Estimate of the environment impact of the ELT instrument MOSAIC

Annemieke W Janssen, Kacem El Hadi, Nazim A Bharmal, Francois Wildi, Cyril Pannetier, Gavin Dalton, Franck Ducret, Marisa Garcia Vargas, Jeff Lynn, Roser Pello, et al.

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► To cite this version:

Annemieke W Janssen, Kacem El Hadi, Nazim A Bharmal, Francois Wildi, Cyril Pannetier, et al. Estimate of the environment impact of the ELT instrument MOSAIC. Modeling, Systems Engineering, and Project Management for Astronomy XI, Jun 2024, Yokohama, Japan. pp.26, 10.1117/12.3018865. hal-04976636

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Estimate of the environmental impact of the ELT instrument
MOSAIC

Annemieke Janssen<sup>a</sup>, Kacem El Hadi<sup>b</sup>, Nazim Ali Bharmal<sup>c</sup>, François Wildi<sup>d</sup>, Cyril Pannetier<sup>e</sup>,
Gavin Dalton<sup>f</sup>, Franck Ducret<sup>b</sup>, Marisa Garcia Vargas<sup>g</sup>, Jeff Lynn<sup>a</sup>, Roser Pello<sup>b</sup>, Eric
Prieto<sup>b</sup>, Mathieu Puech<sup>e</sup>, Myriam Rodrigues<sup>e</sup>, and Ruben Sanchez-Janssen<sup>h</sup>

<sup>a</sup>NOVA optical infrared instrumentation group at ASTRON, Oude Hoogeveensedijk 4 7991
PD, Dwingeloo, The Netherlands.

<sup>b</sup>Aix Marseille Universite, CNRS, CNES, Laboratoire d'Astrophysique de Marseille, UMR 7326, 13388, Marseille, France.

<sup>c</sup>Centre for Advanced Instrumentation, Durham University, South Road, Durham DH1 3LE,
United Kingdom.

<sup>d</sup>University of Geneva, Dept. of Astronomy, Ch. Pegasi 51, CH-1290 Sauverny, Switzerland.

<sup>e</sup>GEPI, Observatoire de Paris, PSL Research University, CNRS, UMR 8111, Place Jules
Janssen, 92195 Meudon, France.

<sup>f</sup>Dept. Physics, University of Oxford, Keble Road, Oxford, OX1 3RH, United Kingdom.

<sup>g</sup>Dpto de Fisica de la Tierra y Astrofisica, Fac. CC Fisicas, Universidad Complutense de
Madrid, Plaza de las Ciencias, 1, E-28040 Madrid, Spain.

<sup>h</sup>UKRI-STFC UK Astronomy Technology Centre, Blackford Hill, Edinburgh EH9 3HJ, United
Kingdom.

ABSTRACT

MOSAIC is an instrument for the Extremely Large Telescope (ELT). The instrument has started phase B, and apart from technical and financial requirements, MOSAIC has the additional requirement to investigate and minimise its environmental impact. The first step is to estimate the carbon footprint (and other effects) in a 'Life Cycle Analysis', for the instrument development up to Provisional Acceptance in Chile. This paper presents a preliminary analysis, aimed at identifying potential contributors to environmental impact. Investigated contributors are: materials, Full-Time-Equivalents, travel, and transport of the instrument. Not yet investigated (due to lack of information or certainty) are: electronics, test facilities and prototyping. Uncertainty in input data and conversion factors leads to error bars of a factor 2 or larger. Therefore, the outcome of the analysis can be used for internal comparison of contributors only, and it should not be used for comparison to other instruments or disciplines.

Keywords: ELT instrument, MOSAIC, sustainability, environmental impact, Life Cycle Analysis

# 1. INTRODUCTION

## 1.1 The reason behind the analysis

The environmental conditions at earth have been very stable for around 10.000 years, but they are now changing due to human activity. Papers 1 and 2 describe for nine processes the 'planetary boundaries' within humanity can operate safely, without causing irreversible changes to these processes. Climate change is one of the processes that receives much attention, but there are more. The nine processes are (as defined by 2 in 2023) : climate change; novel entities, rate of biodiversity loss; interference with the nitrogen and phosphorus cycles; stratospheric ozone depletion; ocean acidification; global freshwater use; change in land use; and atmospheric aerosol loading. Climate change, rate of biodiversity loss, novel entities, change in land use, global freshwater use and interference with the nitrogen and phosphorus cycles are outside the safe region. By doing a LCA, the hope is to find ways to reduce the impact of our instrument development on these processes.

