Abstract
As part of the EU’s climate transition plan and the recently reformed EU Emissions Trading System (ETS)Directive 2023/958, aircraft operators will be required to report their
non-CO2 effects from 2025. A monitoring, reporting, and verification framework (MRV) for these effects is yet to be developed. The aim of a non-CO2 MRV should be to robustly inform policymakers with scientifically validated information to guide future policy frameworks and incentives to minimise non-CO2 effects and their impact on the climate.
Considering this, the primary goal of the future MRV should be to establish an effective evaluation and steering tool based on scientific principles.
To achieve this objective, a four-stage process needs to be pursued:
1. Identification of a calculation model and relevant data
2. Advancement of research
3. Official designation of meteorological/climate models
4. Targeted positive incentives
1. Identification of a calculation model and relevant data: the choice of the calculation model determines the data required e.g. flight trajectory from which atmospheric
effects can be derived, aircraft/engine data, and fuel data. Only a few air transport aircraft have installed sensors that are sufficient for lower atmospheric data collection for assimilation into numerical weather prediction models and to some extent for fog forecasting, but there is not a commercially available humidity sensor that is capable of contributing data for contrail condition determination. Improved sensors and standards will be needed by our industry, and we continue to advocate for such a capability.
2. Advancement of research to understand better the potential effects of non-CO2 emissions (including cooling and indirect effects), the implications of different climate metrics, the accuracy of meteorological models on Ice Super Saturated Regions (ISSR) localisation (and what is needed to improve them), contrail properties resulting from varying fuel properties, engine exhaust conditions, emissions, and the potential of mitigation strategies to minimise their impact considering appropriate time horizons.
3. Official designation of meteorological/climate models and political decisions to be made on climate metrics: The MRV calculation model and the one used by airlines have to be aligned to set common rules for airlines for the avoidance of contrails while minimising other effects, providing the correct incentives in the MRV: we call for the definition of a European “single source of truth” for operational avoidance: approved meteorological models, contrail and climate models, engine exhaust and emissions
data, and the climate metric/time horizon to be used by airlines for avoidance, and by the EU in the MRV.
4. Based on this, targeted positive incentives should be created for airlines and the aviation industry to reduce their non-CO2 impact and make sure that they integrate the use of Sustainable Aviation Fuels (SAF), adopt new technologies to determine contrail properties and the consequences of the additional fuel burn due to contrail avoidance.
If the MRV is poorly implemented, it could result in poor outcomes including:
1. Wrong incentives / No effect: Currently, there is no scientific basis for establishing an MRV for non-CO2 effects. Without a scientifically based calculation model, there is a significant risk that the real problem, the emissions themselves and their potential effects, will not be effectively understood. Consequently, a large administrative burden would be created without any substantial benefit to the climate. The lack of a commonly recognised metric would lead to unnecessary avoidance and uncoordinated operational decisions by aircraft operators.
2. There is no scientifically agreed CO2 equivalent for non-CO2 effects: Introducing a blanket CO2 equivalent without a stronger scientific understanding of the issue is premature. The EU is at risk of failing to minimise the climate impact of aviation due to misinterpreting data which is not fully understood or defined.
3. Required data is not available: Aircraft operators do not incorporate things like humidity data into their flight planning or post-flight-analysis process and cannot easily access it. Their knowledge of non-CO2 effects relies on modelling which is currently highly uncertain. It is essential for all stakeholders to collaborate in the collection and consolidation of such data.
Three types of mitigating solutions are currently considered:
1. Operational avoidance i.e. avoiding the formation of persistent contrails by avoiding flying through areas where persistent contrails form (Ice Super Saturated Regions, or ISSRs) at times where the contrails will have a net warming effect over their lifetime (i.e. pre-dusk and overnight). However, changing the flight level may lead to extra fuel burn, costs, and CO2 emissions. Operational avoidance requires the prediction of contrail formation conditions, persistence, and integrated lifetime radiative properties to minimise unnecessary detours. It also requires clear EU science-based guidelines to arbitrate between CO2 and non-CO2 effects. Accurate predictions depend on atmospheric conditions (such as temperature, humidity, wind, and ambient cloud cover), aircraft characteristics (such as aircraft geometry, weight, engine exhaust conditions and emissions) and fuel characteristics (aromatic content, sulfur content, hydrogen content).
2. Reducing the climate impact of contrails formed by using cleaner fuels such as Sustainable Aviation Fuels (SAF) or low aromatics fuels. More research is needed to
quantify the benefit of SAF or low aromatic fuels in reducing contrail radiative forcing.
3. The application of lean-burn combustion chambers with very low particulate matter emissions.
To provide the right incentives for airlines to minimise non-CO2 effects, the MRV should first set common rules for all flight operators in Europe via a single source of truth (approved meteorological data with ISSR location, appropriate contrail and climate models, engine exhaust and emissions data and climate metric for CO2 vs. non-CO2 arbitration). It should then set the right incentives for operators.