Decarbonization of Shipping and Progressing Towards Reducing Greenhouse Gas Emissions to Net Zero: A Bibliometric Analysis

Abstract

The International Maritime Organization (IMO) is the regulator for the safety and pollution prevention of ships. They have set an ambitious target of driving International Shipping to achieve net-zero greenhouse gas (GHG) emissions in 2050 by the process of decarbonization of shipping. Decarbonization of shipping is integral to sustainability, as it can reduce GHG emissions and provide a clean environment in a world that is conducive to the good health and well-being of our future kith and kin. Decarbonization of shipping may be achieved using alternate low-carbon fuels, a more efficient ship operation to save energy, or redesigning the ship’s hull. The purpose of this article is to conduct a bibliometric analysis of the research papers conducted in the past decade on the initiatives adopted by the shipping industry to work towards the net-zero goal. This study utilizes the Scopus database, renowned for its extensive collection of scientific papers. Moreover, to analyze and visualize the data, the bibliometric software tools VOSviewer 1.6.20, Bibliometrix 4.4.0, and Harzings’ 8.17.4863 have been used. These tools facilitated the assessment of the research output in this bibliometric study. Our findings reveal a steady increase in publications over the years, with a notable rise in research interest from 2015 onward. The most frequently discussed topics include greenhouse gases, emission control, and energy efficiency, with notable contributions from the United Kingdom, China, and Scandinavian countries. The study also highlights the leading journals publishing about this research area. Future research directions include exploring alternative fuels and more inclusive policy frameworks for maritime decarbonization.

1. Introduction

The International Maritime Organisation (IMO) is the United Nations body responsible for the safe operation and pollution prevention of ships. Shipping contributes to 80% of the global transport [1]. IMO has been working effortlessly to reduce GHG emissions from ships. In terms of the proportion of global GHG emissions, the emission from ships is much less, but IMO has been making continuous efforts following the adoption of its policies in 2003 related to the reduction in GHG emissions from ships [2]. In 2018, the global carbon dioxide (CO₂) emissions from shipping amounted to 1.06 gigatons (Gt), representing a share of 2.89% of global anthropogenic CO₂ emissions [3]. According to a recent study, 2023 was the warmest year on record, influenced by multiple warm ocean basins [4].

The Paris Agreement on climate change was agreed in 2015 by parties of the United Nations Framework Convention on Climate Change (UNFCCC) and entered into force in 2016 [5]. The United Nations has called for urgent action to combat climate change and its detrimental effect on the world’s population, which is the main aim of its Goal 13 [6]. According to the sustainable development report of 2023, it is envisaged that the world will exceed 1.5 degrees Centigrade by 2035 and face 2.5 degrees warming by 2100. Hence, the call for all parties is to develop a deep, rapid, and sustained GHG emissions reduction by 43% by 2030 and to net zero by 2050 [7].

In 2018, IMO adopted its initial strategy to reduce GHG emissions from ships [8]. The strategy aimed to reduce the carbon intensity of ships by enforcing compliance with the Energy Efficiency Design Index (EEDI) for new vessels, calculated based on the ship’s power and speed, and to reduce the carbon intensity of international shipping by at least 40% by 2030 and 70% by 2050, relative to 2008 levels. Additionally, the strategy set a target to reduce total annual GHG emissions from international shipping by at least 50% by 2050.

In support of UN Goal 13, the IMO introduced an updated strategy in 2023 to further reduce GHG emissions from ships [9]. This new strategy includes further reducing the carbon intensity of new ships by improving energy efficiency, cutting CO₂ emissions from international shipping by at least 40% by 2030 compared to 2008, and achieving at least a 5% uptake of zero or near-zero GHG emission technologies, fuels, and energy resources by 2030, with an aspiration to reach 10%. The goal is for GHG emissions from international shipping to reach net zero. To achieve this, the strategy sets a series of milestones, with the first checkpoint targeting a reduction in annual GHG emissions by at least 20% by 2030, aiming for 30%, compared to 2008 levels. The second checkpoint seeks to reduce annual emissions by 70% by 2040, with an aspiration to reach 80%, also relative to 2008. It is worth noting that IMO has considered the year 2008 as the basis for all its future directives for the reduction in carbon intensity as well as GHG emissions from ships.

The main objective of this research is to explore the decarbonization of the shipping industry in alignment with IMO’s ambitious targets for reducing GHG emissions, particularly the goal of achieving net-zero emissions by 2050. This study aims to provide a comprehensive analysis of the current approaches, challenges, and advancements in the field, focusing on the effectiveness of these strategies and the role of innovative technologies in the transition towards sustainable shipping. Despite the increasing attention on maritime emissions, there remains a significant research gap in understanding the broader impacts of different decarbonization strategies and their integration into existing international regulations. Therefore, this research is novel in its examination of the IMO’s evolving strategies, the latest GHG reduction technologies, and their potential for widespread adoption.

