| 12 Feb 2026
Concept of risk-aware contrail avoidance strategies
Abstract. Targeted contrail avoidance consists of rerouting aircraft to minimise the formation of contrails whose warming of the climate system can be much larger than that due to the CO2 emitted for some of the flights. A commonly proposed strategy is to reroute all flights for which the trade-off between additional CO2 emissions and reduction in contrail warming leads to a climate benefit. However, current predictions of contrail climate impact are highly uncertain. In this study, we describe a framework to integrate the risk of unintentionally damaging the climate in the contrail avoidance decision-making process, using the Contrail Cirrus Prediction model (CoCiP) and operational ensemble weather forecasts. Optimising trajectories around a best estimate of contrail radiative forcing then including weather and parametric uncertainties in that predicted forcing in a second step reveals that 55 % of the reroutings have a higher-than-5 % risk of unintentionally damaging the climate compared to a standard risk-unaware avoidance strategy. This fraction increases to 76 % when choosing to reject any risk. However, the reroutings that are the least risky to operate are also those with the highest potential climate benefit, often referred to as ‘big hits’. Alternatively, accounting for uncertainties from the start of trajectory optimisation allows to mitigate the risk directly when planning the flight. This strategy would even result in a 52 % higher potential climate benefit compared to the risk-unaware avoidance strategy, when choosing to reject any risk. Our results thus demonstrate that the risk of unintentionally damaging the climate can and should be included in the decision-making of contrail avoidance, in particular in the context of early adoption policies.
Competing interests: Audran Borella and Olivier Boucher are founders and employees of the Klima Consulting company, that aims at reducing the climate impact of aviation, among other objectives. Cameron Steer and Nicolas Bellouin declare that they have no competing interests.
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Review of
"Concept of risk-aware contrail avoidance strategies"
by A. Borella et al.
General impression and recommendation:
The authors use a flight planning tool in combination with numerical weather forecasts and a contrail prediction tool to study three concepts of contrail avoidance with increasing awareness of the risk involved. This is the risk of unintendedly damaging climate when the rerouting required for contrail avoidance leads to higher fuel consumption and emissions such that the increase of the CO2 climate impact is larger than the climate benefit by contrail avoidance. The three concepts are: 1) planning a cost-climate optimal flight, that is, the cost-optimal contrail avodoidant route and fly it; 2) as 1), but then use ensemble weather forecasts to estimate the bandwidth of the potential climate impacts and fly the cost optimal route only if the there is a (positive) climate benefit in almost all ensemble members; 3) optimize the route in all ensemble members (i.e., plan N flights), and combine each of these routings with the remaining members of the ensemble, then select from those flights that have low risk the one with the largest predicted climate benefit.
To my feeling, this is a good strategy of research and the paper is a valuable contribution to the collection of ideas how to deal with the uncertainties caused by the contrail-avoidance vs. fuel consumption trade-off.
In the following I'll give some ideas for further improving the paper. All these are eventually minor comments.
1) My most important comment refers to the description of the "risk-optimal strategy" in Section 2. This description is not good and misleading. The reader gets a full understanding only in Section 6. What I understand from Section 2 is this: For each of N members of an ensemble forecast a climate cost-optimal route is computed. The uncertainty-based risk assessment mentioned in the previous par. ("risk-informed strategy") is then performed on each of the N flight trajectories (if this is correct, please state it clearly). A subgroup M
2) I would also like to suggest to the authors to consider the distribution/clustering of their "big hits". Admittedly, this is not central to this paper, but it is still an important topic that, to my view, has received too little attention in the past. I suggest that the data is screened for big hits, that is, extract the flights with the highest 2% of contrail warming and check where they appear spatially and temporally. I expect some clustering and this should make big contrail prediction easier and with lower risk. It could, however, lead to problems with airspace congestion if a big hit cluster is to be avoided completely. I believe you can here enhance the importance of your paper with little effort.
3) A point for the discussion section is this: The study uses the NAFC as the model region. Do you think that the risk-informed and -optimal methods can be applied to more congested air spaces like Europe. How should the nearby presence of cirrus clouds and other contrails be treated?
4) The expression "to reject any risk" (twice in the Abstract and also later in the text) is surprising. This sounds as you would think of a significance level of zero (that is, probability of an error of first kind is zero). In a system with random elements this is not possible. In aviation this would imply to stay grounded.
5) Line 22: I suggest to replace "contribution" by ERF. Contrails have the largest ERF, but perhaps not the largest contribution (to anything).
6) Line 24: replace "saturation" with "relative humidity". Saturation is 100% and does not exceed it.
7) Section 2, description of the "risk-informed strategy". This was difficult to understand, probably because the additional calculation of the cost-optimal route is mentioned at the beginning. The reader must think that this is the important step, but it is not, I believe. I suggest to present this method as follows: We start as in the risk-unaware method with a climate-cost-optimal routing, then we use the uncertainties to estimate a risk. If the risk is too large, we calculate the usual clost-optimal route and fly it.
8) Section 3.2, last par: Correction of the humidity field is necessary since NWP models often underestimate RHi in ISSRs. Perhaps you should note explicitly that the problem that ISSRs are often not predicted at their actual position cannot be fixed easily and that it needs more data for assimilation.
9) Section 3.3, around line 207: It might be that the uncertainty is not weather-dependent, however, the efficacy factor itself is probably weather dependent. As the efficacy somehow measures the integrated effect of fast feedbacks over the course of weeks or months, it will certainly vary with the weather. I suggest to clarify this point. Otherwise, to simply assume a constant factor for this study is as a first step certainly in order.
10) Section 4.1: I was surprised that flight A that exploits the jet stream needs more fuel then flight B that has strong headwinds, whereas both flights are quite similar in distance. Is this perhaps a consequence of the two different aircraft? Is the B767 more efficient than the B777?
11) Section 4.2, 2nd par: Is there a (tentative) explanation for this surprising finding, namely that flight A is sensitive to wind shear direction, while flight B is sensitive to particle emissions?
12) Section 4.2, around line 328: It is even worse, because the nominal estimates can be very different from the actual weather.
13) Section 4.2, end: Now it becomes clear what you mean with "zero risk". It can be zero because you use a finite sample. Anyway, I suggest a slight rewording, say "negligible risk" and add in brackets where it is mentioned first (with zero climate damages in our finite sample).
14) Section 6, line 446-447: The sentence is easier to read if the last part "when adopting..." is shifted after "number of rerouted flights".
15) Sect. 7: I agree that "the estimation of the climate benefit of reroutings must not be reduced to only one deterministic modelling configuration". I am not sure, but isn't this what the MRV-system is just doing to "determine" the contrail impact of single flights? Do you know more and can you comment on this? I think, your statement is an important recommendation.