the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Feb 2024
The importance of an informed choice of CO2-equivalence metrics for contrail avoidance
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.
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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 comments1) 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 commentsln 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 -
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
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