The target audience for this research includes policymakers, environmental advocates, and maritime industry stakeholders, as well as researchers working on sustainable shipping and climate change mitigation. This work is particularly relevant for those involved in the development of regulations and technologies aimed at reducing emissions in the maritime sector. The findings of this study will contribute to the ongoing discourse on how the shipping industry can meet global climate targets while ensuring operational feasibility and economic sustainability.

Section 2 covers the literature review and looks into various methods available to date for the reduction in GHG emissions from ships, including ship design aspects, selection of alternate fuels to replace the presently used fossil fuels, and application of various innovation technologies encompassing the regulatory requirements. Section 3 explains the methodology adopted in the research, followed by the results in Section 4. Section 5 highlights the key findings, followed by discussions in Section 6, and finally, conclusions in Section 7.

2. Literature Review

The decarbonization of shipping is a primary aspect of global efforts to reduce GHG emissions and achieve net-zero targets. IMO has established ambitious goals of achieving full decarbonization by the century’s end [10]. Achieving this target requires a multifaceted approach, addressing vessel design, operational optimization, alternative fuels, and technological innovations. This literature review synthesizes key studies in these areas, offering insights into the strategies, challenges, and progress in the decarbonization of shipping.

2.1. Vessel Design and Operational Optimization

One of the core strategies for reducing emissions in shipping involves optimizing vessel design and operations. Research has shown that optimizing ship size for cargo volume and reducing operational inefficiencies can significantly reduce CO₂ emissions by as much as 30% [10]. This involves not only optimizing the physical design of ships, such as implementing advanced hull shapes like bulbous bows but also refining operational parameters, including speed and route planning, which are crucial for fuel efficiency [10]. The optimization of shipping routes and the reduction in port waiting times, such as through “Just in Time” port call optimization, are also critical to reducing emissions and fuel consumption [11]. The incorporation of digital tools like real-time data analytics and artificial intelligence further facilitates these improvements.

2.2. Alternative Fuels

The adoption of alternative fuels is another crucial avenue for decarbonization. Hydrogen, ammonia, and biofuels have emerged as promising alternatives to traditional marine fuels. Hydrogen, particularly when produced from renewable sources, can significantly reduce emissions from shipping [12]. Yadav et al. explored the potential of Green Hydrogen as a future fuel for the transportation sector in India, including shipping, which has not received significant attention for decarbonization thus far [13]. The country has ambitious plans to adopt Green Hydrogen in the near future. Similarly, ammonia is gaining attention as a potential future fuel, with projections suggesting it could form a substantial part of the maritime fuel mix by 2050 [14]. However, the widespread adoption of these fuels is hindered by challenges such as the lack of infrastructure, high costs, and safety concerns [15,16]. LNG is currently considered a viable transition fuel, offering a 15–25% reduction in GHG emissions compared to traditional fuels. However, its lower energy density, about 49% lower than conventional marine fuels, requires a larger storage capacity, impacting cargo capacity and increasing operational costs [17]. Life cycle assessments (LCA) of LNG highlight both its potential for emission reduction and the challenges associated with its production and storage. The transition to alternative fuels requires substantial investments in bunkering infrastructure and retrofitting existing ships.

2.3. Technological Innovations

Energy-efficient technologies are another key component of the decarbonization strategy. Solid oxide fuel cells (SOFCs) and liquid organic hydrogen carriers (LOHCs) are being developed to enhance ship propulsion efficiency [12]. Additionally, market-based measures, such as emissions trading systems (ETS), are being explored to incentivize the adoption of these technologies [18]. The IMO’s EEDI and Carbon Intensity Indicator (CII) have been adopted to guide and encourage the development and deployment of cleaner technologies and operational practices [19].

A methodology for predicting the speed/power performance of a ship under various conditions, considering the influence of biofouling on a ship’s hull and propeller [20]. The biofouling development on the ship’s hull is represented by surface roughness, which utilizes Granville’s model for hull resistance and the ITTC model for propeller characteristics. The research applied to a Tanker vessel. As the resistance of the hull and propeller increases, the reserve power will decrease. A suitable numerical model was established to determine the propeller operation point. For a given voyage, increased hull resistance leads to an increase in fuel consumption, which in turn leads to increased GHG emissions. The research provides the capability of analyzing the impact of surface roughness on the ship’s hull and propeller with the minimum amount of ship information. The method developed in this research could be useful in analyzing the influence of surface roughness on vessel performance using publicly available data.

