### Global Warming and Decarbonisation  
The maritime sector is constantly searching for alternatives to fossil fuels with tighter environmental controls in place to aid in reducing climate change. Referring to Table 1, the total shipping activities account for about 3% of all man-made greenhouse emissions [3]. According to the report in [4], it is estimated that 16 ships create approximately the same amount of Sulphur pollution as all the cars in the world. The International Maritime Organization (IMO) has implemented the Initial IMO Strategy on Reduction of Greenhouse Gas (GHG) Emissions from Ships, to reduce gross annual GHG emissions from shipping by at least 50% by 2050, relative to the 2008 emission levels [5]. Several means in place to mitigate the carbon emission from shipping activities are:  
* Energy Efficiency Measurement Index The IMO has implemented the Ship Energy Efficiency in Annex VI of the MARPOL [7] which include the Energy Efficiency Design Index (EEDI) in the ship design state, Ship Energy Efficiency Management Programme (SEEMP) in the ship operational planning stage and Energy Efficiency Operational Index (EEOI) in monitoring the energy efficiency and collection of data for continuous improvement in terms of carbon emission. The proposed ship energy efficiency concept aims to minimize GHG emissions by developing ways to lower fuel usage, more efficient ship design and switching to alternative fuels that emit lesser GHG [8]. * Alternative Marine Fuels To meet the increasingly strict emission regulation, alternative fuels such as liquified natural gas (LNG), ammonia, methanol and liquid hydrogen have become a more important part of the energy mix. LNG trade has expanded dramatically from 100 million tonnes in 2000 to approximately 300 million tonnes in 2017. However, LNG still generates carbon emissions but is significantly lesser than diesel. Ammonia and hydrogen could be produced from hydrocarbons, and green ammonia and green hydrogen which are produced from electrolysis powered by renewables or nuclear are excellent sources of zero-emission fuel [9]. Methanol on the other hand is easier to store and handle than LNG. However, ammonia, hydrogen and methanol have a lower energy content than conventional fuel. * Electrification Electrification of marine vessels is becoming more commercially viable due to increasingly declining battery costs fueled by the growth of electric cars. Several commercial electric vessels have also been built. For example, the 4.3 MWh all-electric ferry, Ellen (Figure 1a), was built in the framework of the EU's Horizon 2020 program and is estimated to save 2000 tons of CO\({}_{2}\) per year in its operation. A small electric cruise ship, Bprime Explorer (Figure 1b), and the Grimaldi GGSG ro-ro freighter (Figure 1c) were built to operate in Norway's fjords and the Mediterranean, respectively [10]. Nevertheless, there are several challenges in marine electrification, especially in the charging infrastructure, voyage distance and weight issues [11]. \begin{table}
\begin{tabular}{l c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Third IMO GHG Study (Million Tonnes)**} & \multicolumn{4}{c}{**ICCT (Million Tonnes)**} \\ \cline{2-10}  & **2007** & **2008** & **2009** & **2010** & **2011** & **2012** & **2013** & **2014** & **2015** \\ \hline Global CO\({}_{2}\) Emissions & 31,959 & 32,133 & 31,822 & 33,661 & 34,726 & 34,968 & 35,672 & 36,084 & 36,062 \\ International Shipping & 881 & 916 & 888 & 773 & 853 & 805 & 801 & 813 & 812 \\ Domestic Shipping & 133 & 139 & 75 & 83 & 110 & 87 & 73 & 78 & 78 \\ Fishing & 86 & 80 & 44 & 58 & 58 & 51 & 36 & 39 & 42 \\ Total Shipping & 1100 (3.5\%) & 1135 (3.5\%) & 977 (3.1\%) & 914 (2.7\%) & 1021 (2.9\%) & 942 (2.6\%) & 910 (2.5\%) & 930 (2.6\%) & 932 (2.6\%) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Breakdown of GHG emissions [6]. There are also other means for mitigating carbon emissions such as wind-assisted propulsion ship [12], use of Flettner rotor [13], slow steaming [14] and many more.