The importance of an informed choice of CO<sub>2</sub>-equivalence metrics for contrail avoidance

Borella, Audran; Boucher, Olivier; Shine, Keith P.; Stettler, Marc; Tanaka, Katsumasa; Teoh, Roger; Bellouin, Nicolas

doi:https://doi.org/10.5194/egusphere-2024-347

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https://doi.org/10.5194/egusphere-2024-347

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22 Feb 2024

| 22 Feb 2024

The importance of an informed choice of CO₂-equivalence metrics for contrail avoidance

Audran Borella, Olivier Boucher, Keith P. Shine, Marc Stettler, Katsumasa Tanaka, Roger Teoh, and Nicolas Bellouin

Abstract. One of the proposed ways to reduce the climate impact of civil aviation is rerouting aircraft to minimise the formation of warming contrails. As this strategy may increase fuel consumption, it would only be beneficial if the climate impact reduction from the avoided contrails exceeds the negative impact of any additional carbon dioxide (CO₂) emitted by the rerouted flight. In this study, we calculate the surface temperature response of almost half-a-million flights that crossed the North Atlantic sector in 2019 and compare to the response of hypothetical rerouted flights. The climate impacts of contrails and CO₂ are assessed through the perspective of CO₂-equivalence metrics, defined here as nine combinations of different definitions and time horizons. We estimate that the total emitted CO₂ and the persistent contrails formed will have warmed the climate by 16.9 µK in 2039, 13.5 µK in 2069, and 14.0 µK in 2119. Under a scenario where 1 % additional carbon dioxide is enough to reroute all contrail-forming flights and avoid contrail formation completely, total warming would decrease by 4.6 (−27 %), 2.4 (−18 %), and 1.8 (−13 %) μK in 2039, 2069, and 2119, respectively. In most rerouting cases, the results based on the nine different CO₂-equivalence metrics agree that rerouting leads to a climate benefit, assuming that contrails are avoided as predicted. But the size of that benefit is very dependent on the choice of CO₂-equivalence metrics, contrail efficacy and CO₂ penalty. Sources of uncertainty not considered here could also heavily influence the perceived benefit. In about 10 % of rerouting cases, the climate damage resulting from contrail avoidance indicated by CO₂-equivalence metrics integrated over a 100-year time horizon is not predicted by metrics integrated over a 20-year time horizon. This study highlights, using North Atlantic flights as a case study, the implications of the choice of CO₂-equivalence metrics for contrail avoidance, but the choice is ultimately political.

Received: 06 Feb 2024 – Discussion started: 22 Feb 2024

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15 Sep 2024

The importance of an informed choice of CO₂-equivalence metrics for contrail avoidance

Audran Borella, Olivier Boucher, Keith P. Shine, Marc Stettler, Katsumasa Tanaka, Roger Teoh, and Nicolas Bellouin

Atmos. Chem. Phys., 24, 9401–9417, https://doi.org/10.5194/acp-24-9401-2024,https://doi.org/10.5194/acp-24-9401-2024, 2024

Short summary

Audran Borella, Olivier Boucher, Keith P. Shine, Marc Stettler, Katsumasa Tanaka, Roger Teoh, and Nicolas Bellouin

Interactive discussion

Status: closed

RC1:
'Review of "The importance of an informed choice of CO2-equivalence metrics for contrail avoidance" by Borella et al., egusphere-2024-347', Anonymous Referee #1, 08 Mar 2024

This is a very interesting, balanced and well thought-out article analysing the use of climate metrics in the context of contrail avoidance. Using the North Atlantic flight tracks from 2019 as a case study, the authors analyse the influence of the choice of climate metric on the decision to re-route around areas of contrail formation. Their conclusion, that the decision to reroute is largely independent of the climate metric, is very interesting and clearly shows that the implementation of contrail avoidance policies need not be unduly hampered by the choice of climate metric. However, as the authors also conclude, since the climate benefit of rerouting depends on the choice of climate metric as well as contrail efficacy and CO₂ penalty, further research analysing specific contrail avoidance strategies is well warranted.
Overall, the article is logically presented, well-written and appropriate for ACP. I recommend publication of this article subject to minor revision. I have provided my specific comments and questions below, followed by a few technical remarks and suggestions for improvement.