E-mail: mosaic_sustainability@lam.fr
## 1.2 Introduction MOSAIC

MOSAIC is an instrument for the Extremely Large Telescope (ELT). The instrument has started phase B in early 2023; a description of the instrument is given in 3. Apart from technical and financial requirements, MOSAIC is the first ESO instrument with the additional requirement to investigate and minimise its environmental impact. The first step is to estimate the carbon footprint (and other effects) in a ‘Life Cycle Analysis’ (LCA), for the instrument development up to Provisional Acceptance (PAC) in Chile.

MOSAIC has a sustainability committee consisting of 5 persons, who do analyses based on input from the technical team, and who can give recommendations to the project office. The committee has decided that to guide the project, we would begin by using the LCA to understand our uncertainties in estimating the environmental impact and then begin to assess strategies and scenarios that could lead to mitigation without affecting the core technical performance or introducing significant, additional costs. In other words, we provide guidance based on assessment which is compatible with the other drivers in the project and so sustainability becomes part of the design and engineering disciplines.

The Life Cycle Analysis of several other instruments have been done, and these serve as a source of information and comparison. 4 discusses the Life Cycle Analysis of the Athena X-ray Integral Field Unit. The European Southern Observatory (ESO) has had a Life Cycle Analysis done for the GRAAL instrument (5). We can use several aspects of that latter analysis for MOSAIC; for example, the average environmental impact of 1 FTE (Full Time Equivalent) at ESO Garching. The main difference is that the MOSAIC analysis is done early in the project, whereas in case of GRAAL the analysis was done afterwards. Doing the analysis early in the project gives the opportunity to identify the main contributors to environmental impact and actively steer the project towards sustainable solutions.

# 2. PURPOSE AND STATUS OF THE ANALYSIS

There are two sources of significant uncertainty in the outcome of the analysis:

* uncertainty in the input data (amount of materials to be used / travels made / energy used).

* uncertainty in the conversion factors from input data to environmental impact.

Yet the calculation is useful if the estimates can be used for comparing the environmental impact of several scenarios within the uncertainties stated.

The current status of the analysis is summarised in Figure 1. The analysis includes the project phases up to Provisional Acceptance in Chile (PAC), as defined in our Statement of Work. Nevertheless, the impact from operations and de-commissioning could be significant and it would be interesting to investigate those as well.

## 2.1 Databases with conversion factors

The analysis requires a database to convert input parameters (kilometers travelled, kg of material used) to output (liters of water used, kg of CO2-equivalents emitted, etc). We have access to the following databases / calculators :

* Ecoinvent is a commercial database. It contains many inputs (materials, energy, transport, processes), and many outputs (climate change, terrestrial acidification, aquatic ecotoxicity, land occupation, terrestrial ecotoxicity, ionising radiation, respiratory effects, human toxicity, ozone layer depletion, photochemical oxidation, non-renewable energy, mineral extraction, aquatic eutrophication, aquatic acidification). The educational license costs 3800 Euros, plus a 750 Euro yearly maintenance fee. For the GRAAL project 5, the most relevant lines were extracted from the database, and these are available. We can thus experiment with a subset of the ecoinvent database. The environmental effects are given in both physical units and scaled to so called 'points', to allow comparison of several environmental effects.
<table><caption>Figure 1. Identified contributions to the instrument environmental impact. The part after Provisional Acceptance Chile (PAC) is not officially part of the analysis, and hence light shaded.</caption><thead><tr><th>Occasion</th><th>Estimated Value / Status</th></tr></thead><tbody><tr><td colspan="2"><strong>Phase B - up to PDR</strong></td></tr><tr><td>FTE</td><td>170</td></tr><tr><td>Travel</td><td>data collected, analysis done</td></tr><tr><td colspan="2"><strong>Phase C - between PDR and FDR</strong></td></tr><tr><td>FTE</td><td>190</td></tr><tr><td>Travel</td><td>data collected, analysis done</td></tr><tr><td>Materials for prototypes</td><td>Not yet investigated</td></tr><tr><td>Test facilities</td><td>Not yet investigated</td></tr><tr><td colspan="2"><strong>channel MAIT - between FDR and CDR</strong></td></tr><tr><td>FTE</td><td>280</td></tr><tr><td>Travel</td><td>data collected, analysis done</td></tr><tr><td>Materials for final hardware</td><td>data collected, analysis done</td></tr><tr><td>Test facilities</td><td>Not yet investigated</td></tr><tr><td colspan="2"><strong>PAE - between CDR and PAC</strong></td></tr><tr><td>FTE (EU and Chile)</td><td>90</td></tr><tr><td>Travel</td><td>data collected, analysis done</td></tr><tr><td>Test facilities</td><td>Not yet investigated</td></tr><tr><td>Instrument transport</td><td>3 scenarios investigated :<br>boat + truck<br>boat + rail + truck<br>flight + truck</td></tr><tr><td colspan="2"><strong>Commissioning and operations - until end-of-life</strong></td></tr><tr><td>FTE (EU and Chile)</td><td>20</td></tr><tr><td>Travel</td><td>data collected, analysis done</td></tr><tr><td>consumables during instrument operation (Electricity, LN2)</td><td>Not yet investigated</td></tr><tr><td>Data storage</td><td>Not yet investigated</td></tr><tr><td>Disposal</td><td>Not yet investigated</td></tr></tbody></table>

