The Impacts of Climate Change on Aircraft Noise near European Airports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The present manuscript tackles a very relevant problem for future civil air transport, which is its change in response to future climate change and its modified impact on inhabited areas in terms of aircraft noise. However, while the climate side is handled reasonably well within the bounds of a single paper, the same is much, much more questionable for the elements related to aircraft performance, noise prediction, and affected population. These questionable elements lead to equally questionable outcomes, from which some strong claims are made without good enough scientific backing. The manuscript has some valid elements, but to make it acceptable for a journal quite a large number of modifications must be made, targeted especially at explaining the many simplifications and toning down the claims.

For these reasons, I recommend a ‘major revision’ to be performed by the authors, highlighting that I’m expecting rather careful work by the authors. Below are my major remarks, followed by some minor comments.

Major remarks:

• Lines 67-72: aircraft performance and airport noise modelling in literature goes well beyond the contributions referenced by the authors, which are only the consolidated tools. Among the many examples see for instance https://doi.org/10.1016/j.apacoust.2017.10.017, https://doi.org/10.3390/aerospace7080104, https://doi.org/10.1186/s12544-019-0389-x, https://doi.org/10.1016/j.trd.2025.104710, from which the authors can also find additional referenced works handling the aforementioned topics. As it stands, the introduction is very poor in that regard.
• Examining the future of aircraft noise goes well beyond the simplifications made by the authors. For a >30-year span, key added elements are the fleet renewal (which has led since the 1970s to a 0.3-0.4 dB/year noise decrease), the air traffic growth (which, on the contrary, leads to a noise increase), and possible modifications of departure flight procedures (which have already occurred to curb noise in certain highly populated areas). One of the sources above also tackles these elements, albeit on a shorter timeframe. The introduction should make it much clearer that this work decides to disregard all these factors, focusing only on climate change effects.
• Sections 2 and 4: throughout this modelling, it appears as if aircraft noise is only a function of the engines. However, even the AEDT manual and ECAC Doc 29, from which IMPACT is formulated, clearly say that airframe noise is a key component of the overall noise spectrum. Yet no mention of this is made in this manuscript. The only saving factor is that, only at take-off, it can be argued that engine noise is predominant. Please at least say something about it, and state that the assumption is to take engine noise as the key contributor to aircraft noise
• Section 3: in light of what I wrote earlier, assuming that T, D, and W at same density (e.g. ISA) will not change 40 years into the future is hardly believable. The necessary conditions that COULD make it acceptable is saying earlier, as I recommended, that the aircraft fleet renewal is here disregarded. And please note once more that this is a huge assumption.
• Global comment up to Section 4: what I cannot understand is what advantages the method proposed by the authors gives compared to, for example, applying IMPACT just with different atmospheric conditions as provided by any SSPx-y scenarios of future climate. Is it less computationally intensive? Is it more accurate? Anything else? This must be made clear (and backed by scientific evidence) in this paper, because this essentially corresponds to the claim of this work being or not valid.
• Section 5: another huge simplification is in the estimates of population affected, because to me it appears that the authors have neglected any change over time in the number and location of people living in their 30 cities. The noise contour area changes can be argued to be credible, though very simplified in this work as discussed, but extending these area changes to the estimation of population affected in the future is quite the stretch. For example, Milan’s population dropped by over 10% since the 1980s, while Vienna’s population has risen by 30% in the same timeframe. And I won’t even start talking about the population patterns (suburbs vs city centres), and future predictions. I hope that the authors are aware of the massive assumptions that they made. The minimum I can think of, again, is clarifying that the population change is disregarded, which of course requires curbing any claims as to the validity of the estimates made throughout the paper, abstract included. The only honest judgement one can make in front of these outcomes boils down to “If nothing else changes apart from climate, a few more people will be annoyed by noise à manufacturers, airlines, and local authorities should keep this into consideration”. In other words, aiming for a more modest target is better than making haphazard claims unsupported, at the moment, by facts.
• On a clearly positive note, the level of English is very high, and the paper flows quite well.