2.4. Policy and Regulatory Frameworks

Effective policy frameworks play a crucial role in driving the decarbonization of shipping. Studies indicate that the IMO’s regulations, such as the “Fit for 55” legislation in the EU, which mandates the use of cold ironing (CI) or shore-side electricity in port, are essential in reducing emissions during port operations [21]. The ships could put off their auxiliary engines in ports and use CI to reduce emissions in ports. The energy supplied in the port could include a combination of LNG and RES (Renewable Energy Sources). The research produced an integrated ship energy flowchart (ISEF) that could estimate the power needs of the vessel, obtained from the vessel’s AIS (Automatic Identification system). The ISEF provides an estimation of the ship’s emissions like CO₂, NOx, and CO and calculates the benefits of CI in terms of reducing GHG and hazardous emissions. Market-based measures like the emissions trading system (ETS) and well-defined regulatory standards for alternative fuels and technologies are critical to creating a conducive environment for investment and innovation in the sector [22]. IMO’s 2020 regulations were aimed at reducing GHG emissions from ships by replacing HFO fossil fuel with a more sustainable fuel like LNG, which is a cleaner burning fuel. However, on account of its cryogenic nature, LNG requires new infrastructure and protocols for bunkering. Sundaram et al. estimated the time, material costs, and truck-to-ship and ship-to-ship emissions during the bunkering process [23]. It was concluded that in maritime countries like Norway, Hong Kong, and Singapore, purging and inserting bunkering lines can impact emissions and material costs significantly. Reducing GHG emissions comes at a cost that needs to be borne by all stakeholders, including ship owners, ship managers, and charterers, in the best interest of the maritime community at large. Collaborative efforts among governments, shipping companies, and research institutions are necessary to accelerate the transition to low-carbon shipping.

2.5. Barriers to Decarbonization

Despite the progress being made, several challenges hinder the decarbonization of shipping. The high costs of developing and deploying new technologies, as well as the lack of infrastructure to support alternative fuels, remain significant barriers for shipping companies, especially smaller operators [24]. Moreover, the competition for investment in green technologies from other sectors further complicates efforts to secure funding for decarbonization initiatives [22]. Furthermore, the energy density of alternative fuels, such as LNG and ammonia, presents challenges in terms of storage and fuel efficiency. For example, LNG’s lower energy density affects the cargo capacity of ships, making it an ideal solution primarily for LNG carriers, where boil-off gas can be utilized efficiently [25].

2.6. Life Cycle Assessment and Environmental Impact

The environmental impact of carbon mitigation technologies must be assessed holistically, considering not only operational emissions but also the emissions associated with the production, installation, and disposal of these technologies [26]. This life cycle perspective ensures that decarbonization strategies do not inadvertently increase emissions in other phases of a ship’s life cycle. For instance, studies on LNG’s life cycle emissions reveal that the production phase of LNG can be responsible for a larger portion of its emissions compared to its operation in the ship’s engine [17]. This highlights the need for more stringent measures in the production phase to fully realize LNG’s potential as a low-carbon fuel.

The research involved gathering data from 4151 ships that included fuel consumption and the corresponding exhaust emissions of CO₂, N₂O, and CH₄. The results indicated that CO₂ accounted for 98% of the GHG emissions, whereas N₂O and CH₄ were insignificant at 1.5% and 0.5%, respectively [27]. The types of ships included in the above research were cargo ships, chemical tankers, oil tankers, and gas carriers. This is especially useful research relating fuel consumption to GHG emissions from certain classes of vessels. It will be useful to compare these results with other classes of ships, especially bulk carriers, container ships, and passenger ferries.

IMO has designated the North Sea, Baltic Sea, and the English Channel as NOx emissions control areas (NECA) starting January 2021 [28]. A study was conducted to estimate the NOx emissions in the North Adriatic Sea by [29]. They described the current methodologies for estimating emissions from ships as bottom–up and top–down methods. They devised a Ship Emissions Assessment Method (SEA), which is claimed to be a hybrid innovative method that fills the gap between the current methods. They used publicly available data to estimate cost-effective emissions. The study was conducted to estimate costs in crucial container ports Trieste, Rijeka, and Venice. The SEA method is an innovative method. This could be used for other ports around the globe where the datasets are publicly available and emissions estimated.

2.7. Technological Trends and Advancements

The decarbonization of shipping is an ongoing area of research, with numerous studies exploring innovative solutions to reduce emissions. For example, Sogihara et al. investigated the life cycle savings associated with the use of wind propulsion rotors on bulk carriers [26]. However, fitting rotors to larger bulk carriers presents operational challenges, particularly during loading and unloading, requiring further research to optimize their implementation. Studies on zero-emission ships (ZES), such as those by Kim et al., show that while ZES can drastically reduce GHG emissions, they are currently more expensive than conventional ships [30]. Nevertheless, they hold promise for future adoption, particularly in small vessels operating in coastal waters.

