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

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
==========
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
==========
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.