 

Minor comments:

• Lines 50-52: noise exposure in take-offs is primarily a function of distance and thrust (level), not distance and sound intensity. Noise exposure = cumulation of sound intensity over time.
• Line 89: add a reference for Latin hypercube sampling technique.
• Lines 123-124: the atmospheric absorption coefficient is heavily dependent on frequency, and aircraft noise is a wide spectrum from 50 to 10k Hz. What justifies your chosen alpha_0? Please expand on this choice, quite important since your manuscript focuses on climate change.
• Line 127: point source with a size? What does it mean? Do the authors want to say that noise should not be computed if r is under 10m from the source? Please clarify in the paper.
• Figures 5 and 8 are lacking in resolution, the dpi needs to be increased especially to make out the scatter points inside the ‘violins’.
• In figure 7 the white squares are often invisible, and in each subplot the map is uselessly much larger than the square, please fix. A suggestion is changing the colour palette (e.g. no yellows)
• Figure 8 has no y-axis labels. If, as I suppose, they are the people affected, I highly recommend to use percentage increments. 60 or 250 more people is useless if one does not know the starting point (i.e. 1k people historically? Bad, high increase. 1M people? Absolutely inconsequential).
• Line 268: please indicate the population density as ‘people * km-2’, or ‘people/km2’, or something similar, but don’t use only ‘km-2’, which is misleading.
• Line 380: acronym IMPACT has a ‘68’ in its extended definition, I assume it’s a typo.

Author Response

We are grateful to the reviewers for taking the time to review our work and feel that having taken all their comments and suggestions into account that the work is substantially improved. Please find our detailed responses in the attached file.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Attached

Comments for author File: Comments.pdf

Author Response

We are grateful to the reviewers for taking the time to review our work and feel that having taken all their comments and suggestions into account that the work is substantially improved. Please find our detailed responses in the attached file.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript concerns a very interesting issue: the impact of climate change on noise emissions in the vicinity of airports. The authors correctly assumed that an increase in average air temperature would result in a decline in aircraft performance, which would result in a reduction in the flight path angle during climb. Consequently, ambient noise exposure would increase. I find the idea for this type of analysis very interesting and original.

The work uses the proven IMPACT noise modeling method. The influence of temperature changes on the change of the flight path angle was clearly explained by the authors in chapter 3 and presented in the form of a simple relationship (11). The noise contours obtained from the model were compared with the results of other works and similar results were obtained.

I have some reservations and comments regarding the presentation of the calculation and simulation results:

• Figure 3 - three graphs are presented, but only the one on the left has a description of the vertical axis - please describe the axes on the other two graphs,
• Figure 5 is one of the key figures for the work, but it contains too many miniature graphs - I think it would be worth selecting an example of one airport from it and placing it in a separate, additional figure with detailed explanations,
• I think that, following the example of Figures 5 and 8, it is worth adding graphs illustrating the change in the flight path angle in response to climate change - this will also be very interesting for the reader.

I noticed only minor editorial and formal errors in the work:

• the IPCC acronym should be explained in the work (I assume it stands for Intergovernmental Panel on Climate Change),
• no period at the end of the sentence on line 104,
• unnecessary dot after the IMPACT abbreviation on line 72,
• I have doubts about placing the table caption below it.

Author Response

We are grateful to the reviewers for taking the time to review our work and feel that having taken all their comments and suggestions into account that the work is substantially improved. Please find our detailed responses in the attached file.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

Line 5: What is a multi-engine capability with regards to noise modeling?

Line 9 and throughout paper: Mention clearly earlier that it is the 50 dB L_den contour.

Line 28: You're missing 20 years of data here. The last 10 years have been significantly warm and could have an effect on both your historical results if you include the missing years as well future projections. How are the missing years accounted for in the results?

Line 35: Clarify that you mean the 60 dB Lden contour.

Line 36: “contour around Chios airport has increased by almost 5% in the 25 year period up to 2022” – That is a curious observation as aircraft have become considerably quieter over the past 25 years and the contours around most airports have become smaller unless the number of flights increased. I'm not sure how much of the observed increase can with certainty be attributed to increased temperatures.

Line 54: Local population density does not affect sound transmission but number of people affected by noise. Urban artifacts such as buildings or trees can affect sound propagation through screening or reflections but that is not meant in the text.

Line 123: “α₀ = 0.0001 dB · m−1, which is a good approximation for, say, 1 kHz and 50% relative humidity.” – The absorption coefficient also differs with temperature, which is the subject of this paper. How is the effect of temperature on atmospheric absorption taken into account?

Figure 1: What are the numbers 40-55 in Figure 1? Mention that in the caption along with the units.

Equation 9: Equation 9 is confusing. Is it meant that the relation holds true for both past and future conditions or is some combined relation intended? Perhaps split the equation or abandon the 1,2 formulation.

Line 191: Use ‘broadly representative for’ instead of equivalent to.

Line 209: Not sure about age of the engine but more whether an older or more recent engine is used on the aircraft. Newer engines (certainly GTF) are quieter than older engines. I don't expect that the noise deteriorates with age for a given type of engine.

