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

We would like to thank Mr Markus Funke and Dr Beatrice Smyth for their kind comments about our research. Overall, the reviews agree that our initial submission
is of importance to the research community, but that it could benefit from some changes. Below, we respond to each individual comment.

Mr Markus Funke (Review #1)
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.
Thank you for the time that you have spent reviewing our research, and the insightful comments about it.
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. 

Response: Thank you for highlighting this issue. Whilst the calculator is not currently able to provide users with information on any journey between any two locations, it is
designed in such a way that it is scalable with further data. In particular, calculating the carbon footprint of any journey within the UK+EU would be relatively trivial.
However, we stopped short of fully automating this process due to the narrow scope of the project regarding COP 27. We have clarified the abstract (line 12) and the model
description (lines 217-221) to better reflect this.

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 the form of, e.g., a diagram.

Response: We agree and have moved the appendix to a dedicated ‘Methodology and Data’ section.

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.
Response: Where all possible avenues to reduce emissions have been taken, we see carbon offsets as a last resort to minimise the environmental impact of travel. Whilst there
are many issues related to carbon offsets, we attempt to address only one – namely the imbalance between the global North and South with regards to who legislates
carbon markets, who benefits from them, and who receives the secondary benefits provided by carbon offsetting schemes in terms of energy, health, education, food
and water security. This is why we recommend offsetting schemes that contribute to the UN’s Sustainable Development Goals, such as those offered by the Gold
Standard. We have slightly edited this section to further emphasise the importance of emissions reduction in lines 178-184, but we welcome suggestions to explain this
stance in more detail if it is felt necessary.

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.
Response: Thank you for the suggestion. We have reworked lines 217-221 to include mention of the application of this approach to further COPs and international conferences. The
intention is to continue to update the model each year prior to COP. 

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.
Response: We agree and have expanded the “Carbon Footprint Calculators” section to include a more detailed discussion of existing calculators and how our calculator contributes to
this space. However, due to the enormous variety in approaches to carbon footprinting, we feel that reviewing the full extent of existing calculators would merit a separate paper and we instead cite ref 18 for further reading on this topic.

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.
Response: The routes were chosen to represent varying degrees to which convenience might be traded off for carbon efficiency. The direct flight is assumed to be the most carbon
intensive option, with shorter flights (and a greater proportion travelled by road or rail) assumed to be more efficient. As the calculator shows, however, this is not
necessarily the case. We have clarified this in the text in lines 212-217.

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. 

Response: Thank you for this feedback. The threshold of 30 kgCO2e hr-1 is very much a subjective threshold and we have further emphasised this in the text. However, one
could view 30 kgCO2e hr-1 as the rough carbon savings made by taking the train to COP 26 instead of flying. We propose that readers can use this as a reference point
for assessing the travel options for COP 27. Visitors to COP that saw the COP 26 train option as worthwhile should be encouraged to take any route option to COP 27 that is
below or to the left of the red threshold in figure 3, as these would be equivalent or better in terms of their personal sacrifice and environmental benefit. However, we
note that travel cost would also confound those decisions and discuss this in the further work section. We have further clarified these points in lines 466-475.
Unfortunately, it would be impossible to compare both COPs in the way proposed, as there is no route to COP 27 that uses exclusively rail or exclusively road. However, we
feel that the two COPs are still comparable in figure 3, since each route, regardless of modes of transport, is fundamentally about a trade off between convenience (time)
and carbon footprint. In the case of COP 26, the trade-off is purely a question of mode of transport; for COP 27, the modes of transport are mixed, but the trade-off is made
in terms of the way-points of London, Brussels, Milan, or Istanbul.

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?
Response: Yes, certainly – we have adapted the figure to remove some of the less important information and convey the main message (that direct CO2 emissions are but a
fraction of the possible overall forcing related to aircraft) more clearly. Please find the adapted figure in the paper.

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".
Response: We have made your recommended change, thank you. 

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.
Response: We use “carbon accounting framework” to mean the legal and financial systems that allow for the measurement and offsetting of carbon emissions. However, we have
rephrased lines 16-18 of the abstract to be less ambiguous.

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.
Response: hank you for highlighting this issue. We have cleaned up the language in all these cases to be more consistent.

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".

Response: We have removed explicit mention of the Gold Standard and simply referred to “Carbon offsetting schemes which promote the UN’s Sustainable Development
Goals”. As well as avoiding ambiguity, we feel this is also less reductive. With the movement of the appendix to the central methods section, LTO and CCD are
now introduced at a much earlier point. However, we welcome suggestions to explain these terms further if it is deemed necessary.

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."

Response: We agree and have replaced instances of “green” transport with softer language, thank you.