Main comments
1) Choice of background scenario

The authors calculate a total temperature increase for all flights of 16.9 μK in 2039, 13.5 μK in 2069 and 14.0 μK in 2119 (ln. 19). The late increase in temperature between 50 and 100 years after the emission in my opinion requires further description and justification. In Table S1 and Figure 4(a) (third panel), this temperature increase is attributed to CO₂. I assume that the emission inventory is treated as a pulse emission in the year 2019? In that case, I can only attribute this temperature increase to a decreasing background CO₂ concentration. Looking at Meinshausen et al. (2020), the background CO₂ concentration of SSP4-3.4 (ln. 168) does seem to peak in around the year 2080. I thus have a few questions:

    1a) SSP4-3.4 is characterised by a low CO₂ concentration in comparison to the other pathways. Why was SSP4-3.4 chosen for this analysis?

    1b) I can imagine that the high lingering temperature as a result of CO₂ emissions has a significant influence on the weighting of CO₂ compared to that of contrails. In particular when more fuel is burned for contrail avoidance. Have the authors considered different background CO₂ concentrations?

    1c) (How) would the results and conclusions presented in this article change if different background CO₂ concentrations were to be used? Or in other words, what is the impact/sensitivity of the choice of background CO₂ concentration?
2) Clarification of the methodology

The methodology and analysis used in section 5.2 "Imperfect contrail avoidance" is not clear to me. Is extra CO₂ emitted for rerouting, as in the previous sections? How are the number of "imperfect avoidance" flights in Table 1 defined and calculated? How is the rerouting efficiency used to obtain these results?

Minor comments
ln 35: "Uncertainties [...] and the subject of [...]"
ln 88: "It includes a sensitivity analysis [...]"
ΔF is used to denote "radiative forcing" in the definition of the AGWP in ln. 110, but then also to denote "instantaneous contrail RF" in ln. 137. Since these two definitions are not equivalent, I would suggest to use a different symbol for the calculation of EF_contrail.
In the climate metrics formulae (ln. 110-123), I would recommend performing the integrals from t_0 -> t_0 + H rather than 0 -> H, where t_0 is the emission year (2019). Or, H should be defined more clearly in the text to make the connection between H and t more clear.
The ATR was initially introduced by Dallara et al. (2011), but has subsequently been modified and is generally used as in this article. In the description of the ATR (ln. 118-122), I would therefore make a brief reference to this.
ln. 170: The number of simulations used to provide the best estimate and standard deviation, 1726, is very precise. Why was this specific number of simulations used?
Fig 1: I suggest to use the same units in both the caption and the figure (pW and pK)
Fig. 5 and corresponding text: I believe the term "detrimental" is too strong to be a good antonym of "beneficial". It conveys a very strong sense of harm or negative impact, which in my opinion is not appropriate in this context, in particular in light of the uncertainties involved in contrail avoidance. I would advocate for the use of different term here such as "harmful" or equivalent.
ln. 381-382 & ln. 404: "few orders of magnitude more warming" from contrails than from CO2 is ambiguous. How is warming defined in these instances? At what time horizon?
ln. 410-411: "[...] like AGWP20 or ATR20, give a much greater climate benefit [...]" I do not think give is the right verb here. I would recommend "suggest" or "calculate".
ln. 460: "inflicted by uncertainties" in regards to the ATR is a very loaded wording choice, which I do not think is appropriate. I recommend using more neutral phrasing.

Citation: https://doi.org/10.5194/egusphere-2024-347-RC1
- AC1: 'Reply on RC1', Audran Borella, 25 May 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-347/egusphere-2024-347-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-347-AC1
RC2:
'Comment on egusphere-2024-347', Michael Ponater, 17 Apr 2024

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-347/egusphere-2024-347-RC2-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2024-347-RC2
- AC2: 'Reply on RC2', Audran Borella, 25 May 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-347/egusphere-2024-347-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-347-AC2

Interactive discussion

Status: closed

RC1:
'Review of "The importance of an informed choice of CO2-equivalence metrics for contrail avoidance" by Borella et al., egusphere-2024-347', Anonymous Referee #1, 08 Mar 2024

