The aviation industry and policymakers are advocating Sustainable Aviation Fuels (SAF) as one of the main pillars for making the aviation industry sustainable. However, regulatory frameworks like CORSIA and the EU Renewable Energy Directive often exclude the climate impact from in-flight non-CO2 emissions (e.g., NOx, H2O, and soot emissions), which is important in determining the effect of SAF in reducing the climate impact of aviation. To bridge this gap, we evaluate the total global warming effects of SAF from a well-to-wake analysis, which includes the climate effects from CO2 emissions of the well-to-wake combined with the non-CO2 emissions of the pump-to-wake (i.e., inflight). We quantify the climate impact of NOx, H2O and contrails and convert them to a CO2 equivalence (CO2e) factor based on a climate metric, for instance, the Average Temperature Response over a given time horizon (i.e., 20, 50 and 100 years). The resulting well-to-wake CO2e values for SAF vary from about 150 to 250 g/MJ, depending on the specific fuel pathways. Our analysis shows that the maximum reduction in CO2e emissions when using SAF is less than 50% compared to conventional jet fuel, mainly due to the inflight NOx and contrail effects.

Aviation is an important economic sector that provides a fast and reliable means of transportation. However, in 2018, the sector was responsible for the emission of almost 1 Gt CO, equivalent to 2.4% of global anthropogenic CO emissions including land use change. Lee et al. (2021) estimated that the share of anthropogenic effective radiative forcing from aviation was around 4%, including both CO and non-CO effects.

The non-CO climate impact stems primarily from the releases of nitrogen oxides (NO), water vapour (HO), and particle emissions. These non-CO effects have strong spatial and temporal dependencies and are associated with a high degree of uncertainty. NO emissions can lead to positive radiative forcing (warming effect) as they serve as a precursor for short-term ozone (O) production but also cause a cooling effect as they destroy background methane (CH) and the associated ozone (named as Primary Mode Ozone, PMO). Water vapour has a negligible greenhouse gas (GHG) effect when emitted in the lower levels of the troposphere due to its short lifetime, but when emitted at high altitudes (near the tropopause or in the stratosphere), its GHG effect becomes stronger due to its increased residence time. Soot and sulfur particulate emissions have relatively small direct climate effects, but when HO condenses onto these particles and freezes to form a contrail, this can lead to a significant positive forcing, especially at night. In total, the non-CO effects represented roughly two-thirds of total aviation's radiative forcing in 2018 considering the best estimate.

One way to reduce aviation's climate impact is by using sustainable aviation fuel (SAF). For instance, aircraft operators are allowed to use SAF to reduce their carbon offsetting requirements through the Carbon Offsetting & Reduction Scheme for International Aviation (CORSIA) and the EU Emissions Trading Scheme (ETS). The carbon footprint of SAFs is often quantified in grams of CO equivalence per megajoule (gCO e/MJ) by means of a Lifecycle Analysis (LCA), which allows the user to identify the environmental benefit of SAF compared to Conventional Jet Fuel (CJF). Based on these findings, a certain type of SAF can be promoted or discouraged by decision-makers. As mandated by regulatory frameworks, e.g., CORSIA, the Renewable Energy Directive (RED) and the Low Carbon Fuel Standard (California), the current LCAs focus mainly on CO, CH and NO emissions produced from Well to Wake (WtW). The WtW scope is composed of two stages: (1) fuel production and distribution (Well-to-Pump (WtP)) and (2) fuel combustion (Pump-to-Wake (PtW)). Accordingly, 100% SAF has the potential to reduce the lifecycle GHG emissions by up to 94% compared to CJF, depending on the feedstocks and technology pathways, e.g., carbon capture.

When it comes to the PtW non-CO emissions, burning SAF can reduce soot particles and thereby reducing contrail radiative forcing. Meanwhile, SAF has little effect on the NO emissions. Therefore, it is necessary to include the climate impact of the non-CO emissions from burning SAF when quantifying the climate impact benefits of SAF. The previous work analyzed the inflight non-CO climate effects of SAF using the Global Warming Potential (GWP) with horizons of 20-100-500 years. It was concluded that the incorporation of these non-CO effects reduced the relative merit of using SAF, as SAF could not mitigate the non-CO climate impact. However, the analysis did not consider the fact that SAF is able to reduce the contrail climate impact because of the reduced soot emissions compared to CJF. Moreover, recent work from Megill et al. tested climate metrics against requirements, such as the consistency between the climate impact evaluation based on a requirement and a scenario analysis. The analysis showed that Average Temperature Response (ATR) as a climate metric was better suited for aviation than, e.g. GWP.

In this study, we perform the Life Cyle Climate Impact Analysis (LCCIA) to evaluate the climate benefit of SAF. To do so, we quantify the climate impact of NO, HO and contrails from the PtW stage and convert them to the gCOe/MJ value based on two climate metrics, GWP and ATR, respectively for time horizons of 20-50-100 years. The resulting gCOe/MJ values are then combined with the Lifecycle GHG emissions of the CORSIA Eligible Fuels to obtain the overall WtW GHG emissions of SAF. Accordingly, we reflect on environmental benefits of SAF and show in the discussion that the GHG emission reductions can be increased by allocating SAF to long range flights. The overall approach includes an integrated modelling chain of emission inventory generation, climate impact calculation and Monte Carlo simulations. The details are described in the method section.