In addition, the use of biofuels and methanol as alternative fuels is being studied for its potential to reduce shipping’s environmental footprint. Voniati et al. assessed methanol and ammonia’s viability, suggesting that more research is needed, particularly for large vessels [31]. Additionally, research by Thiaucourt et al. on optimizing fuel efficiency in head waves demonstrates the potential of simulation tools to model the impact of different operational scenarios on GHG emissions [32]. Two engine control models were developed: the standard constant rotational speed mode and an innovative constant fuel rack approach. This model was coupled to a ship’s simulator, and the fuel consumption was compared between the two modes. The results showed that the constant fuel rack approach reduces fuel consumption by 1.6%. Ship simulators and engine simulators are extremely useful tools for conducting simulations of an engine and ship’s behavior in various load and speed conditions. The above research could be particularly useful in assessing the impact of different fuels on the reduction of GHG emissions from ships.

An interesting study was conducted by Martinez et al., entitled “Impact of blue economy sectors using causality, correlation and panel data models” [33]. The study investigated the causal relationship between various blue economy factors that included ships and GHG emissions, amongst others. They applied multivariate Granger causality theory, correlation analysis, and panel data techniques. The study concluded that GHG emissions reduction is one of the good factors for stimulating economic growth. This research could be useful for future work in correlating numerous factors related to ship operations that could have an impact on the reduction in GHG emissions. For engine performance considerations, the ship crew’s attitude and human factors could be vital factors.

2.8. Eco-Friendly Alternatives

Gucma et al. considered the challenges and need to provide environmental solutions for maritime shipping [34]. The study evaluated individual solutions and their potential use in the global maritime field. The interesting part of the study was the usage of eco-friendly solutions on ships and how the ship’s crew and owners would react to their implementation in future use, leading toward conceptual solutions. Shipowners should consider the additional responsibility that the crew has to undertake for an effective solution. Allowance should be made for training the crew to achieve conceptual solutions and provide incentives to the crew for the implementation of these ideas, which could lead to a better environment for the global community.

Residual fuel oil (RFO) is the fuel used on marine engines at present. RFO contains high sulphur content, which is harmful to the environment as emissions from marine engines produce oxides of sulphur that are detrimental to the environment, causing major respiratory problems, heart disease, and cancer in human beings. An interesting study was conducted by Khurmy et al., where they developed two process models to co-produce methanol and hydrogen from vacuum refinery waste that can minimize sulphur and carbon emissions [35]. The research shows a potential for reduction in GHG emissions, but a cost-benefit analysis is required for refineries to implement this conversion process.

2.9. Economic Implications

The economic implications of decarbonizing shipping are vast and multifaceted. The transition to alternative fuels, such as LNG and eventually hydrogen or ammonia, requires significant investments in new ship designs, bunkering infrastructure, and compliance with evolving regulations [36]. While LNG reduces emissions compared to heavy fuel oil, its long-term viability is still debated, and further investments are necessary for sustainable solutions [37]. Shipping companies face higher operational costs, potentially raising freight rates and impacting global trade [38]. Additionally, policy frameworks such as carbon taxes and emissions trading schemes will shape shipping costs and market dynamics, urging adaptation to a more sustainable regulatory environment [18].

Meng et al. investigated the relationship between the carbon finance market and the shipping industry [39]. The research followed the 2020 Fourth Greenhouse Gas study of IMO, coupled with the economic development of India, China, and Africa. The research discovered a long-term dependence between the carbon finance market and shipping that is strengthened by global crises like Brexit in 2018 and COVID-19. A similar study was conducted to evaluate the economic implications of the Emissions Trading System on the energy supply chain through marine transport (ETS) [40]. The study highlighted the paramount importance of economics and finance in the European shipping market. These studies are useful for governments to include shipping emissions trading schemes for environmental sustainability.

The shift to decarbonization also affects supply chains, with practices like “slow steaming” reducing emissions and costs but potentially increasing delivery times [41]. Innovations in port operations, such as shore power and digital technologies like AI and IoT, offer further efficiencies but require significant investment [42]. These changes could reshape global trade patterns, especially with the emergence of new shipping routes due to climate change [43]. Developing nations may face greater challenges in adapting to stricter regulations, necessitating financial support from developed countries to ensure equitable access to green technologies [41]. As such, a coordinated approach involving technology, policy, and global cooperation is essential for achieving sustainable decarbonization without undermining the economic stability of the shipping industry [44].

3. Methodology

In this study, a bibliometric analysis was conducted to explore the reduction in GHG emissions from ships, with a focus on scientific papers published from 2008 onwards. IMO has been addressing exhaust emissions since 2000, but it uses 2008 as the baseline year for reducing GHG emissions in the maritime sector. To ensure the relevance of the analysis, the authors chose to review papers from 2008 to 2023. Figure 1 Represents the Flow diagram of the summary of study selection and filtering.

The bibliometric analysis was carried out using the Scopus database, known for its comprehensive collection of peer-reviewed literature. To visualize and analyze the data, three bibliometric software tools were employed: VOSviewer, Bibliometrix, and Harzing’s Publish or Perish. PRISMA guidelines were followed in the initial stages of study selection and article filtering. This combination ensured both rigorous inclusion and exclusion criteria, as well as effective visualization of research patterns. A search query using the terms “GHG and ships” was applied to article titles, abstracts, and keywords. This search initially yielded 694 articles, which were screened for non-English (10) and retracted articles (2). Screening based on title/abstract was performed on 682 records; 78 were removed that were not relevant to the topic. Finally, 604 were subjected to full screening, and 59 were excluded based on irrelevance to the shipping industry or not including GHG emissions.

