Review of 'Navigating the Climate Conferences: Comparing the Carbon Footprint of Private Jet Travel and Other Modes of Transport to COP28'

Summary
==========

This work heavily builds up on previous results from Barnsley et al. 2022, presenting an extension of Barnsley et al.'s carbon footprint calculator by incorporating travel emissions arising from private jet travel to the UN Climate Change Convention Conference of the Parties (COP) 2023, held in Dubai, UAE. The results of the calculation emphasise the high environmental impact and significant low efficiency of private jet travel compared to commercial flights and ground travel.

In general, the topic of considering private jets as a travel option is of great importance to both the research community and the actual participants attending conferences. Research in that direction helps (i) to raise awareness and (ii) to provide a mechanism to actually measure the impact. Also, the language used in the article is of good quality and easy to follow.

While the paper addresses an interesting research angle, the lack of methodological rigour poses challenges to the validity of the results, and, most importantly, the discussion appears to be insufficient and inadequate, which may impact the attainment of a comprehensive and balanced research outcome. Given these major concerns, I recommend rejecting the paper in its current form.

Detailed comments
==========

Major concern I - methodological rigour
+++++++++

The methodology section consists of several shortcomings resulting in an overall unclear and intransparent study design.

- To compute the private jet travel data, Flightradar24 was used. However, as the work clearly states, this platform allows "aircraft owners privacy options to restrict information" and "Data gaps are, however, present due to reduced coverage over the region". Even though this is indeed a valid point, it needs to be discussed rigorously as a potential threat to validity and whether or what kind of strategy has been implemented to mitigate this threat.

- Considering the main contribution of this research, the methodology provided for the calculation of private jet travel emissions is not sufficient. On page 6, the methodology is briefly outlined. It is stated that the UCL carbon footprint was used to calculate the emissions. However, the work introduces the "load factor" as an additional variable. It is not clear how this load factor influences the overall calculation and how it was incorporated into the existing calculator.

- also, it raises the question whether the aircraft type influences the calculation or not. It remains unclear if a private jet has the same emissions as a commercial airplane. This circumstance should be considered and discussed. For instance, a concrete example such as Cessna versus Boeing 777 would help.

- Further, the method uses the "average occupancy of private flights in Europe as 4.7 passengers per flight". Apparently, this year's COP edition is based in Dubai, UAE, Asia. It remains unclear whether the factor is also valid for flights outside Europe and intercontinental flights. Again, it needs to be discussed rigorously as a potential threat to validity and whether or what kind of strategy has been implemented to mitigate this threat.

- The same holds for the "average capacity (7.4 passengers) of the most popular aircraft in Europe".

- Beyond the different factors, it is not apparent how the formula for the "private jet passenger load factor" on page 6 emerged. It should be explained whether the formula is based on other research or other basic mathematical models. In its current form, it is presented only without rationale. Also, a concrete example with actual numbers might help to fit it better into the context.

- As stated multiple times, the present work extents on the UCL carbon footprint calculator. However, one of the main contributions of the work by Barnsley et al. 2022 is the "carbon-time efficiency". It remains unclear why the present work does not consider this factor when calculating the footprint of private jets.

Major concern II - insufficient and inadequate discussion
+++++++++

The most important concern arises in the discussion section.

- Section Environmental impact of aviation: while the discussion presents indeed interesting and sound arguments around the impact of flying, the link to the present research and its results is completely missing. Paragraphs 2, 3 and 4 of this section do not even mention the results around the emissions of the private aviation sector. This does not represent an appropriate and analytical interpretation of the results obtained. It remains as background or introduction to the research topic.

- Section The location and context of COP28: while the first paragraph discusses the difficulties to travel to the COP28 location, the second paragraph remains as a social-political reflection which is out of scope of this study. The mentioned aspects are not related to the study at hand and do not reflect on the results obtained.

- Removing these two sections, the section Private jet travel to climate meetings remains: While this section fairly discusses the results obtained, some passages raises some questions. For instance, "[...] use of private jets by high-profile individuals clearly undermines this goal. Such use is a symbol of excess and disregard for the environment, undermining the credibility of the attendees and the conference itself. Showcasing a lack of commitment to sustainable practices shapes public opinion and influences the behaviour of others." While these are fair points, they remain again as social-political reflection without link to the results at hand.

Other comments
==========

- The abstract clearly states the intention of an updated version of "UCL's carbon footprint calculator". Also, the Methodology - COP27 private jet data section indicates an incorporation of "various updates". However, throughout the study, it becomes not clear which concrete parts of the calculator have actually been updated.  It appears that the calculator was reused and the routes to Dubai manually adjusted. Additionally, the private jet travel data has been calculated. From my point of view, I would argue that this research is an instantiation of the footprint calculator with an extension for private jet travel data.

- At several points throughout the study, COP 27 is considered and recalculated. It is not clear why COP 27 was considered for this study in the first place, as the work clearly states the focus of COP 28. Indeed, a comparison to COP 27 might be useful; however, the methodology about private jet data on page 4 are introduced based on COP 27. This remains unclear and should be better explained in the beginning of the paper.

- What is the acronym MLAT on page 4? Acronyms should always be introduced before using them.

- On page 5 "FlightConnections" is used. It remains unclear what this term refers to.

- On page 6, Results, the work underlines the data peak on the 9th November 2022. It would be interesting to discuss or at least outline what events happened at this date to provide an interpretation of this data peak. An idea could be to consider the agenda for COP27 (if available) and interpret/compare the events on this particular date. This might answer the question "why this peak?"

- Figure 4 and the text on page 7 are not in line. While page 7 states "The total carbon footprint of private jet flights to and from Cairo International Airport", figure 4 states "Comparison of carbon footprint per passenger". This results in "total carbon footprint" vs. "footprint per passenger". This problem should be solved.

- On page 10, 2nd paragraph it states "Based on the most frequently flown private aircraft types to COP27, a private jet journey to COP28 would release between 7735 to 11,154 kgCO2e per passenger (Figure 6)." It is not clear why the data are based on COP27 and the journey to COP28 "_would_" release a certain impact? I thought COP28 was particularly calculated. This should be revised.

- The term "carbon intensity" is used on page 10 but was not introduced/defined before. The term should be defined before.

- The last paragraph on page 10 appears to be the most important take-away message among all the results. This should be better highlighted. Maybe it can be better positioned and extended?

- All figures on page 11 do not consider private jet travelling. It remains unclear why these figures are presented. Can the data from private jet be incorporated?