Beyond the conventional GWP: Exploring alternative climate metrics for aviation policy and aircraft design
June 5, 2024
Beyond the conventional GWP: Exploring alternative climate metrics for aviation policy and aircraft design
A growing body of research has highlighted the major contribution of aviation to anthropogenic climate change. Next to carbon dioxide (CO₂), the primary contributions of aircraft are the emissions of nitrogen oxides (NOₓ), water vapour (H₂O) and aerosols, as well as the formation of persistent condensation trails (contrails). Since these latter non-CO₂ effects have been attributed to around two thirds of the total warming from aviation, regulators are increasingly interested in considering these effects in climate policy such as the EU Emissions Trading System (ETS). To do so, non-CO₂ effects must be brought onto a common scale with CO₂ emissions using a so-called climate metric.
However, aviation non-CO₂ effects have highly varying atmospheric lifetimes and efficacies, are dependent on the emission time, altitude and location, and their impacts on the climate have a high degree of uncertainty. Therefore, establishing adequate equivalence between non-CO₂ effects and CO₂ emissions is not trivial and there is currently no consensus on which climate metric is most appropriate for aviation.
Researchers at the DLR Institute for Atmospheric Physics and at the TU Delft Faculty of Aerospace Engineering set out to find a suitable climate metric for both aviation policy and aircraft design. Using the DLR's climate response model AirClim, they explored the neutrality, temporal stability, compatibility and simplicity of a range of climate metrics. As part of their research, they generated and subsequently analysed the climate impact of 10,000 potential future narrowbody aircraft designs powered by conventional kerosene as well as Sustainable Aviation Fuel (SAF) and hydrogen.
Comparison of two aircraft designs using different climate metrics
Pairwise comparison of any two aircraft designs using different climate metrics with 100-year time horizons. An example aircraft pairing is shown in the bottom-right panel: Neutrality is shown for the pairing of aircraft A and aircraft C but not for pairing A-B, since the climate metric value is higher but peak temperature is lower. An ideal climate metric would show results a diagonal line through the bottom-left to top-right quadrants. The EGWP* is the most neutral climate metric, but is found not to be compatible with existing climate policy. The ATR and EGWP perform best across all analyses.
The researchers concluded that transitioning from the conventional Global Warming Potential (GWP) to the Average Temperature Response (ATR) or Efficacy-weighted Global Warming Potential (EGWP) would enable a more accurate assessment of existing and future aircraft. Their analysis into the influence of the time horizon suggests that time horizons larger than 70 years are most suitable for these climate metrics. Finally, the authors found the recently proposed GWP* method to be unstable and inappropriate for use in aviation policy. For his work, the leading author, Liam Megill, received the Ivar Isaksen Early Career Researcher Prize at the 4th ECATS Conference in Delft in October 2023.
Table: Overview of the performance of the analysed climate metrics with respect to each requirement. From Megill et al. (2024); CC BY 4.0.
Requirement
Neutrality
(REQ 1)
Stability
(REQ 2)
Compatibility
(REQ 3)
Simplicity
(REQ 4)
RF
Very low neutrality
Generally stable
Compatible
Simple to understand and implement
GWP
Low, but consistent neutrality
Stable
Compatible (standard climate metric)
Complex to understand, simple to implement
EGWP
High neutrality
Stable
Compatible
Complex to understand and implement
GTP
Low, but inconsistent neutrality
Generally stable
Compatible
Simple to understand, complex to implement
ATR & iGTP
High neutrality
Stable
Compatible
Simple/complex to understand, complex to implement
GWP*
Low, but consistent neutrality
Highly unstable
Not compatible
Highly complex to understand and implement
EGWP*
High and consistent neutrality
Highly unstable
Not compatible
Highly complex to understand and implement
The research was funded by the DINA2030+ project of the German Federal Ministry for Economic Affairs and Climate Action (BMWK), the GlowOPT project of the European Union's Horizon 2020 research and innovation programme, and the Airbus-DLR OpenAirClim project.
Links:
Megill, L., Deck, K. & Grewe, V. Alternative climate metrics to the Global Warming Potential are more suitable for assessing aviation non-CO₂ effects. Commun Earth Environ 5, 249 (2024). https://doi.org/10.1038/s43247-024-01423-6
