the existing fleet of aircraft, however. Limits to the supply of battery cells could hamper their aviation adoption, as they compete with other industries like electric vehicles. Lithium-ion batteries have proven fragile and fire-prone and their capacity deteriorates with age. However, alternatives are being pursued, such as sodium-ion batteries. Hydrogen-powered aircraft In 2020, Airbus unveiled liquid-hydrogen-powered aircraft concepts as zero-emissions airliners, poised for 2035. Aviation, like industrial processes that cannot be electrified, could use primarily Hydrogen-based fuel. A 2020 study by the EU Clean Sky 2 and Fuel Cells and Hydrogen 2 Joint Undertakings found that hydrogen could power aircraft by 2035 for short-range aircraft. A short-range aircraft (< ) with hybrid Fuel cell/Turbines could reduce climate impact by 70-80% for a 20-30% additional cost, a medium-range airliner with H2 turbines could have a 50-60% reduced climate impact for a 30-40% overcost, and a long-range aircraft (> ) also with H2 turbines could reduce climate impact by 40-50% for a 40-50% additional cost. Research and development would be required, in aircraft technology and into hydrogen infrastructure, regulations and certification standards. Sustainable aviation fuels (SAF) Electrofuels (e-fuels) The Potsdam Institute for Climate Impact Research reported a €800–1,200 mitigation cost per ton of for hydrogen-based e-fuels. Those could be reduced to €20–270 per ton of in 2050, but maybe not early enough to replace fossil fuels. Climate policies could bear the risk of e-fuel uncertain availability, and Hydrogen and e-fuels may be prioritised when direct electrification is inaccessible. Economic measures Emissions trading ICAO has endorsed emissions trading to reduce aviation emission, guidelines were to be presented to the 2007 ICAO Assembly. Within the European Union, the European Commission has included aviation in the European Union Emissions Trading Scheme operated since 2012, capping airline emissions, providing incentives to lower emissions through more efficient technology or to buy carbon credits from other companies. The Centre for Aviation, Transport and Environment at Manchester Metropolitan University estimates the only way to lower emissions is to put a price on carbon and to use market-based measures like the EU ETS. Taxation and subsidies Financial measures can discourage airline passengers and promote other transportation modes and motivates airlines to improve fuel efficiency. Aviation taxation include: air passenger taxes, paid by passengers for environmental reasons, may be variable by distance and include domestic flights; departure taxes, paid by passengers leaving the country, sometimes also applies outside aviation; jet fuel