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.
Response: Thank you for the suggestion – we have replaced all images with SVGs.

Dr Beatrice Smyth (Review #2)

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.

Response: Thank you for the time that you have spent reviewing our research, and the insightful comments about it.

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 flowchart showing the steps and/or a diagram
showing the boundaries of the analysis would be a useful addition.

Response: This feedback is common between both reviewers, so we have moved the appendix to follow the introduction as a dedicated methods section. We have also added a
flowchart diagram overview of the model. Rather than include flowcharts for each model component, we have opted to bullet point the important features of each
component in this diagram, but we welcome feedback to how this could be improved if necessary.

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.

Response: We have softened the language in the abstract to no longer claim that climate justice has been incorporated into the calculator. Instead, we have tried to highlight that the
calculator exists with the context of COP 27, during which climate justice was to be a key focus. This work has also been carried out in tandem with a second paper
published last month (https://www.doi.org/10.14324/111.444/000180.v2), which focuses much more on climate justice and is referenced on line 81. In the interest of keeping this
paper focused on the carbon footprint calculator, we are hesitant to expand on our discussion of climate justice here. We have mentioned it throughout the paper to provide
context for the calculator, but feel that readers are better served by exploring the literature we have referenced for a more detailed discussion.

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.
Response: Thank you, we have now explained this in the figure caption and have also improved clarity in the model introduction (lines 212-217) so that it is more clear to readers what the
routes represent.

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?
Response: The route CO2 and route time values in this equation are meant to represent the total carbon footprint and travel time for the journey, including sections that involve flight.
Thank you for highlighting the ambiguity – we have included some lines after the equation to clarify this to readers.

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.
Response: We have amended all instances of these to CO2e for consistency, except for where they refer explicitly to carbon-dioxide emissions only, such as in the discussion on indirect
aircraft emissions.

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.

Response: Both reviewers have highlighted this as an issue and we agree that the initial explanation of this threshold felt arbitrary. We used the threshold as it closely matches the efficiency
of the COP 26 train option. This allows readers to judge the routes to COP 27 against something familiar. It encourages readers to consider a travel decision they may have
made in the past (to fly or take the train from London-Glasgow?) and use their beliefs about this journey to inform their decision on how to travel to COP 27. We suspect that
many climate-conscious UK readers would opt to take the train or coach to COP 26 and so figure 3 explores whether a similar carbon saving can be made in the case of COP 27.
However, the figure shows that no such route exists. We have added lines 466-475 to better justify this in the text.

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.
Response: We have edited this section to make the referencing style consistent, thank you.

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.
Response: We agree that this comes across as confusing for the reader. Our intention was to translate the uncertainties in the indirect effects of flight into actionable advice for the
reader. As we note in the text, we don’t feel comfortable recommending offsetting according to the upper bound (95th percentile) of footprint estimates because we are
conscious of how uncertainty could be weaponised to undermine confidence in the calculator. However, we also want to give the option for readers to account for the full
possible effects (4.5x the direct CO2 effects) if they should so choose. To make this seem less contradictory, we have simply changed ‘double’ to ‘at least double’ in the text.
Although this is a small change, we feel that it properly conveys that ‘at least double’ is the recommendation, whereas ‘4.5x’ is an optional extra. 

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.

Response: Although we would value the space saved by removing the figure, we feel that it saves enough words to warrant its inclusion here. However, we agree with a comment from
another reviewer that it includes excessive information for our purpose and can be improved. We feel that comments from both reviewers are best addressed by adapting the
figure into one of our own, which trims down the information to focus on the points important to our argument.

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.
Response: Thank you for pointing out some of these inconsistencies. The offsetting by the Egyptian government is mentioned very briefly in section ‘A brief history of Carbon Neutral COPs’,
but only to say that has not been made public at the time of writing. However, measurement and offsetting by previous COPs is discussed at length. We have made sure
to now include discussion of the conflict in Iraq and Libya much earlier in the paper. Reference to financial investment in this section is meant to refer to purchasing carbon
credits, which is also discussed briefly in section ‘Carbon Footprint Calculators’. We are hesitant to delve too deeply into climate finance, as the focus of the paper should be on
the calculator itself. However, due to the context of COP 27 and the relevance of climate justice at this particular COP, it felt important to mention this in the Introduction and again
in the conclusion.

