Location location location: A carbon footprint calculator for transparent travel to COP27

Overall, the comments have been addressed well and the readability of the paper is improved. The inclusion of the methods section in the main body of the paper helps with the flow of the document.

However, I still recommend some restructuring of the document somewhat. At a bit over a quarter of the length of the paper, the introduction section is quite long and some of the content would be a better fit in the discussion section. In particular, I’d recommend moving the content on climate justice, the brief history of carbon neutral COPs, and the information on recommendations for virtual attendance to the discussion section. The introduction section should bring the reader to the aim of the paper and, while interesting and important, these topics do not lead directly to the aim, but rather arise through the discussion of the results (e.g. recommending virtual attendance before the analysis has been conducted seems a bit premature).

Other minor comments are given below.

Line 8 – repetition – delete “often”

Line 10 – typo – delete “in”

Line 14 – I would recommend using “lessened” rather than “mitigated”, as “to mitigate” is usually used to mean “to make less severe” (and benefits aren’t severe)

Line 43 – What does the 29% refer to?

Mention in the methods section that London is the starting point for the analysis.  The first mention of London is on line 354 in the results.

Figure 1 The first CO2e in the top left-hand corner of the calculator box looks like a zero is used instead of an “O”, i.e. C02e vs CO2e

Line 194 – typo – is vs are, i.e. “carbon benefits are explored” or “carbon benefit is explored”

Line 205 – typo – the paths of motorways and railways are …

Line 265 – typo – subscript needed for CO2e

Table 3 – Does this table cover all the countries considered? E.g Bulgaria is mentioned on lines 364-365, but is not included in the table.

Line 358 – typo – space needed between number and unit, i.e. 300 km

Figure 2 – For clarity, I’d recommend adding a footnote to explain that a flight is assumed from the destination noted, e.g. Brussels, Milan etc.

Figure 2 – Have you explained how the emissions were calculated from London to e.g. Brussels, e.g. if travelling by coach, was this assumed to be via the Eurotunnel (train) or by ferry between England and the continent? I don’t see this in the methodology, although maybe I missed it? Please update the methods section if needed to explain how this calculation was done.

Line 404/405 – There is a slight inconsistency as the text refers to “carbon-time efficiency”, but the equation below calls it “CO2e saved per hour”.

Line 449, 456 and 463, caption for Figure 5 – delete “ref” and rephrase.

There is an equation after the conclusions. Is this meant to be here?

Check the reference list. There are a few references which I think should have weblinks and dates of access, but these are not currently included.

Summary
==========
This work presents the results of calculating and evaluating the carbon footprint of two consecutive editions of the UN Climate Change Conference (COP) in the years 2021 in Glasgow (COP 26) and 2022 in Sharm El-Sheikh (COP 27). The results of the carbon footprint calculation emphasise the high environmental impact of air travel compared to bus and train travel. Further, the work provides a definition for the "carbon-time efficiency" for a given travel route, leading to a metric for comparing the trade-off between time and carbon emissions for time-sensitive travel.
Overall, the work is of great importance to the research community, but particularly to the organisers of large conferences such as the COP and beyond. The language used in the paper is of good quality and easy to follow. Nevertheless, the overall quality could be improved by considering the detailed comments below.

Detailed comments
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General
- It appears that the work provides a method of calculating the footprint for two specific conferences, rather than providing an overarching carbon footprint calculator that is universal. When reading the abstract, this aspect could be misleading, as it states that the work presents "UCL's own carbon footprint calculator." However, as the paper progresses, it becomes clear that the carbon footprint calculator needs to be extended to find the most carbon-efficient route between _any_ locations. Furthermore, all routes (air, car, train, bus) for these two conferences are calculated manually and need to be recalculated for each additional conference. This aspect should be highlighted earlier in the abstract and/or introduction to avoid confusion.

Methodology
- As the methodology of the carbon footprint calculation is one of the main contributions of the work, it is not clear why it was moved to the appendix instead of adding a dedicated Methodology section. Adding such a section would emphasise the contribution. Moreover, by reading the methodology, a high-level overview is missing. It would be helpful to provide such an methodology overview in form of, e.g., a diagram.