To ensure the articles were relevant to the topic, each paper was carefully reviewed. Those that did not pertain directly to GHG emissions in the context of shipping and maritime industries were excluded. For example, studies such as Lightburn et al. (2023) [45], which focused on global warming without specific relevance to ships, and Alola et al. (2023) [46], which examined waste management of industrial and agricultural GHG emissions, were excluded. Articles on the adaptive reuse of historic buildings [47] or the environmental assessment of corrugated carton recycling [48] were also discarded due to their lack of connection to maritime GHG emissions.

In total, 59 articles were excluded based on these criteria, leaving 545 articles for the final bibliometric analysis. This selection process ensures that only research directly related to the reduction in GHG emissions from ships was included in the analysis. Only the most pertinent and foundational studies that directly informed the bibliometric analysis and contributed significantly to the topic were discussed, as not all of the 545 articles directly impacted the analysis or conclusions of this study. The search results are valid as of 19 November 2024.

4. Results

4.1. Main Information About Data

In their ambitious target of driving international shipping to achieve net-zero GHG emissions in 2050, IMO has taken the base year as 2008 for comparison of their short-term, medium-term, and long-term strategies. From the above Figure 2, it can be observed that the documents published in the IMO’s base year 2008 was 3 and rose steadily to 96 in 2023. All stakeholders of the maritime world continue to make their best efforts to achieve the target set by IMO.

In their regulation to

Table 1. Main information about data.

Description	Results
Timespan	2008:2023
Sources (Journals, Books, etc.)	272
Documents	531
Annual Growth Rate %	25.99
Document Average Age	4.8
Average citations per doc	22.69
References	0
Document Contents
Keywords Plus (ID)	3077
Author’s Keywords (DE)	1458
Authors
Authors	1531
Authors of single-authored docs	54
Authors Collaboration
Single-authored docs	73
Co-authors per Doc	3.69
International co-authorships %	25.8
Document Types
article	297
article	2
book	1
book chapter	30
conference paper	162
conference paper article	1
conference paper conference paper	1
conference review	4
editorial	1
erratum	1
note	2
review	28
review article	1

below shows the key information gathered from the bibliometric data. The time span for the data ranges from 2008 to 2023. The sources that included journals, books, and conference presentations were 489, which resulted in the extraction of 531 documents. The annual growth rate of the documents happened to be 25.99 %, with the average age of the document being 4.8. The author’s keywords were recorded as 1458. The authors who contributed to the topic of GHG connected to the shipping industry were recorded as 1531.

4.2. Keywords Used in This Research

Table 2. List of keywords used.

Most Relevant Words	Occurrences
greenhouse gases	344
ships	286
gas emissions	162
emission control	127
energy efficiency	127
greenhouse gas	113
carbon dioxide	97
ship propulsion	81
shipping	72
GHG emission	57
climate change	52
greenhouse gas emissions	51
international maritime organizations	51

shows the most frequently used keywords extracted from the bibliometric search of “GHG and ships,” which constitute this study. We can also see from Figure 3 the word frequency over time. The most frequently accruing words were greenhouse gas, which appeared 344 times in the research documents, followed by ships, which occurred 286 times, further followed by gas emissions, which occurred 162 times. Emission control and energy efficiency each occurred on 127 occasions. Also, Figure 3 displays the word frequency over time.

Figure 4 shows the tree extracted from the bibliometric analysis of this study. From the Word Tree, we can observe the words in terms of percentage, the highest being greenhouse gas at 13%, followed by ships at 11%.

4.3. Co-Authorship and Countries

Since the reduction in GHG from ships is a global phenomenon, it would be prudent to consider the total strength of co-authorship of each country with other countries. A minimum of five documents for each country from 32 countries was available. As depicted in Figure 5, there are six clusters, each showing the co-authorship between countries. Cluster 1, shown in green, comprises nine countries, including India, Indonesia, Poland, South Korea, Spain, Sweden, Taiwan, Türkiye, and the United Kingdom.

Cluster 2 has eight countries, including Australia, China, Denmark, Egypt, Hong Kong, Malaysia, Saudi Arabia, and Singapore. Cluster 3, with six countries, includes Belgium, Finland, France, Germany, Greece, and Italy. Cluster 4 comprises Croatia, Japan, and Norway. Cluster 5 has Canada, Portugal, and the United States, and finally, Cluster 6 has Brazil and The Netherlands. The diameter of the circle displaying the country proportionately indicates the documents published and citations, and the thickness of the connecting lines indicates the total link strength. The United Kingdom produced 60 documents with 1920 citations and a link strength of 62. This was followed by China, which had 69 publications, 1611 citations, and a total link strength of 61. Morocco was at the bottom of the table, with seven publications and 30 citations.

