experienced significant demand reduction upon HSR service commencement ( [[#Lawrence--2019|Lawrence et al. 2019]] ). In some instances, negative effects on air traffic, air fare, and flight frequency have occurred at medium-haul distances such as HSR services in China on the Wuhan–Guangzhou route (1069 km) and the Beijing–Shanghai route (1318 km) ( [[#Fu--2015|Fu et al. 2015]] ; [[#Zhang--2016|Zhang and Zhang 2016]] ; [[#Chen--2017|Chen 2017]] ; [[#Li--2019|Li et al. 2019]] ; [[#Ma--2019|Ma et al. 2019]] ). This competition at medium-haul distances is contrary to that which has been experienced in European and other markets and may be attributable to China having developed a comprehensive network with hub stations, higher average speeds, and an integrated domestic market with strong patronage ( [[#Zhang--2019a|Zhang et al. 2019a]] ).

The LCA literature suggests that the GHG emissions associated with HSR vary depending on spatial, temporal, and operational specifics ( [[#Åkerman--2011|Åkerman 2011]] ; [[#Baron--2011|Baron et al. 2011]] ; [[#Chester--2012|Chester and Horvath 2012]] ; [[#Yue--2015|Yue et al. 2015]] ; [[#Hoyos--2016|Hoyos et al. 2016]] ; [[#Jones--2017|Jones et al. 2017]] ; [[#Robertson--2016|Robertson 2016]] ; [[#Robertson--2018|Robertson 2018]] ; [[#Lin--2019|Lin et al. 2019]] ). These studies found a wide range of approximately 10 to 110 gCO 2 pkm –1 for HSR. This range is principally attributable to the sensitivity of operational parameters such as the HSR passenger seating capacity, load factor, composition of renewable and non-renewable energy sources in electricity production, rolling stock energy efficiency and patronage (i.e., ridership both actual and forecast), and line-haul infrastructure specifics (e.g., tunnelling and aerial structure requirements for a particular corridor) ( [[#Åkerman--2011|Åkerman 2011]] ; [[#Chester--2012|Chester and Horvath 2012]] ; [[#Yue--2015|Yue et al. 2015]] ; [[#Newman--2018|Newman et al. 2018]] ; [[#Robertson--2018|Robertson 2018]] ). The prospect for HSR services providing freight carriage (especially online purchases) is also growing rapidly ( [[#Strale--2016|Strale 2016]] ; [[#Bi--2019|Bi et al. 2019]] ; [[#Liang--2019|Liang and Tan 2019]] ) with a demonstrated emissions reduction potential from such operations ( [[#Hoffrichter--2012|Hoffrichter et al. 2012]] ). However, additional supportive policies will most likely be required ( [[#Strale--2016|Strale 2016]] ; [[#Watson--2019|Watson et al. 2019]] ). Limiting emissions avoidance assessments for HSR modal substitution to account only for CO 2 emissions ignores aviation’s non-CO 2 effects ( [[#10.5.2|Section 10.5.2]] ), and likely results in an under-representation of the climate benefits of HSR replacing flights.

HSR modal substitution can generate a contra-effect if the air traffic departure and arrival slots that become available as the result of the modal shift are simply reallocated to additional air services ( [[#Givoni--2006|Givoni and Banister 2006]] ; [[#Givoni--2013|Givoni and Dobruszkes 2013]] ; [[#Jiang--2016|Jiang and Zhang 2016]] ; [[#Cornet--2018|Cornet et al. 2018]] ; [[#Zhang--2019a|Zhang et al. 2019a]] ). Furthermore, HSR services have the potential to increase air traffic at a hub airport through improved networks but this effect can vary based on the distance of the HSR stations from airports ( [[#Jiang--2014|Jiang and Zhang 2014]] ; [[#Xia--2016|Xia and Zhang 2016]] ; [[#Zhang--2019b|Zhang et al. 2019b]] ; [[#Liu--2019|Liu et al. 2019]] ). Such rebound effects could be managed through policy interventions. For example, in 2021 the French government regulated that all airlines operating in France suspend domestic airline flights on routes if a direct rail alternative with a travel time of less than 2.5 hours is available. Other air travel demand reduction measures that have been proposed include regulations to ban frequent flyer reward schemes, mandates that all marketing of air travel declare flight emissions information to the prospective consumer (i.e., the carbon footprint of the nominated flight), the introduction of a progressive ‘Air Miles Levy’ as well as the inclusion of all taxes and duties that are presently exempt from air ticketing ( [[#Carmichael--2019|Carmichael 2019]] ). Moreover, China has the highest use of HSR in the world in part due to its network and competitive speeds and in part due to heavy regulation of the airline industry, in particular restrictions imposed on low-cost air carrier entry and subsidisation of HSR ( [[#Li--2019|Li et al. 2019]] ). These air travel demand reduction strategies may induce shifts to other alternative modes in addition to stimulating HSR ridership.