This is a very interesting, balanced and well thought-out article analysing the use of climate metrics in the context of contrail avoidance. Using the North Atlantic flight tracks from 2019 as a case study, the authors analyse the influence of the choice of climate metric on the decision to re-route around areas of contrail formation. Their conclusion, that the decision to reroute is largely independent of the climate metric, is very interesting and clearly shows that the implementation of contrail avoidance policies need not be unduly hampered by the choice of climate metric. However, as the authors also conclude, since the climate benefit of rerouting depends on the choice of climate metric as well as contrail efficacy and CO₂ penalty, further research analysing specific contrail avoidance strategies is well warranted.
Overall, the article is logically presented, well-written and appropriate for ACP. I recommend publication of this article subject to minor revision. I have provided my specific comments and questions below, followed by a few technical remarks and suggestions for improvement.

Main comments
1) Choice of background scenario

The authors calculate a total temperature increase for all flights of 16.9 μK in 2039, 13.5 μK in 2069 and 14.0 μK in 2119 (ln. 19). The late increase in temperature between 50 and 100 years after the emission in my opinion requires further description and justification. In Table S1 and Figure 4(a) (third panel), this temperature increase is attributed to CO₂. I assume that the emission inventory is treated as a pulse emission in the year 2019? In that case, I can only attribute this temperature increase to a decreasing background CO₂ concentration. Looking at Meinshausen et al. (2020), the background CO₂ concentration of SSP4-3.4 (ln. 168) does seem to peak in around the year 2080. I thus have a few questions:

    1a) SSP4-3.4 is characterised by a low CO₂ concentration in comparison to the other pathways. Why was SSP4-3.4 chosen for this analysis?

    1b) I can imagine that the high lingering temperature as a result of CO₂ emissions has a significant influence on the weighting of CO₂ compared to that of contrails. In particular when more fuel is burned for contrail avoidance. Have the authors considered different background CO₂ concentrations?

    1c) (How) would the results and conclusions presented in this article change if different background CO₂ concentrations were to be used? Or in other words, what is the impact/sensitivity of the choice of background CO₂ concentration?
2) Clarification of the methodology

The methodology and analysis used in section 5.2 "Imperfect contrail avoidance" is not clear to me. Is extra CO₂ emitted for rerouting, as in the previous sections? How are the number of "imperfect avoidance" flights in Table 1 defined and calculated? How is the rerouting efficiency used to obtain these results?

Minor comments
ln 35: "Uncertainties [...] and the subject of [...]"
ln 88: "It includes a sensitivity analysis [...]"
ΔF is used to denote "radiative forcing" in the definition of the AGWP in ln. 110, but then also to denote "instantaneous contrail RF" in ln. 137. Since these two definitions are not equivalent, I would suggest to use a different symbol for the calculation of EF_contrail.
In the climate metrics formulae (ln. 110-123), I would recommend performing the integrals from t_0 -> t_0 + H rather than 0 -> H, where t_0 is the emission year (2019). Or, H should be defined more clearly in the text to make the connection between H and t more clear.
The ATR was initially introduced by Dallara et al. (2011), but has subsequently been modified and is generally used as in this article. In the description of the ATR (ln. 118-122), I would therefore make a brief reference to this.
ln. 170: The number of simulations used to provide the best estimate and standard deviation, 1726, is very precise. Why was this specific number of simulations used?
Fig 1: I suggest to use the same units in both the caption and the figure (pW and pK)
Fig. 5 and corresponding text: I believe the term "detrimental" is too strong to be a good antonym of "beneficial". It conveys a very strong sense of harm or negative impact, which in my opinion is not appropriate in this context, in particular in light of the uncertainties involved in contrail avoidance. I would advocate for the use of different term here such as "harmful" or equivalent.
ln. 381-382 & ln. 404: "few orders of magnitude more warming" from contrails than from CO2 is ambiguous. How is warming defined in these instances? At what time horizon?
ln. 410-411: "[...] like AGWP20 or ATR20, give a much greater climate benefit [...]" I do not think give is the right verb here. I would recommend "suggest" or "calculate".
ln. 460: "inflicted by uncertainties" in regards to the ATR is a very loaded wording choice, which I do not think is appropriate. I recommend using more neutral phrasing.