4.4. Maximum Citations and Corresponding Authors

This study investigated the author names with maximum citations globally and reviewed the papers published by the corresponding authors. With reference to Figure 6, the size of the circle proportionately shows the maximum number of citations in the documents published on the reduction in GHG emissions from ships. The circle displays the author and the year of publication. The authors’ top four publications in this category were reviewed and presented.

Balcombe et al. had 390 citations in their publications studying the options for fuel technologies and policies to decarbonize International Shipping. The highlight of their research claimed that LNG must be combined with various efficiency measures to meet IMO’s 50% climate target. Further, the authors suggested that slow steaming and wind resistance are key efficiency measures to reduce fuel consumption, thereby reducing carbon emissions [44,45].

The next document with a maximum citation of 242 was by Lindstad et al., which looked at the reduction in GHG emissions and cost reduction on ships [49,50]. The model proposed by the authors was based on reducing the speed of the vessel, thereby reducing fuel consumption, which in turn reduces cast simultaneously. The results shown by the authors conclude that a mere reduction in speed results in an annual reduction in emissions by 28% in terms of millions of tons of CO₂ per year at zero abatement cost.

A study by Wan et al., with maximum citations of 128, suggested solutions and policy recommendations for the decarbonization of the international shipping industry. The paper depicted technical, operational, and market-based solutions for reducing GHG emissions from ships [51]. The authors argue that there are loopholes in the performance-based index, necessitating revisiting IMO’s GHG reduction target and its legal implications by adopting the advancement in technological solutions. The authors conclude that slow steaming is not the only solution for the reduction in GHG emissions, which could be achieved even at faster speeds.

A similar review could be carried out for any of the authors, as seen in Figure 6.

4.5. Bibliographic Coupling of Selected

The implementation of IMO’s regulations is an international issue. Therefore, it will be useful to investigate the most influential countries that publish research papers on the topic and the strong link between the selected countries and the impact they have on the maritime world. A bibliographic coupling of the most influential countries from the 545 publications was conducted based on the method of occurrence of a minimum of five publications from each country. This method resulted in a display of the visualization map showing the strength of the link, respective publications, and citations of each country, as shown in Figure 7 and

Table 3. Countries with Link Strength.

S. No	Country	Documents	Citations	Total Link Strength
1	Australia	14	397	2573
2	Belgium	6	103	617
3	Brazil	8	60	1636
4	Canada	22	590	2813
5	China	69	1611	8070
6	Croatia	10	84	563
7	Denmark	34	183	6149
8	Egypt	11	296	1466
9	Finland	12	290	2418
10	France	14	376	4043
11	Germany	21	488	2281
12	Greece	35	1340	4680
13	Hong Kong	7	167	1487
14	India	11	152	4309
15	Indonesia	10	63	747
16	Italy	34	634	3124
17	Japan	24	304	4287
18	Malaysia	9	218	1504
19	Morocco	7	30	77
20	Netherlands	13	302	4373
21	Norway	50	1849	8063
22	Poland	5	71	1503
23	Portugal	8	81	3345
24	Saudi Arabia	5	117	1069
25	Singapore	30	922	4716
26	South Korea	27	763	2983
27	Spain	23	469	5596
28	Sweden	36	1357	8763
28	Taiwan	7	139	1736
30	Türkiye	15	159	2498
31	United Kingdom	60	1920	9232
32	United States	37	1637	8243

below. Figure 7 displays 7 clusters comprising 32 countries. Cluster 1, shown in maroon, has eight countries: Australia, Brazil, China, Egypt, Malaysia, Saudi Arabia, Singapore, and the United Kingdom. Cluster 2 in green has seven countries, including India, Poland, South Korea, Spain, Sweden, Taiwan, and Türkiye. Cluster 3 in blue has six countries, including Canada, Finland, Germany, Greece, Japan, and the United States. Cluster 4 in yellow shows Croatia, Indonesia, Morocco, and Norway, and Cluster 5 has Belgium, France, and Italy. Cluster 6 has The Netherlands and Portugal, and finally, Cluster 7 has Denmark and Hong Kong.

Table 3 shows the countries with their corresponding publications, citations, and link strength, revealing some interesting observations. The United Kingdom, with 60 publications, had the largest number of citations in 1920 and a total link strength of 9232. China, with 69 publications and 1611 citations, has a link strength of 8070, closely followed by Norway, which has a link strength of 8063. The contribution of Scandinavian countries— including Norway, Sweden, Denmark—China, the United Kingdom, and the United States ranks higher in their pursuit of the reduction in GHG emissions from ships compared to other maritime nations.