* ADEME offers a free database. It contains many inputs (materials, energy, transport, processes), and many outputs (ozone layer depletion, aquatic eutrophisation, terrestrial eutrophication, photochemical ozone formation, particular matter, ionising radiation, minerals and metal resource use, land use). The outputs are given in physical units. Although quite complete, the database does not always contain an applicable entry. There are for example many entries regarding 'steel', however most are 'steel in household electronics, substitution benefits included'. Including the 'substitution benefits' results in a negative carbon footprint, which is not what one expects for a raw material. The closest match to steel as a raw material was 'steel unspecified, for furniture', and the closest match to aluminium as a raw material was 'sheet' or 'extrusion profile'. The ADEME database was in the end not used for calculating the impact from materials.

* IRAPs travel-footprint-calculator calculates the CO2-eq emission of air travel.
* Ecopassenger compares the CO2-eq emission of several travel options including car, train, and air travel.

* Circulaire Cijfers 6 is a free database that lists CO2-eq emission, energy use, and water use, for the production of raw materials.

These databases and calculators were compared to each other, based on their output CO2-eq emission (the only common parameter among all these databases). As table 1 and 2 show, there are sometimes large differences. In case of the train, these are caused by differences among countries, in case of the flight, these are caused by taking radiative forcing into account or not. (see reference 7) By taking an average and producing an uncertainty, we quantify both a usable average which converges on a European average for rail-relevant for MOSAIC since the bulk of those project members traveling by this mode are all based in Europe-and for flights represents a value which embraces the complex uncertainties in feedbacks and additional forcings by this mode of transport.

<table><caption>Table 1. Conversion factors for train, air, bus, and car travel. In case different values were given for short-haul, medium-haul, and long-haul flights, the medium-haul value is taken. The contrails produced by airplanes cause climate change, usually a factor ~ 2 is used to correct for this, but it is an uncertain number.</caption><thead><tr><th>Database</th><th>Train factor<br>CO2eq<br>kg/km</th><th>Flight factor<br>CO2eq<br>kg/km</th><th>Bus factor<br>CO2eq<br>kg/km</th><th>Car factor<br>CO2eq<br>kg/km</th><th>Comments</th></tr></thead><tbody><tr><td>ADEME (DE)</td><td>0.002-0.08</td><td>0.1-0.14</td><td>0.03</td><td>0.22</td><td>flight without<br>radiative forcing from contrails</td></tr><tr><td>Ecoinvent (DE)</td><td>0.07</td><td>0.1</td><td>0.11</td><td>0.33</td><td>flight without<br>radiative forcing from contrails</td></tr><tr><td>IRAP (FR)</td><td>0.023</td><td>0.17-0.30</td><td>-</td><td>-</td><td></td></tr><tr><td>Ecopassenger (UK)</td><td>0.017</td><td>0.24</td><td>-</td><td>0.17-0.23</td><td></td></tr><tr><td>Average</td><td>0.037</td><td>0.175</td><td>0.07</td><td>0.25</td><td></td></tr></tbody></table>

