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

=== 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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'''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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