4.6. Identification of Core Resources by Using Bradford’s Law in Bibliometrix

Bradford’s law was formulated in 1930. In their work, the authors [52] offer strategies for identifying key journals in a pertinent area of research. Bibliometrix research on the reduction in GHG emissions from ships identified core journals using Bradford’s law, which is displayed in

Table 4. Journals and their ranking as per Bibliometrix.

Journal	Rank	Pubs	Impact Factor
Transportation research part d: transport and environment	1	25	7.4
Ocean engineering	2	23	4.6
Sustainability (Switzerland)	3	20	3.3
Journal of marine science and engineering	4	19	2.7
Energies	5	11	3.0
Proceedings of the international conference on offshore mechanics and arctic engineering—OAME	6	11	-
Advances in intelligent systems and computing	7	9	6.8
Energy policy	8	7	9.3
IOP conference series: earth and environmental science	9	7	-
Maritime policy and management	10	7	3.7
Motor ship	11	7	-
Applied energy	12	6	10.1
Journal of cleaner production	13	6	9.8
Proceedings of the international offshore and polar engineering conference	14	6	-
Progress in marine science and technology	15	6	-
Energy	16	5	9.0
Journal of physics: conference series	17	5	-
Marine pollution bulletin	18	5	5.3
Transactions-society of naval architects and marine engineers	19	5	-
Computer-aided chemical engineering	20	4	3.9
E3s web of conferences	21	4	-
Environmental science and technology	22	4	10.9
IEEE electrification magazine	23	4	2.5
Marine policy	24	4	3.5
Maritime economics and logistics	25	4	4.1
Renewable and sustainable energy reviews	26	4	16.3
SNAME 8th international symposium on ship operations, management, and economics, some 2023	27	4	-
WMU journal of maritime affairs	28	4	2.4
Brodigan	29	3	-
Cleaner logistics and supply chain	30	3	6.9

, with their specific ranking.

The three-field plot, developed in Bibliometrix and shown in Figure 8, combines three parameters of the research: the authors are shown in the middle field; the left field shows the document and year of publication; and the right field shows keywords used in the article. Each of the parameters may be varied to generate a new three-field plot. In the above plot, the author considered a combination of authors, 34 references, and 32 keywords. This specific interesting research work was conducted by Lindstad et al. [50] using energy efficiency as the keyword.

4.7. Trending Topics of Reduction in GHG Emissions from Ships

Table 5. Trend topics.

Item	Frequency	Year q1	Year-Median	Year q2
cost-benefit analysis	7	2010	2011	2015
ship speed	5	2010	2011	2012
Kyoto protocol	6	2012	2012	2014
speed reduction	6	2011	2012	2018
oil tankers	5	2011	2013	2018
profitability	5	2012	2013	2013
marine engines	12	2013	2014	2016
design	5	2013	2014	2015
marine engineering	14	2011	2015	2019
biomass	10	2014	2015	2015
biofuel	8	2014	2015	2019
transportation	10	2014	2016	2017
united states	10	2014	2016	2021
methane	9	2015	2016	2020
energy efficiency design indices (EEDI)	13	2015	2017	2019
speed	9	2012	2017	2018
energy use	7	2014	2017	2019
fuels	40	2015	2018	2020
air pollution	19	2016	2018	2019
shipbuilding	19	2014	2018	2020
ship propulsion	81	2017	2019	2022
GHG emission	58	2013	2019	2021
global warming	51	2016	2019	2022
greenhouse gases	350	2017	2020	2022
ships	290	2017	2020	2022
gas emissions	165	2017	2020	2022
maritime transportation	50	2019	2021	2022
life cycle	44	2018	2021	2022
environmental impact	34	2017	2021	2022
greenhouse gas emissions	51	2021	2022	2023
Carbon	34	2019	2022	2022
alternative fuels	31	2020	2022	2023
case-studies	7	2023	2023	2023
integer programming	7	2020	2023	2023
intensity indicators	6	2022	2023	2023

above and Figure 9 below identify the trend topics in the bibliometric research on the reduction in greenhouse gas emissions from ships. The plot shows that between 2010 and 2012, the focus was on cost benefits and ship speed, with apparently slow steaming of ships being the initial focal point. The trend of the research focal point started with alternate fuels, which is also a topic that is currently being considered. The topic most frequently used was gas emissions, ships, and GHG in 2020. At present, researchers are investigating topics like integer programming, case studies, and intensity indicators.

5. Key Findings

The decarbonization of shipping has acquired significant focus in recent years as the global maritime industry strives to meet the IMO’s target of achieving net-zero GHG emissions by 2050. This bibliometric analysis, based on an extensive review of the Scopus database and using tools such as VOSviewer, Bibliometrix, and Harzing’s Publish or Perish, has uncovered critical insights into the ongoing research trends, methodologies, and emerging topics within the field.

