Aviation accounted for almost 2.8% of global energy-related CO2 emissions in 2017, and air passenger traffic is expected to more than double to almost 16 000 billion km/yr by mid-century under current trends. Efficiency improvements should reduce energy consumption and slow the increase in energy demand, but alternative fuels will eventually be needed to avoid increases in emissions from the sector. Advanced biofuels and hydrogen-based fuels are leading options.

Besides on-board use of hydrogen in aviation, hydrogen is already used today in a few auxiliary power units that generate electricity when the jet engine is not running. Such units, which usually run on natural gas, can account for up to 20% of ground aircraft emissions.