Consolidating and stepping up aviation&rsquo;s climate ambition: a comprehensive definition of a climate neutral air transport system

Peerlings, Bram; Paletti, Ligeia

doi:10.5194/jecats-2026-10

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https://doi.org/10.5194/jecats-2026-10

Preprints

03 Jul 2026

| 03 Jul 2026

Status: this preprint is currently under review for the journal JECATS.

Consolidating and stepping up aviation’s climate ambition: a comprehensive definition of a climate neutral air transport system

Bram Peerlings and Ligeia Paletti

Abstract. To date, commercial aviation contributed approximately 4 % of global anthropogenic climate change through the emission of greenhouse gases during flight and operational non-CO₂ climate effects. Over the last few years, numerous aviation-related organisations have set goals to reduce aviation’s climate impact. These goals, however, lack alignment, are poorly or ambiguously defined, or are internally inconsistent. This increases uncertainty about what aviation should work towards, how various stakeholders can contribute, and introduces problems with respect to accountability. In order to address this issue, this paper presents a comprehensive definition of a climate neutral air transport system as an “air transport system of which the climate effects of all its greenhouse gases and non-CO₂ effects throughout the entire life cycle of each element of the system is balanced”. The proposed definition spans relevant systems and encompasses all life cycle phases. To achieve a climate neutral air transport system by 2050, all life cycle greenhouse gas emissions and non-CO₂ climate effects remaining after in-sector reduction should be neutralised, as should all remaining non-CO₂ climate forcing from emissions prior to 2050. Clarity on governance is furthermore needed, as the goal and associated targets proposed should be adopted globally.

Received: 01 May 2026 – Discussion started: 03 Jul 2026

Competing interests: At least one of the (co-)authors is a member of the editorial board of Journal of Environmentally Compatible Air Transport System..

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Bram Peerlings and Ligeia Paletti

Status: final response (author comments only)

RC1:
'Comment on jecats-2026-10', Thomas Planès, 27 Aug 2026
General comment
The paper addresses an important and timely question, namely how to define a climate neutral air transport system. It provides a useful overview of existing sectoral objectives, and then propose a definition and recommendations for climate neutral air transport. However, in my view, the manuscript currently reads more as a simplified proposal for a framework to define climate targets for aviation than as a comprehensive state-of-the-art review of the scientific and conceptual challenges associated with climate neutrality. In particular, the proposed definition of climate neutrality may oversimplify the fundamentally different responses of CO₂ and non-CO₂ effects to changes in emissions and their associated climate impacts. Further clarification is also needed regarding the scope of the system considered, including life-cycle emissions, non-in-flight activities, and the role of removals and offsetting, as well as the allocation of global or Paris-compatible climate objectives to aviation. I therefore suggest expanding on the simplified definition of 'climate-neutral air transport' to emphasise the specific features mentioned in the paper and the review. I highlight in particular the next three points.
The distinction between Sections 2 and 3 is unclear. Indeed, Table 1 in Section 3 summarises the commitments mainly from Section 2, and Section 3 discusses the definition of GHGs for aviation in the context of political texts and IPCC reports. I suggest merging these two sections and concluding the new section with a discussion on the definition of GHGs for aviation.

L225/L345: I am sceptical about the use of these concise definitions. Indeed, balancing CO₂ effects amounts to stabilising the rise in temperature, whereas balancing most (short-term) non-CO₂ effects amounts to rapidly achieving a net zero effect on the rise in temperature (cooling compared with today). Furthermore, this definition could exclude the preservation of cooling effects, which may be beneficial. Should we not, on the contrary, seek to set a dual definition rather than oversimplifying matters: carbon neutrality and specific definitions of “neutrality” for non-CO₂ effects (e.g. zero induced net forcing, constant net forcing...)?

L446: this objective is very ambitious and open to debate – though desirable – due to the fact that it is not the case for CO2 (e.g. stabilising the RF for CO2 means approximatively net zero CO2 emissions, whereas stabilising the RF for contrails means approximatively a stabilisation of contrails number). You should mention it in the main text.

Specific comments
L33: I suggest moving the comment on the estimation of CO₂-equivalent metrics to the end of the paragraph (L42), in order to distinguish between uncertainties relating to the physics of non-CO₂ effects and the challenges involved in quantifying these metrics. This will also help to avoid confusion when providing quantifications of the aviation sector's contribution to climate change, such as effective radiative forcing or temperature change.

L45 (“targets differ in the extent to which they allow for offsetting…”): I recommend to move the end of the paragraph in a last dedicated one, due to the fact that it applies to all commitments (and not only European research establishments).

Global comment on offsetting/removal: is it relevant to talk about carbon (or climate) neutrality for a sector? It is well defined at the global (or regional) scale, but the “removal” part could be complex to define properly and could lead to carbon accounting issues. I think this issue should be mentioned in the paper

L252-L277: Although it is relevant to take the entire life cycle into account, I feel that this passage tends to overemphasise these impacts. I think the statement should be more measured: all LCA studies show that in-flight emissions are by far the dominant factor in terms of climate impacts (or are there references that show different results?)

L278-281: That is an interesting point. Regarding the distinction between concepts of neutrality (see my second main comment), should we not define a third level, namely “GHG emissions from non-in-flight phases”, in order to incorporate this into the objectives and avoid accounting issues?

L332: regarding the “Paris-compatible” definition, the allocated target to aviation is a key question (e.g. should we allocate 1%, 2%, 5%... of the temperature target to aviation?). I think you should mention this issue. Feel free to read/cite the following paper (Section 1.4) for more information: https://doi.org/10.1016/j.paerosci.2023.100920

Figure 1: you should mention in the text that the coloured boxes are just some examples. You could in particular add a box for the fuel production because it is a significant contributor to aviation GHGs emissions. You should also verify the figure (e.g. the lighter blue box seems in the wrong place).

L395: “two objectives that could be affected by a climate-neutral air transport system” (to avoid suggesting that safety and climate neutrality are incompatible).

L445: in order to avoid the issues regarding cooling non-CO2 effects, should you replace “neutralised” by “neutralised (or net negative)”?

L468: you should add 1990 in the target (currently you have no information on the reference year).

L483: the comment on contrails is useless here because you only provide an explanation on NOx emissions.

Table 3 (line 2040): I assume that there is a small mistake on a figure (90% instead 60% or other for CO2 emissions, 90% being the target for 2050).

Sections 6 and 7 could be merged.

Technical corrections
L233: breath instead of breadth (section title)

L254: two commas
Citation: https://doi.org/10.5194/jecats-2026-10-RC1
RC2: 'Comment on jecats-2026-10', Anonymous Referee #2, 29 Sep 2026

Please see attached document for review

Citation: https://doi.org/10.5194/jecats-2026-10-RC2

Bram Peerlings and Ligeia Paletti

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Short summary

Aviation organisations have set varied climate goals, but misalignment, ambiguity and internal inconsistencies hinder real progress. This paper addresses this by proposing a comprehensive definition of a climate neutral air transport system: “air transport system of which the climate effects of all its greenhouse gases and non-CO₂ effects throughout the entire life cycle of each element of the system is balanced”.


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