# Corpsia--A Feasible Second Best Solution?
## 2 Institutional and Technical Background
### Technical Boundaries Justifying Offsetting  
In the past, fuel efficiency in aviation has increased by around two percent per year. These environmental benefits have been outpaced by sustained growth in air traffic, which is likely to double in the next decade [12]. Under such growth scenarios, sector-wide net emission reductions are very ambitious, especially because advanced technologies for clean flying such as synthetic fuel are not yet in operation. Offsetting seems to be particularly attractive for the air transport industry. This industry is considered to be a 'hard(er)-to-abate sector', both for technological and economic reasons. Marginal abatement costs in the aviation sector are considerably higher than in other (transport) sectors. This is because fuel costs have a share of about one third of total production costs in air transport; the sector has constantly been trying to cut fuel costs for decades. Therefore, it is reasonable to assume that reducing fuel costs has been in the very interest of the aviation industry for many years, and relatively less expensive solutions have mostly been implemented. Hydrocarbons, which are the root cause of the carbon emissions of air transport, can be regarded for aviation applications as optimal energy carriers. They feature an optimal combination of chemical characteristics, first and foremost energy density, along with further properties (freezing point, handling safety, storability) and economic characteristics such as ubiquitous availability and affordability. Alternative energy carriers and propulsion technologies, which can be introduced in ground transport with relative ease, do not work as technological equivalents in air transport. Batteries allow electrical aircraft for the foreseeable future to be operated with only a small payload over distances of less than 500 km. Unless not currently foreseeable progress in battery technology is made, long-haul flights will remain unfeasible on battery power. While hydrogen would be technologically feasible, it is associated with numerous disadvantages. First, an enormous amount of green electricity would be required to produce hydrogen for the air transport sector alone. Second, storage and distribution is  
Figure 1: Principal design of the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA). Notes: This figure highlights the principle design of CORSIA in 2021. Source: Own Figure [PERSON] et al., Options to Continue the European Union Emissions Trading System for Aviation in a CORSIA-World, Sustainability, 2019.  
challenging due to its physical properties, such as the propensity to diffuse through tank walls. Third, the low overall efficiency measured as a proportion of final energy content available for propulsion to the energy input required to produce green hydrogen will likely lead to relatively high prices. Fourth, the size and weight of hydrogen tanks would reduce the energy efficiency of aircraft. Last, emissions of water vapor, contrails, and nitrogen oxides (in the case of direct hydrogen combustion in gas turbines) at high altitudes occur with hydrogen powered aircraft and contribute to so-called non-CO\({}_{2}\) climate effects. Aviation contributes to climate change by both CO\({}_{2}\) and non-CO\({}_{2}\) effects, such as ozone and methane changes from NOx emissions or contrails and contrail cirrus. The climate-relevant effects from non-CO\({}_{2}\) species can be even larger than the effects from CO\({}_{2}\)[13]. Hence, even with an air transport sector hypothetically relying on hydrogen as energy carrier, the need to offset climate impacts will be present to a certain extent. This similarly applies to a situation where hydrocarbons synthetized from water and CO\({}_{2}\) captured from ambient air under utilization of green electricity would be utilized.