<table><caption>Table 2. Conversion factors for raw materials</caption><thead><tr><th>Database</th><th>Aluminium<br>CO2eq kg/kg</th><th>Steel<br>CO2eq kg/kg</th><th>Glass<br>CO2eq kg/kg</th></tr></thead><tbody><tr><td>ADEME</td><td>3.2232</td><td>1.5109</td><td>1.6172</td></tr><tr><td>Ecoinvent</td><td>7.6038</td><td>5.167</td><td>1.0756</td></tr><tr><td>Circulaire Cijfers</td><td>7.83</td><td>2.3</td><td>2.3</td></tr></tbody></table>

# 3. DATA COLLECTION AND ANALYSIS

## 3.1 Travel

All project managers and the board members were asked to list their travels for the project from the start of phase B (March 2023) until April 2024, and to estimate their travel until the end of the project. The travelled distance, transport means (car, train, bus, plane), and the number of hotel nights were collected. In several cases, the impact of accommodation can be equal to travel (see e.g. reference 8 for the magnitude of values. We use 34 kg CO2-eq per night, see 5 ) hence we include this factor in our travel calculations.
Adding up all expected and already realised travel from the start of phase B up to PAC, results in : ~
4 × 10⁵ km train, ~ 3 × 10³ km bus, ~ 2 × 10⁴ km car and ~ 2 × 10⁶ km flight, and ~ 1700 nights in a hotel.

The related estimated CO2-eq emission, based on the average conversion factors in table 1 is ~ 3 – 4 × 10⁵ kg, of which 10-20 % is caused by overnight stays.

## 3.2 FTE

A Full Time Equivalent is one person working full time for one year. It comes with an environmental impact from commuting, a work desktop / laptop, the canteen, heating the institute, etc. This impact could well differ significantly from institute to institute. But with 31 Institutes in the MOSAIC consortium, we did not attempt to calculate this for each institute separately. Instead, the conversion factor for 1 FTE at ESO in Garching is taken from the GRAAL report, which uses the ecoinvent database: 3400 kg CO2-eq / FTE. This number includes staff commute, office heating, office electricity, office amortization, staff consumables, and waste. The report also gives a number for an FTE in Paranal, but this is not relevant for our analysis as by far most of the FTEs up to PAC are spent in Europe.

Table 3 shows an estimate of the amount of FTE required per project phase. The total is 750 FTE (lower limit). This results in a ~ 2.5 × 10⁶ kg CO2 equivalent emission related to FTEs. This is a very uncertain number; we expect the actual emission to be between 25% and 200% of this number.

<table><caption>Table 3. Estimate of FTE per project phase, as of now.</caption><thead><tr><th>Phase</th><th>Year</th><th>Year</th><th>FTE</th></tr></thead><tbody><tr><td>KO</td><td>Q1 2023</td><td>2023</td><td>10</td></tr><tr><td rowspan="2">SAR</td><td rowspan="2">Q4 2024</td><td>2024</td><td>40</td></tr><tr><td>2025</td><td>50</td></tr><tr><td rowspan="2">PDR</td><td rowspan="2">Q4 2026</td><td>2026</td><td>70</td></tr><tr><td>2027</td><td>90</td></tr><tr><td rowspan="3">FDR</td><td rowspan="3">Q4 2028</td><td>2028</td><td>100</td></tr><tr><td>2029</td><td>100</td></tr><tr><td>2030</td><td>100</td></tr><tr><td>MAIT</td><td>Q4 2031</td><td>2031</td><td>80</td></tr><tr><td rowspan="2">PAE</td><td rowspan="2">Q4 2032</td><td>2032</td><td>50</td></tr><tr><td>2033</td><td>40</td></tr><tr><td>PAC</td><td>Q4 2034</td><td>2034</td><td>20</td></tr><tr><td>Total</td><td></td><td></td><td>750</td></tr></tbody></table>

## 3.3 Materials

The technical team made an initial estimate of the amount of (raw) materials in the final instrument, and used for AIT tools, see Figure 2. Our ecoinvent database was used to translate this into environmental impact. Because it does not include conversion factors for carbon fiber, the carbon footprint for carbon fiber was taken from circulaire cijfers. Moreover, 'electronics' is excluded from the analysis, because ecoinvent has separate entries for 'active electronic component' and 'passive electronic component', with quite different impacts. We do not yet have detailed enough information to calculate the impact from electronics.