Carbon Offsetting
- It is appreciated that the work critically examines the topic of carbon offsetting. However, it remains unclear why the work still "recommend Gold Standard certified offsetting schemes that support sustainable development". This sounds to be contradictory to what is explained in the reflection on carbon offsets. This ambiguity should be clarified.

Generalisability beyond the COP context
- Given the importance of this work, it remains unclear why there is less emphasis in the paper on reusing this approach for other conferences and face-to-face meetings outside the COP context. Only in the section on future work it is mentioned briefly that there is an idea to extend the capabilities to also calculate the footprint between any two locations. However, to increase awareness, it would be beneficial to also describe the potential applications of such a methodology in a broader context as well.

Related work and/or background
- The paper loosely compares the present carbon calculator approach to other carbon footprint calculators. However, it remains unclear why the paper does not include a section on related work to provide critical considerations of related carbon footprint calculators and to emphasize the need for development of the present calculator. Since the appendix mentions certain improvements over other methods, a specific section on related work would help the reader find such considerations and arguments.

Figures
- Figure 1: the motivation for choosing the selected journeys is not clear. Is it based on the trips with the greatest impact? Or based on the most interesting findings? This should be clarified.
- Figure 3: the definition of the carbon-time efficiency is highly appreciated. However, the visualisation in figure 3 is not clear, especially the red line threshold. Why is the threshold set to 30 kgCO2eq? This is not clear. Additionally, it is confusing why figure 3 (a) shows the different routs per mode of transportation and (b) shows the different mulit-stops-routes. Due to that, it is not possible to compare COP26 and COP27. It is suggested to explain the threshold with more care and use the same data points for both COPs.
- Figure 4: the added value of this figure is not clear given its high complexity. In addition, it appears that the figure is reused from ref. 25. It is therefore suggested to re-evaluate the figure. Would it be possible to show only a subset of the information shown in the figure to reduce complexity?

Minor suggestions
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Title and Abstract
- The use of the abbreviation "COP27" in the title may be inappropriate for the general audience. Consideration could be given to replacing it with a more general valid term, e.g. "[...] to the UN Climate Change Conference 2022".
- It is not clear in the Executive Summary what the "carbon accounting framework" is. If the "Carbon Footprint Calculator" is meant here, this term should be used for consistency.

Language
- Consistent use of abbreviations and terms should be considered. For example, the paper uses both "COP 27" and "COP27" (with and without spaces); or "Carbon Accounting Framework" vs. "Carbon Footprint Calculator" vs. "Carbon Footprint Tool"; or "CO2" vs. "CO2e" vs. "CO2-eq", etc.
- Terms which are not generally valid should be described and introduced with more care. For example, "Gold Standard certified offsetting schemes", or "LTO" and "CCD".
- The Carbon Footprint Tool section states that the goal is to raise public awareness by comparing direct flights to _green_ alternatives. However, these alternatives are far from green. This language issue should be addressed by using the term "green" carefully (even without giving a clear definition of "green"). I suggest softening the tone, e.g., " more green alternatives," or changing it, e.g., to "using less carbon-intensive modes of transportation."

Figures
- All figures could benefit from using vector-based graphics (e.g., PDFs) instead of pixel-based graphics (e.g., PNG). This would allow high resolution figures and prevent blurry figures, which could be especially important for Figure 4.

This is an interesting article that compares the carbon footprint of various options for travel to the upcoming COP in Egypt.  The article is timely, highly relevant and of importance in providing an evidence base to support decision making and counteract greenwashing.  I would recommend a few changes to the article before it is finalised.  Comments and suggestions are provided below.

My main comment is that the structure of the article is a bit unusual in that it does not follow the standard scientific format of introduction – methods – results – discussion – conclusions.  While the article loosely follows this layout, there is no dedicated methodology section.  Information on the methods is given in an appendix and in places throughout the main text, but I found it a bit disjointed, with queries often occurring that were then answered later in the text.  I would recommend preparing a dedicated methods section to appear after the introduction and to contain relevant information relating to the assumptions, boundaries and methods.  If the authors decide to keep the appendix, then I would still recommend a dedicated methods section that gives an overview of the steps and assumptions, and that points clearly to the appendix.  An algorithm or flow chart showing the steps and/or a diagram showing the boundaries of the analysis would be a useful addition.