Despite the risk of a rebound effect, and due to the probable reality of an incremental adoption of sustainable aviation fuel technology in the coming decades, the commencement of appropriate HSR services has the potential to provide, particularly in the short- to medium-term, additional means of aviation emissions mitigation.

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=== 10.5.4 Assessment of Aviation-specific Projections and Scenarios ===

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The most recent projection from ICAO (prior to the COVID-19 pandemic) for international traffic (mid-range growth) is shown in Figure 10.11. This projection shows the different contributions of mitigation measures from two levels of improved technology, as well as improvements in air traffic management and infrastructure use. The projections indicate an increase in CO 2 emissions by a factor of 2.2 in 2050 over 2020 levels for the most optimistic set of mitigation assumptions. The high/low traffic growth assumptions would indicate increases by factors of 2.8 and 1.1, respectively in 2050, over 2020 levels (again, for the most optimistic mitigation assumptions).

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[[File:aa69c1279b7f4c5ac09340bf7671e2c5 IPCC_AR6_WGIII_Figure_10_11.png]]

'''Figure 10.11 | Projections of international aviation emissions of CO''' 2 '''.''' '''Data in Mt yr''' –1 ''', to 2050, showing contributions of improved technology and air traffic management and infrastructure use to emissions reductions to 2050.''' Data from [[#Fleming--2019|Fleming and de Lépinay (2019)]] ; projections made pre-COVID-19 global pandemic.

The International Energy Agency has published several long-term aviation scenarios since AR5 within a broader scope of energy projections. Their first set of aviation scenarios include a ‘reference technology scenario’, a ‘2°C Scenario’ and a ‘Beyond 2°C Scenario’. The scenarios are simplified in assuming a range of growth rates and technological/operational improvements ( [[#IEA--2017b|IEA 2017b]] ). Mitigation measures brought about by policy and regulation are treated in a broad-brush manner, noting possible uses of taxes, carbon pricing, price and regulatory signals to promote innovation.

The IEA has more recently presented aviation scenarios to 2070 in their ‘Sustainable Development Scenario’ that assume some limited reduction in demand post-COVID-19, and potential technology improvements in addition to direct reductions in fossil kerosene usage from substitution of biofuels and synthetic fuels ( [[#IEA--2021b|IEA 2021b]] ). There is much uncertainty in how aviation will recover from the COVID-19 pandemic but, in this scenario, air travel returns to 2019 levels in three years, and then continues to expand, driven by income. Government policies could dampen demand (12% lower by 2040 than the IEA ‘Stated Policies Scenario’, which envisages growth at 3.4% per year, which in turn is lower than ICAO at 4.3%). Mitigation takes place largely by fuel substitution with lower-carbon biofuels and synthetic fuels, with a smaller contribution from technology. Approximately 85% of the actual cumulative CO 2 emissions (to 2070) are attributed to use of fuel at their lowest technology readiness level of ‘Prototype’, which is largely made up of biofuels and synthetic fuels, as shown in Figure 10.12. Details of the technological scenarios and the fuel availability/uptake assumptions are given in [[#IEA--2021b|IEA (2021b)]] , which also makes clear that the relevant policies are not currently in place to make any such scenario happen.

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[[File:150f22aaf9dec7fc6b57a2dbe588a82f IPCC_AR6_WGIII_Figure_10_12.png]]

'''Figure 10.12 | The International Energy Agency’s scenario of future aviation fuel consumption for the States Policies Scenario (‘STEPS’) and composition of aviation fuel use in the Sustainable Development Scenario.''' Source: adapted from [[#IEA--2021b|IEA (2021b)]] .

