Playing field for bio-jet fuels

What emissions and what climate impact does today's aviation have? It is important to create a consensus around this in order to be able to both compare the effects of different fuels and other mitigation measures and to relate the impact of aviation to e.g., other modes of transport. Emissions from aviation are inventoried and reported today at several different levels. Currently the main driver of CO2 emission inventories are regulations targeting emissions of greenhouse gas emissions. This report provides a brief overview of the methods and data used, which stem from various regulations and initiatives. A number of emission calculators is driven by the demand for data to report climate impact from air travel and freight transport and includes emission or climate calculators that focus on emissions of CO2 or CO2 equivalents assigned to a passenger or to volume or mass of cargo on a given route or nominal distance. There is a number of such calculators that use different emission factors, flight parameters, aircraft occupancy and contributions from high-altitude impacts, and thus generate different results. Examples are those of ICAO (International Civil Aviation Organization, 2019), NTM (Network for Transport Measures, 2019), IATA (International Air Transport Association, 2019), Atmosfair (Atmosfair, 2019) or Flight Emission Map (Flight Emission Map, 2019). A survey of data and assumptions that form the basis for aviation greenhouse gas emissions and climate calculators and a validation of these by means of data on reported fuel consumption during flights was carried out in the project. For most of the calculators there is a good agreement with the fuel consumption data when the variability of the fuel consumption due to different aircraft types, occupancy, etc., is taken into the account. Three calculators show substantially higher emissions and an analysis indicates that the reason is that they are using obsolete emission factors. The biggest difference between calculators arise from the calculation of CO2 equivalents in which case all use radiation forcing index (RFI) as a measure. The study also included a comparison of SMHI's air emission model with fuel consumption data.

The high-altitude effects of SLCP are crucial in minimizing the climate impact of aviation – for combustion engines these effects will remain even with use of fossil-free fuel. The first important questions associated with the high-altitude effects are their quantification and reduction of uncertainties of the climate impact of the SLCP. RFI used by many climate calculators is a blunt tool if the aim is to target the high-altitude effects as such, as it is related solely to CO2 emissions and the relation to the SLCP radiative forcing is through impact of the historic emissions of aviation up to the date for which the FI is calculated. More appropriate are forward looking metrics considering forcing from actual SLCP species emitted during the flight as global warming potential (GWP) or global temperature potential (GTP). The most important climate forcing components are emissions of CO2 and formation of contrails and contrail cirrus. Sustainable aviation fuels (SAF) with high hydrogen and low aromatic content emits substantially less soot particles which reduces radiative forcing of the contrails. Model simulations of full implementation of SAF in the current aviation fleet would lead to 20-50 % reduction of RF from contrails and contrail cirrus. A combination of the use of SAF, engine technology with low emissions of soot and NOx and route climate optimisation has the potential to substantially reduce the high-altitude effect.

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