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)
Response: Thank you, we’ve added the requested references. Taking an economy flight from London to Sharm El-Sheikh as an example, our calculator finds that the least-emitting flight is
from Gatwick on a Boeing 737 MAX 8, with a footprint of 253 kgCO2e. The BEIS approach uses a simple conversion factor of 0.074345 kgCO2e per passenger-km for economy.
London to Sharm El-Sheikh is ~3881 km, therefore the BEIS estimate would be ~288 kgCO2e. However, the BEIS conversion factor does not specify an aircraft. Our calculator
also finds flights from London to Sharm El Sheikh on an Airbus A320, Airbus A321, and Boeing 737-800, with footprints of 290, 286, and 303 kgCO2e respectively. If it were to
average across aircraft, our calculator would therefore roughly agree with the BEIS. There is possibly an argument that our calculator should show the footprint of the
average aircraft and not the minimum, as it does currently. This would certainly increase the footprint of the direct flight option. However, we feel that users are likely to be
motivated to minimise their flight footprint after they have chosen a route. This is relatively easy for them to do, since Google Flights and Skyscanner – two popular
websites for booking flights – will direct users towards flights with comparatively lower footprints. We see the benefits of both approaches, but feel that the specific context of
COP27 and its typical climate-conscious visitor supports our minimising approach.

The text immediately above Table 1 in the appendix states that fuel is converted to GWP “as described”. Where is this described?
Response: It is described in the first paragraph of the flights section: “Greenhouse gas emissions are […] converted into CO2e by the Global Warming Potential (GWP) factors outlined in the
IPCC’s sixth assessment report”. However, we have now edited this line to be more explicit.

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. 
Response: Whilst this is helpful feedback, we’ve since moved the appendix into the main text, so it no longer applies.

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).
Response: The passenger car component of the model does in fact account for fuel vehicles in addition to hybrid and electric. However, we see how the phrasing at the beginning of this
section could imply that they are excluded. We have added a line mentioning fuel vehicles  to make it clear to readers that they are included.

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?
Response: Regarding road travel – yes, the UK emissions factor was used for Cars and specific countries were used for rail. As we discuss in the text, the choice of emissions factor in
the case of electric cars depends strongly on where the car is charged. It’s conceivable that an electric vehicle could traverse a country without ever charging from its electricity
grid. There is an argument that the footprint should be weighted by the distance it spends in each country along its route. This would provide a footprint based on the average
behaviour of drivers, assuming they charge their cars at random (although they typically don’t, as they are motivated by prices). We decided against this option largely due to
scalability of the calculator. As the number of possible routes increase, the calculator would need to consider a much larger number of roads and calculate the proportion of the
journey spent in each country. Rail networks, on the other hand, are comparatively simple, with only a handful of high-speed international routes, allowing this work to be
done manually. Whilst we haven’t accounted for varying emission factors in this version of the calculator, we do agree that it would improve the model and will work to include these effects into
future versions of the calculator if possible. Regarding rail travel – yes, all railway routes are assumed electric, since the routes selected to COP 27 are all along high speed railway lines that have been electrified. The rail electricity consumption data in the model includes both passenger and freight. Since the calculator aims to find the footprint of a journey that is guaranteed to be a passenger
journey, it is necessary to approximate the share of the electricity consumed by passenger rail. Since freight electricity consumption is negligible (in the UK, at least), we approximate the passenger share as ~100%. Similarly, the passenger-km data includes both diesel and electric trains. Since the calculator aims to find the footprint of a journey that is guaranteed to be electric (due to
the routes chosen), it is necessary to approximate the share of passenger-km accounted for by electric trains (~80%). Failing to do this would erroneously attribute some of the
electricity consumed by electric trains to diesel trains, giving a lower carbon footprint for our journeys than would be accurate. The value of 0.8 was used to represent the 80%
share, as is estimated by the European Commission.

Please explain the terms tier 1, tier 2 etc at first mention.
Response: We have added lines 237-242 to introduce all tiers together at the beginning of the Model description so that their differences are clear to readers.

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

Thanks

Addressing the large carbon footprint of conferences such as the UN Climate Change Convention Conference of the Parties (COP) will be important for maintaining public confidence in climate policy. Transparency is also a vital aspect of creating equitable outcomes in climate policies, as often those most likely to be affected or who can create change on the ground are often unable to attend in person because of the high financial costs as well as having a large carbon footprint. The selection of host locations for the regular meetings of the UN Climate Change Convention is based on a rotation in amongst the five UN regions, which for 2022 is Africa. Here, we present a carbon footprint calculator for travel to COP 27 in Sharm El-Sheikh, Egypt, weighing the benefits of certain routes and modes of transport. The calculator demonstrates the well-known carbon-efficiency of coach and rail over flights but shows that these benefits are partly mitigated in the case of COP 27 due to insufficient transport links from Europe to the conference location. However, we also highlight some of the benefits of hosting a COP in the global South, particularly in the context of climate justice. Users of the calculator are invited to consider all their options for travel and acknowledge the issue of climate justice through careful selection of carbon offsets.