Within the Coupled Model Intercomparison Project Phase 6 emissions database, a range of aviation emissions scenarios for a range of Shared Socio-economic Pathway (SSP) scenarios are available (Figure 10.13). This Figure suggests that by 2050, direct emissions from aviation could be 1.5 to 6.5 (5–95th percentile) times higher than in the 2020 model year under the scenarios that exceed warming of 4°C during the 21st century with a likelihood of 50% or greater (C8). In the C1 (which limit warming to 1.5°C (&gt;50%) during the 2st century with no or limited overshoot) and C2 (which return warming to 1.5°C (&gt;50%) during the 2st century after a high overshoot) scenarios, aviation emissions could still be up to 2.5 times higher in 2050 than in the 2020 model year (95th percentile) but may need to decrease by 10% by 2050 (5th percentile).

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[[File:6e530a54288ed53fd3717c9f0b91d5df IPCC_AR6_WGIII_Figure_10_13.png]]

'''Figure 10.13 | CO''' 2 '''emissions from AR6 aviation scenarios indexed to 2020 modelled year.''' Data from the AR6 scenario database.

The COVID-19 pandemic of 2020 has changed many activities and, consequentially, associated emissions quite dramatically ( [[#Le%20Quéré--2018b|Le Quéré et al. 2018b]] ; [[#Friedlingstein--2020|Friedlingstein et al. 2020]] ; [[#Liu--2020c|Liu et al. 2020c]] ; [[#UNEP--2020|UNEP 2020]] ). Aviation was particularly affected, with a reduction in commercial flights in April 2020 of about 74% over 2019 levels, with some recovery over the following months, remaining at 42% lower as of October 2020 ( [[#Petchenik--2021|Petchenik 2021]] ). The industry is considering a range of potential recovery scenarios, with the International Air Transport Association (IATA) speculating that recovery to 2019 levels may take up until 2024 ( [[#Earley--2021|Earley and Newman 2021]] ) (Cross-Chapter Box 1 in Chapter 1). Others suggest, however, that the COVID-19 pandemic and increased costs as a result of feed-in quotas or carbon taxes could slow down the rate of growth of air travel demand, though global demand in 2050 would still grow 57%–187% between 2018 and 2050 (instead of 250% in a baseline recovery scenario) ( [[#Gössling--2021a|Gössling et al. 2021a]] ).

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=== 10.5.5 Accountability and Governance Options ===

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Under Article 2.2 of the Kyoto Protocol, Annex I countries were called to ''‘…pursue limitation or reduction of emissions of GHGs not controlled by the Montreal Protocol from aviation and marine bunker fuels, working through the International Civil Aviation Organization and the International Maritime Organization, respectively.'' ’ The Paris Agreement is different, in that ICAO (and the IMO) are not named. As a result, the Paris Agreement, through the NDCs, seemingly covers CO 2 emissions from domestic aviation (currently 35% of the global total from aviation) but does not cover emissions from international flights. A number of states and regions, including the UK, France, Sweden, and Norway, have declared their intentions to include international aviation in their net zero commitments, while the EU, New Zealand, California, and Denmark are considering doing the same ( [[#Committee%20on%20Climate%20Change--2019|Committee on Climate Change 2019]] ). The Paris Agreement describes temperature-based goals, such that it is unclear how emissions of GHGs from international aviation would be accounted for. Clearly, this is a less than ideal situation for clarity of governance of international GHG emissions from both aviation and shipping. At its 40th General Assembly (October 2019) the ICAO requested its Council to ‘ ''…continue to explore the feasibility of a long-term global aspirational goal for international aviation, through conducting detailed studies assessing the attainability and impacts of any goals proposed, including the impact on growth as well as costs in all countries, especially developing countries, for the progress of the work to be presented to the 41st Session of the ICAO Assembly'' ’. What form this goal will take is unclear until work is presented to the 41st Assembly (Autumn, 2022). It is likely , however, that new accountability and governance structures will be needed to support decarbonisation of the aviation sector.

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