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Ever since the Wright Brothers' first powered flight in 1903, commercial aircraft have relied on liquid hydrocarbon fuels. However, the need for greenhouse gas emission reductions along with recent progress in battery technology for automobiles has generated strong interest in electric propulsion in aviation. This work provides a first-order assessment of the energy, economic, and environmental implications of all-electric aircraft. We show that batteries with significantly higher specific energy and lower cost, coupled with further reductions of costs and CO 2 intensity of electricity, are necessary for exploiting the full range of economic and environmental benefits provided by all-electric aircraft. A global fleet of all-electric aircraft serving all flights up to a 400-600 nmi (741-1,111 km) distance would demand an equivalent of 0.6-1.7% of worldwide electricity consumption in 2015. Whereas lifecycle CO 2 emissions of all-electric aircraft depend 2 on the power generation mix, all direct combustion emissions and thus direct air pollutants and direct non-CO 2 warming impacts would be eliminated.

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In response to strong growth in air transportation CO 2 emissions, governments and industry began to explore and implement mitigation measures and targets in the early 2000s. However, in the absence of rigorous analyses assessing the costs for mitigating CO 2 emissions, these policies could be economically wasteful. Here we identify the cost-effectiveness of CO 2 emission reductions from narrow body aircraft, the workhorse of passenger air transportation. We find that in the US, a combination of fuel burn reduction strategies could reduce the 2012 level of CO 2 emissions per passenger-km by around 2% per year through mid-century. These intensity reductions would occur at zero marginal costs for oil prices between $50-100 per barrel. Even larger reductions are possible, but could impose extra costs and require the adoption of biomassbased synthetic fuels. The extent to which these intensity reductions will translate into absolute emissions reductions will depend on fleet growth.

Aviation emissions are not on a trajectory consistent with Paris Climate Agreement goals. We evaluate the extent to which fuel pathways-synthetic fuels from biomass, synthetic fuels from green hydrogen and atmospheric CO 2 , and the direct use of green liquid hydrogen-could lead aviation towards net-zero climate impacts. Together with continued efficiency gains and contrail avoidance, but without offsets, such an energy transition could reduce lifecycle aviation CO 2 emissions by 89-94% compared with year-2019 levels, despite a 2-3-fold growth in demand by 2050. The aviation sector could manage the associated cost increases, with ticket prices rising by no more than 15% compared with a no-intervention baseline leading to demand suppression of less than 14%. These pathways will require discounted investments on the order of US$0.5-2.1 trillion over a 30 yr period. However, our pathways reduce aviation CO 2 -equivalent emissions by only 46-69%; more action is required to mitigate non-CO 2 impacts.

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Policies aimed at influencing air transportation must operate in a complex, interacting global system of passengers, airlines, airports and other stakeholders. Tools which are capable of assessing policy outcomes in this situation are vital. Given the high uncertainty about future demand, costs and technology characteristics on policy-relevant timescales, such tools also need to allow the evaluation of outcomes from a wide range of plausible futures. This paper presents the validation study and initial baseline results from a comprehensive, open source update of the global AIM aviation systems model. We show that running the model from 2005 to 2015 using 2005 base year data reproduces well the observed demand levels and patterns of growth. Running from a 2015 base year, we project global demand in 2050 of between 13,800 billion and 46,000 billion revenue passenger kilometres (RPK), respectively 2.2 and 7.4 times year 2015 values, depending primarily on the future scenario for population, income and oil price assumed. Absent any radical change in aircraft technology, this would lead to global direct CO2 emissions from aviation of between 876 and 2,500 Mt, or 1.5 to 4.4 times the year-2015 level. This wide level of baseline variation may present a challenge for long-term aviation policy and its adaptability to different futures.