# Forecast Analysis of Pollutant Emissions of Cruise Ship Routes in Western Mediterranean
## 5 Discussion  
Some discussions are derived from the results shown above. First, most cruise ships have Wartsila engines installed, which represents a dominant position in the shipbuilding industry of cruise ships. This dominant position could reduce competition between engine manufacturers in terms of SFC efficiency, which can affect overall gas emissions. New participants could promote new innovations that can push the efficiency of main engines in terms of their SFC. Second, data reveal that more than 25% of routes concentrated more than 80% of connections. This indicates that there are dominant ports and routes in the Western Mediterranean. An example of these could be the ports of Barcelona, Palma de Mallorca, Valencia, and Marseille due to their high hub index, as [PERSON] and [PERSON] pointed out [46]. Additionally, considering the research carried out in [PERSON] and [PERSON] [47], the port of Barcelona has a cooperative relationship with the ports of Civitavecchia, Palma de Mallorca, Marseille, Savona, and Genoa, all of which are major cruise ports in the Western Mediterranean with homeport operations. Furthermore, [PERSON] et al. [48] highlighted the importance of cruise activity in the ports of Barcelona, Civitavecchia, and Palma de Mallorca, as they registered positive passenger shifts every year for the period of 2011-2017. Considering the European Maritime Transport Environmental Report 2021 [49], cruise activity contributed to 1.1% of NO\({}_{x}\) emissions and 0.6% of SO\({}_{2}\) emissions of the total emissions from ships in 2019 in the Mediterranean. Furthermore, regarding the entirety of the emissions associated with maritime transport at the European level in 2019 [49; 50], the estimations reveal that the emissions of the cruise activity assessed represented approximately 0.58% of NO\({}_{x}\), 0.28% of SO\({}_{2}\), and 1% of CO\({}_{2}\). Figure 8: CO\({}_{2}\) gas emissions for the five scenarios of evolution of cruise traffic considering an SFC \(\pm\)10% with respect to the reference SFC: (**a**) SFC 10% higher than the reference SFC and (**b**) SFC 10% less than the reference SFC. Third, the sailing distance of the routes and the number of connections are key factors in the overall quantity of gas emissions from the navigation of cruise ships in the Western Mediterranean. For instance, a high number of connections with low sailing distances (i.e., Barcelona-Marseille) produce higher gas emissions, and longer routes (i.e., Malaga-Funchal) with a low number of connections produce higher gas emissions. Fourth, the methodology proposed for this research could be a valuable tool for assessing the outcome of operational changes or projecting the impact of policy measures in terms of polluting gas emissions. This leads to accurate results of the overall gas emissions considering most of the key factors involved: type of installed machinery, sailing distance, engine regime, type of fuel consumed, or number of connections per route. The forecast analysis of the best scenario shows that there will be an increase in pollutant gas emissions up to 16.1% for NO\({}_{\rm x}\) and CO\({}_{2}\) in 2025. The implementation of the SECA will reduce the overall SO\({}_{2}\) emissions by up to 76.8%. The five scenarios considered show that CO\({}_{2}\) and NOx emissions range between \(-\)5.0% and +16.1% if they are compared with the 2019 reference year. Variations in SFC were also considered. They showed that CO\({}_{2}\) emissions can vary by up to 18.1%, for instance. As a key performance indicator, the cruise activity of regular routes of the Western Mediterranean in the reference year had an emission gas factor of 22.0 kg/nm of NOx\({}_{\rm x}\), 3.8 kg/nm of SO\({}_{2}\), and 1248.2 kg/nm of CO\({}_{2}\). Regarding the repositioning routes, the results exhibit higher emission gas indicators, achieving 25.8 kg/nm of NOx\({}_{\rm x}\), 4.5 kg/nm of SO\({}_{2}\), and 1443.2 kg/nm of CO\({}_{2}\).