Citation: https://doi.org/10.5194/egusphere-2024-347-RC1
- AC1: 'Reply on RC1', Audran Borella, 25 May 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-347/egusphere-2024-347-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-347-AC1
RC2:
'Comment on egusphere-2024-347', Michael Ponater, 17 Apr 2024

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-347/egusphere-2024-347-RC2-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2024-347-RC2
- AC2: 'Reply on RC2', Audran Borella, 25 May 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-347/egusphere-2024-347-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-347-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Audran Borella on behalf of the Authors (26 May 2024) Author's response

EF by Vitaly Muravyev (10 Jun 2024) Manuscript Author's tracked changes

ED: Referee Nomination & Report Request started (10 Jun 2024) by Martina Krämer

RR by Michael Ponater (18 Jun 2024)

RR by Anonymous Referee #1 (20 Jun 2024)

Suggestions for revision or reasons for rejection

I thank the authors for their work and detailed replies to my comments. All of my previous comments were sufficiently addressed. I especially appreciate the detailed response to the dependence of the results on the background emissions scenario. This is interesting work which in my opinion could warrant a description in the supplement, but I leave that decision to the authors.

Overall, I commend the authors on their interesting work and recommend publication of this article subject to one further minor revision, which I have outlined below.

**Minor revision**

In Section 5.4 (previous Section 5.3), the authors vary the contrail efficacy. In the preprint, a best estimate of 0.35 was used, which was now modified to 0.37. The authors used this best estimate for normalisation in Figure 6. However, it seems that there may have been a mistake in the normalisation or visualisation of the results, since in the new Figure 6 unity is shown for an efficacy of 0.31. Could the authors please check their (visualisation) code to ensure that the correct best estimate is used for all results?

**Technical comments/suggestions**

ln. 82: "time horizon" rather than just "horizon"

ln. 116-7: "AGWP is a time-integrated metric, and because it is based on radiative forcing, it is not an explicit measure of the climate response." Does this final clause refer to the temporal integration or to the radiative forcing? What is meant by an "explicit measure"?

ln. 120: In line with the other definitions, I would suggest ΔT(t_0 + H) here as well

ln. 390: I thank the authors for including an extra sentence here describing the rerouting efficiency factor. Since a rerouting efficiency factor of 100% corresponds to the case that all contrails are avoided, I suggest the following modification: "For each rerouting, contrail energy forcing is *inversely* scaled by this rerouting efficiency factor" (or words to this effect). I believe this makes the definition of the factor more clear.

Hide

ED: Publish subject to minor revisions (review by editor) (24 Jun 2024) by Martina Krämer

AR by Audran Borella on behalf of the Authors (01 Jul 2024) Author's response Author's tracked changes Manuscript

ED: Publish as is (09 Jul 2024) by Martina Krämer

AR by Audran Borella on behalf of the Authors (10 Jul 2024) Manuscript

Journal article(s) based on this preprint

15 Sep 2024

The importance of an informed choice of CO₂-equivalence metrics for contrail avoidance

Audran Borella, Olivier Boucher, Keith P. Shine, Marc Stettler, Katsumasa Tanaka, Roger Teoh, and Nicolas Bellouin

Atmos. Chem. Phys., 24, 9401–9417, https://doi.org/10.5194/acp-24-9401-2024,https://doi.org/10.5194/acp-24-9401-2024, 2024

Short summary

Audran Borella, Olivier Boucher, Keith P. Shine, Marc Stettler, Katsumasa Tanaka, Roger Teoh, and Nicolas Bellouin

Supplement

https://doi.org/10.5194/egusphere-2024-347-supplement

Audran Borella, Olivier Boucher, Keith P. Shine, Marc Stettler, Katsumasa Tanaka, Roger Teoh, and Nicolas Bellouin

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This work studies how to compare the climate impact of the CO₂ emitted and contrails formed by a flight, applied to contrail avoidance strategies that would decrease the climate impact of flights by changing the trajectory of aircraft to avoid persistent contrail formation, at the risk of increasing CO₂ emissions. We find that different comparison methods lead to different quantification of the total climate impact of a flight, but lead to similar decisions about rerouting an aircraft or not.


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The importance of an informed choice of CO2-equivalence metrics for contrail avoidance

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Interactive discussion

Interactive discussion

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Journal article(s) based on this preprint

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The importance of an informed choice of CO₂-equivalence metrics for contrail avoidance