Line 209: The noise metric doesn't change the sound intensity. It is a means of converting the sound intensity into an understandable and relatable form that can be used for different purposes (certification, community noise impact etc.).

Line 217: Flight operation or climb angle?

Line 220: Add a reference for this 2dB reduction. I assume it is an interpolation of the 3 dB reduction for halving of the sound power or intensity.

Line 221: Explain how the sample size is chosen and why the values are so different for the different samples. It is for instance unclear how or why sample 6 results in the lowest difference in area.

Figure 3c: Better to reformulate as 'The larger the climb angle the smaller the A50 value' as it relates better to the intended optimization for area.

Table 1: Is the unit of the angle not degrees?

Caption Figure 7: Why was San Sebastian airport chosen as an example in the earlier section if not all data is available for this airport? Why not use an airport for which you have all the data?

Figure 8: What is the 'value' in figure 8? It looks to be number of additional residents within the contour area. Better to put this clearly in the figure.

Line 293-300 can be moved to recommendations. The text doesn't really fit below figure 8.

Conclusions: What is perhaps lacking in the conclusions is the impact of the observations on for instance spatial planning and airport operations. So if 300 people fall within contours in the future, what does that mean for policy makers and for the aviation sector. In effect, what are the real-world consequences of this study and the findings therein? What actions would be needed to counter any negative effects? I see that the study was done but why exactly and what the potential real-world consequences are don’t come through very clearly. Should be improved in the text.

Line 328-345: From here you're talking about recommendations for future work.

Comments for author File: Comments.pdf

Author Response

We are grateful to the reviewers for taking the time to review our work and feel that having taken all their comments and suggestions into account that the work is substantially improved. Please find our detailed responses in the attached file.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have addressed all my comments with sufficient care, providing a revised manuscript of significantly higher quality than the original one. Additionally, the claims of the paper are more  contained and consistent with the methodology proposed by the authors. I believe that the manuscript can be accepted in this present, revised form. Best of luck to the authors for their future endeavours.

Author Response

We are delighted that this reviewer has accepted our additional work and deems it suitable for publication! 

Reviewer 2 Report

Comments and Suggestions for Authors

In attached file

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Thank you for your response and for incorporating my comments into the manuscript. In my opinion, the article is suitable for publication in its current form.

Author Response

We are delighted that this reviewer has accepted our additional work and deems it suitable for publication! 

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

Despite the authors’ explanations, I still do not see their assumption that the temperature effect does not influence aircraft noise at the source as sufficiently validated. In lines 180-207, the following reasoning is provided:

- Temperature increase reduces jet thrust, so to maintain constant ground acceleration the thrust settings have to be changed, which leads to jet noise increase.

- Temperature increase reduces the fan’s tip Mach number as per Eq. (7), which leads to fan noise decrease.

- Jet noise increase and fan noise decrease are approximately equal and thus compensate for each other.

However, this explanation is not satisfactory. Indeed, such an approach implies that fan and jet are not related, so that jet thrust can be changed without changing fan rotation speed (which in turn changes its tip Mach number). In fact, the RPM should actually change (the jet speed has changed!), therefore, the assessment of the contribution of the fan, which compensates for the increased jet noise, was performed incorrectly. In addition, the noise of the jet itself with different velocity should be evaluated more carefully. Thus, in my opinion, the authors’ assumption that the engine noise does not depend on air temperature has not yet been validated.

I do agree with the authors that turbofan engine noise is a very complex phenomenon and its description is associated with non-trivial uncertainties. That is why I have proposed in the previous reviews to either provide some experimental data for the temperature effect on engine noise or make use of some regression models that contain temperature as an input variable (if such models exist).

Yet another approach could consist in modeling turbofan engine parameters with an open-source software such as PyCycle, when air temperature differs while total thrust remains the same. This would give necessary input parameters to calculate turbofan noise (e.g. via Heidmann’s model for fan noise, Stone’s model for jet noise etc). Thus, a quantitative estimation of the temperature effect on engine noise could be obtained. This can be done for a generic, not commercial engine to avoid proprietary problems, but such (or similar) thing has to be done to account correctly for interdependencies of jet and fan noise, thus fixing the current explanation in the manuscript.

I would like to stress that the discussion concerns a key assumption of the study. Although the specific value of IdB,0 is just an adjustment parameter to provide a good agreement with IMPACT model, keeping the value constant for other temperatures is not an obvious step at all. And unlike many other simplifications of the study, this assumption can significantly alter its results and conclusions.

Author Response

Please see the attachment.

Author Response File: Author Response.docx