Apart from the mass of the materials going into the final instrument plus what is used for AIT tools, more material is needed for prototyping and manufacturing.
* Prototyping is not taken into account. Although some prototypes have been defined (for example, for testing the distributed AIV) the list may not yet be complete.

* No correction factor for spares or manufacturing losses has been applied.

<figure><img src="image_3.png" /><figcaption>Figure 2. Initial estimate of the amount of raw materials, tools and electronics in MOSAIC.</figcaption></figure>

## 3.4 Transport of instrument

The MOSAIC instrument consists of several systems, being: the visible channel (fibers and spectrograph), the near-infrared channel (fibers and spectrographs), Electronic Instrument Control System (EICS), Ground Layer Adaptive Optics (GLAO), and finally the front-end including Instrument Core Optical System (ICOS), optical relay system, fiber positioner, calibration unit, instrument support structure, and cable de-rotator. Figure 3 shows the instrument at the ELT Nasmyth platform. All subsystems have to be transported to Cerro Armazones in Chile. The ICOS system is being designed and manufactured in Brazil, and it would save a lot of effort, risk, and environmental impact, if this system could be tested in Brazil and then shipped directly to Chile.

Three Transport scenarios were investigated, as summarised in Table 4. All assume the transport routes as shown in figure 5. In short, the systems are being integrated and tested at the institutes of the managing consortium partner for that system. Then all, except for ICOS, are shipped to LAM in Marseille, for preliminary acceptance in Europe. Some mass dummies will have to be shipped to Brazil, in order to test the ICOS subsystem. Then all systems are shipped to Chile. This can be done in the following ways :

* Scenario 1 : Truck + Rail + Boat.

* Scenario 2 : Truck + Boat.

* Scenario 3 : Truck + Flight. The flights in this scenario are from Groningen-Marseille, Madrid-Marseille, Newcastle (U.K.)-Marseille, Barcelona-São Paulo, Barcelona-Madrid-Viru Viru, and Sao Paulo - Viru Viru Airport - Cochabamba.

## 3.5 Total

Figure 6 shows an overview of the CO2-equivalent footprint of the contributors that were investigated. The error bars show the range where the actual emission (equivalent) is expected. These error bars are at least a factor 2, and larger if we think that is appropriate. In particular:

* for materials, the ADEME and ecoinvent resulted in an outcome of more than a factor 2 difference.

* FTE is strongly country- and institute-dependent.
<figure><img src="image_4.png" /><figcaption>Figure 3. MOSAIC at the telescope.</figcaption></figure>

<table><caption>Figure 4. Overview of transport scenarios and masses to be transported.</caption><thead><tr><th rowspan="3"></th><th rowspan="3">Sub-sytem</th><th rowspan="3">Mass (tons)</th><th rowspan="3">Route</th><th colspan="9">Distances considering transportation mode (kms)</th></tr><tr><th colspan="3">Truck+Rail+boat (1)</th><th colspan="2">Truck+boat / only Truck (2)</th><th colspan="2">With flight (3)</th></tr><tr><th>Truck</th><th>Rail</th><th>Boat</th><th>Truck</th><th>Boat</th><th>Truck</th><th>Flight</th></tr></thead><tbody><tr><td rowspan="4">AIT</td><td>VIS</td><td>7,67</td><td>Netherlands Marseille</td><td>20</td><td>1400</td><td>0</td><td>1300</td><td>0</td><td>60</td><td>1200</td></tr><tr><td>NIR</td><td>8,27</td><td>Madrid Marseille</td><td>30</td><td>1200</td><td>0</td><td>380</td><td>653</td><td>30</td><td>892</td></tr><tr><td>GLAO</td><td>0,8</td><td>Durham Marseille</td><td>20</td><td>1620</td><td>0</td><td>1650</td><td>0</td><td>60</td><td>1573</td></tr><tr><td>ICOS dummies</td><td>4,2</td><td>Marseille Itajuba</td><td>20</td><td>350</td><td>9200</td><td>280</td><td>9200</td><td>700</td><td>9400</td></tr><tr><td rowspan="2">AIV</td><td>Part 1 to Chile</td><td>23</td><td>Marseille Armazones</td><td>142</td><td>1500</td><td>13379</td><td>1200</td><td>13379</td><td>2000</td><td>9600</td></tr><tr><td>ICOS to Chile</td><td>12,9</td><td>Itajuba Armazones</td><td>142</td><td>4156</td><td>0</td><td>3695</td><td>0</td><td>1350</td><td>1500</td></tr></tbody></table>

## 4. CONCLUSION

An order-of-magnitude calculation shows that material, FTEs and Travel have a similar CO2-eq impact. Transport of the instrument could be similar or an order-of-magnitude lower, depending on the chosen transport strategy. Other contributors, like running lab facilities in the MAIT phase, have not yet been investigated.