The other main comment I have is regarding the mention of climate justice and related issues.  The abstract infers that the principles of climate justice were incorporated into the carbon footprint calculator, but I’m not sure that this has been done.  The conclusions section also raises the importance of supporting sustainable development and promoting equity and climate justice, but this hasn’t really been addressed in the main paper.  Due to the stated importance, I would suggest expanding on the topic in the main paper.  The topic is outside my main area of research, but as a suggestion you could look at this work which I undertook with colleagues on the principle of justice in the renewable energy area: https://doi.org/10.1016/j.jclepro.2021.129280 .  It might give you some ideas.

A few minor comments are below:

• Figure 1.  I assume that this graph shows the results for train from London to e.g. Brussels and then a flight to Egypt.  Could you please clarify this in the figure title.
• For the equation that appears above Figure 3, could you please clarify what is meant by “route”?  Is this the part of the journey that does not involve flights, or is this the entire journey including both flights and other means of transport?
• The terms CO2, CO2e, CO2-eq and CO2-equivalent are used variously throughout the paper.  Please check that the terms are used consistently.  Should all values be in CO2e?  If results are mixed and reported in both CO2 and CO2e, then I would advise amending the calculations so that all values are reported in CO2e.
• In Figure 3, the red line indicates points equal to 30 kgCO2e/hr.  The text below the figure states that journeys “beneath this line are considered worthwhile by an individual within this threshold”.  I find this a bit confusing.  Why is this threshold assumed?  Please expand on this.
• In the section on indirect effects, the referencing is done by name and year in places rather than by number.  Please keep the referencing style consistent.
• Also in this section, the text recommends that users double the amount of carbon offsets they purchase.  The next sentence states that the carbon footprint of the flight should be multiplied by 4.5.  Please expand on this to fully explain the points being made here, as the text appears contradictory.
• Is copyright permission required for Figure 4?  I also wonder if the image is really needed for your paper.  It’s an interesting image, but it might be better to use the space for your own results.
• The conclusions section raises interesting points, but a few of the issues mentioned have not been covered in the main text.  Generally, in academic writing, there should be nothing new in the conclusions section.  Examples are the references to financial investment, which wasn’t investigated in this paper (but the text in the conclusions implies that it was), along with conflicts in the Middle East.  These are both important issues, so I would suggest including discussion in the paper to address them.  The conclusions section also refers to the carbon footprinting and offsetting already undertaken by the Egyptian government; again, I don’t think this is covered in the main text, but it would be useful for it to be included.
• In the appendix in the section under flights, please give the reference for the average taxi time and the approach used by BEIS.  How do your results compare to the values in BEIS?  Also in this section, please provide the references for the EEA equivalent, the ICAO calculator and the values reported by Boeing and Airbus (a reference is give in the table caption, but not in the text).
• The text immediately above Table 1 in the appendix states that fuel is converted to GWP “as described”.  Where is this described?
• I suggest numbering tables and figures that appear in the appendix using a different system than is use for those in the main text, e.g. Table 1 vs Table A1.  Check the journal guidelines, but table numbers and captions usually appear above the table rather than below.
• In the section on passenger cars, it appears that all vehicles were assumed to be hybrid or electric.  I think this is the first time this has been mentioned.  Please explicitly state the assumptions (as per the comments on a suggested methodology section above).
• In the main text above Figure 1, the text states that electricity in France is carbon cheap, while in the appendix the text states that the UK emissions factors were used for all journeys.  Were UK emissions factors used for cars and specific country emissions factors used for rail?  If this is the approach taken, is it reasonable (as it appears to be a bit inconsistent)?  Was all rail travel assumed to be electric?  While electric rail might be more common than diesel overall, is it more common for the long-distance journeys investigated here?  The text states that it was assumed that electricity consumption is dominated by passenger rail as opposed to freight.  Please explain the relevance of this assumption.  Why was the value of 0.8 used?
• Please explain the terms tier 1, tier 2 etc at first mention.

Overall, this was a very interesting article and I would be keen to see an updated version.