## 1 Introduction  
### Research Background  
Over the last few decades, extensive use of fossil fuels has resulted in increased Greenhouse Gases (GHGs) in the atmosphere. These emissions refer to the emissions of carbon dioxide (CO\({}_{2}\)), methane (CH\({}_{4}\)), nitrous oxide (N\({}_{2}\)O), and fluorinated gases at low concentrations. They form a thin layer in the atmosphere that prevents solar irradiation from being reflected from the Earth's surface to space, and as a result, it causes the greenhouse effect, which leads to global warming [1]. The Glasgow Climate Pact of 2021 is the most recent climate agreement, which reaffirmed the Paris Agreement's ambitions for limiting the global average temperature increase to 1.5 \({}^{\circ}\)C above pre-industrial levels [2,3]. To combat the global warming problem, some urgent measures need to be taken, including a sharp GHG reduction in all sectors of the economy, including the shipping sector.  
According to the Fourth GHG Study of the International Maritime Organisation (IMO), the share of shipping GHG emissions in global anthropogenic GHG emissions in 2018 was 2.89%, an increase of 9.6% compared to 2012. Without any emission reduction measures, the study predicts a great increase in emissions by the end of 2050 [4]. Due to that, the IMO set the Initial Strategy for GHG reduction, which is in line with the Paris Agreement temperature goals. The strategy has three levels of ambitions: reduction in carbon intensity (CO\({}_{2}\) emissions per transport work) through the implementation of further phases of the Energy Efficiency Desing Index (EEDI) for new ships; reduction in carbon intensity by at least 40% by 2030 and 70% by 2050; and reduction in total annual GHGs by at least 50%, compared to 2008 levels [5]. In order to achieve IMO decarbonisation goals, the strategy indicated measures with the following timelines: short-term (2018-2023), mid-term (2023-2030), and long-term (2030-) measures. Short-term measures represent the start of reducing shipping emissions with national plans, a tighter EEDI, the Ship Energy Efficiency Management Plan (SEEMP), speed reduction, etc. [6]. As a mid-term measure, the IMO has adopted new ship energy efficiency regulations for existing ships, i.e., the Energy Efficiency Existing Ship Index (EEXI) and Carbon Intensity Indicator (CII), which entered into force on the 1 st of January 2023. The EEXI is a technical measure of energy efficiency related to the design of a ship, while the CII represents the operational measure of energy efficiency embedded into the SEMP, and it measures CO2 emissions per transport work for cargo, cruise, and ro-ro passenger ships over 5000 gross tonnage (GT). Like its predecessor, the EEDI, the EEXI should be applied for all ships above 400 GT, and their calculated, i.e., attained EEXI needs to be less than or equal to the required EEXI [7; 8].  
The mid-term measure whose implementation represents an incentive towards zero-carbon technologies is the inclusion of the shipping sector in the Emission Trading System (ETS). As of 2024, commercial cargo and passenger ships of above 5000 GT operating in the European Union will be required to purchase carbon allowances for each ton of released CO2 emission [9].