This paper only publishes the CO2-eq emission of the instrument development, although the databases also provide numbers on other environmental effects (like ozone layer depletion, ionising radiation, land use, etc). Once we have gained more confidence in the accuracy of the outputs of other environmental effects, we can also think of strategies to minimise these.

Although this analysis can be used to track the environmental impact of the instrument over time, it should not be used for comparison to other instruments or fields, simply because the uncertainties are too large. On top of uncertainties in the input data, the outcome depends on the database of choice. If a more detailed analysis is needed, this requires more resources (skill, people, money for licenses).

Knowing that travel is an important impact, the committee and the project office already took measures to reduce the amount of people travelling to progress meetings, without hampering the communication within the team.
<figure><img src="image_5.png" /><figcaption>Figure 5. Transport routes of the instrument subsystems to the Extremely Large Telescope on Cerro Armazones.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Figure 6. Overview of contributors to the total CO2-equivalent footprint. Values for electronics, test facilities and prototypes still need investigation.</figcaption></figure>

## Disclosures

The authors have no relevant financial interests in the manuscript and no other potential conflicts of interest to disclose.

## Code, Data, and Materials Availability

The ecoinvent database is available only with a license. The Graal report 5 is not available online. Please contact the authors if you are interested, we need to discuss with ESO and the author if we can share the report.
Acknowledgments

We owe many thanks to Claudio Melo from ESO, for his support in starting up this exercise that is new to all of the MOSAIC sustainability committee members. Moreover, we thank the MOSAIC consortium for providing us with input on material and travel, during a very busy time. In France, MOSAIC has received funding support from the French “ANR-Programme d’Investissements d’Avenir”, referenced as ANR-21-ESRE-0008 and french CSAA/ENG.

REFERENCES

[1] Rockström, J., Steffen, W., Noone, K., and Persson, A., “A safe operating space for humanity,” *Nature* **461**, 472–475 (2009).

[2] Richardson, K., Steffen, W., lucht, W., and Bendtsen, J., “Earth beyond six of nine planetery boundaries,” *Science advances* **9** (2023).

[3] Pello, R., Puech, M., Prieto, E., Rodrigues, M., Sanchez-Janssen, R., Dalton, G., Ducret, F., Hadi, K. E., García-Vargas, M. L., Lynn, J., Bharmal, N. A., Chapuis, D., Dupieux, M., Hottier, C., Larrieu, M., Martin, L., Mohamed, M., Morris, T., Pérez, A., Seifert, W., Xu, W., Morris, S., Kaper, L., and Gallego, J., “MOSAIC at the ELT: a unique instrument for the largest ground-based telescope,” in [*Ground-based and Airborne Instrumentation for Astronomy X*], SPIE (2024).

[4] Barret, D., Albouys, V., Knödlseder, J., and et al., “Life Cycle Assessment of the Athena X-ray Integral Field Unit,” *Experimental Astronomy* **57** (2024).

[5] Capurro, A., “Carbon footprint of an astronomical instrument and carbon budget of the European Southern Observatory,” *Thesis at EPFL (private communication)* (2022).

[6] Bakker, N. and Van der Mei, A.J., “Circular Statistics.” https://www.circulairecijfers.nl (2021).

[7] Lee, D., Fahey, D., Skowron, A., Allen, M., Burkhardt, U., Chen, Q., Doherty, S., Freeman, S., Forster, P., Fuglestvedt, J., Gettelman, A., De León, R., Lim, L., Lund, M., Millar, R., Owen, B., Penner, J., Pitari, G., Prather, M., Sausen, R., and Wilcox, L., “The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018,” *Atmospheric Environment* **244**, 117834 (2021).

[8] Ricaurte, E. and Jagarajan, R., “Hotel sustainability benchmarking index 2023.” https://ecommons.cornell.edu/items/f50b30f1-40ea-4c87-95d0-83c8009f6497 (2023).