Our analysis reveals that the majority of research publications in the past decade have concentrated on alternative fuels, energy-efficient ship operations, and the optimization of ship designs to achieve emission reductions. Among the alternative fuels, biofuels, hydrogen, and ammonia have emerged as prominent candidates due to their potential for significantly reducing GHG emissions in maritime operations. However, challenges such as fuel availability, economic feasibility, and infrastructure development remain substantial hurdles to the widespread adoption of these technologies.

Additionally, studies on energy efficiency measures, such as hull modifications, the use of renewable energy sources (e.g., wind and solar power), and propulsion system innovations, are becoming increasingly prevalent. These technologies not only contribute to lowering fuel consumption but also enhance the operational efficiency of vessels, further aligning with the goal of decarbonizing shipping. Despite the substantial progress observed in these areas, the literature highlights the need for more interdisciplinary research that brings together maritime engineering, environmental sciences, economics, and policymaking to facilitate a holistic approach to decarbonization.

6. Discussion

The decarbonization of the shipping industry has emerged as an urgent and multifaceted challenge, necessitating a comprehensive understanding of the factors influencing GHG emissions, innovative practices, and regulatory frameworks aimed at achieving net-zero emissions by mid-century. This discussion synthesizes a significant body of literature to assess the progress and challenges associated with the decarbonization of shipping. The bibliometric analysis reveals converging themes, including regulatory compliance, renewable energy adoption, stakeholder engagement, and the implications for operational efficiency and economic viability.

A considerable proportion of greenhouse gas emissions from the maritime industry originates from the combustion of fossil fuels, accounting for about 2–3% of global emissions. According to the IMO, the global shipping industry’s carbon dioxide emissions exceeded one billion tons in 2022, confirming its significant contribution to global emissions [53]. The IMO’s 2050 GHG strategy underscores the necessity for urgent action to reduce emissions from ships. Recent studies highlight the inadequacies within current regulations and the pressing need for robust enforcement mechanisms to ensure compliance across international waters [54,55,56]. The challenges of enforcement in varying jurisdictions can undermine global efforts, emphasizing the role of governmental partnerships and effective international agreements in framing sustainable shipping practices [54,56]. The authors believe that it would be highly beneficial to the shipping community if all the major players of the shipping industry came on a common platform for a more collaborative approach to address and resolve this major challenge of arriving at net zero by 2050.

The transition towards renewable energy technologies represents a pivotal strategy in mitigating GHG emissions. Integrating renewable energy sources into the shipping industry’s operational frameworks is crucial for achieving substantial emission reductions [57]. Furthermore, green ports need to leverage advanced energy technologies to minimize fossil fuel dependency and enhance operational efficiencies through solar, wind, and other renewable energy systems [58]. The utilization of innovative solutions such as battery-electric vessels and hybrid models has been shown to enhance energy efficiency, thereby contributing to overall emissions reduction [59]. The authors also believe that there is no one size that fits all. We need to use a combination of technologies that uses wind power and solar power, improving the design of the ship’s hull and utilizing various alternate fuels available today for the propulsion of the main engines on board the vessel. In the opinion of the authors, it is also necessary to focus on carbon capture on board ships as a strategy to reduce GHG emissions.

The comparative effectiveness of various decarbonization practices reveals a spectrum of outcomes depending on the strategic choices of shipping companies. For instance, ref. [60] describes how a sustainable shipping management framework can facilitate operational efficiency while achieving compliance with environmental regulations. This framework prioritizes eco-design, stakeholder engagement, and the integration of renewable energy, providing shipping firms with a comprehensive approach to sustainability. Moreover, the role of technological innovations, particularly in automation and artificial intelligence, is increasingly recognized for its potential to streamline maritime operations and enhance fuel efficiency [61].

Regulatory measures play a crucial role in guiding the maritime industry towards sustainable practices. The introduction of the EU Emissions Trading Scheme (ETS) is a noteworthy development, promising to create market incentives that encourage emission reductions [62]. This approach aligns with findings from Reza et al., who elucidates the importance of customer-driven sustainable business practices and their relationship with environmental performance across the European shipping industry [63]. The need for shipping companies to adapt to increasing demands for transparency in sustainability efforts has led to proactive voluntary initiatives whereby firms independently adopt greener practices well ahead of regulatory requirements [64].

Another critical aspect of decarbonization is the role of stakeholder engagement in shaping meaningful sustainability efforts. As Fontoura et al. and Ashrafi et al. articulate, collaborative approaches within the shipping supply chain that include all stakeholders, ranging from regulatory bodies to consumers, are essential for fostering innovation and commitment to environmental stewardship [65,66]. This sentiment resonates with the work of [67], who advocate for enhanced stakeholder collaboration as a strategy for mitigating ecological impacts. The necessity of awareness and education regarding sustainable practices cannot be overstated, particularly for industry actors directly responsible for emissions.

The implications of sustainable practices extend beyond environmental benefits to economic considerations. Ref. [68] argue that while transitioning to sustainable shipping entails upfront costs, the long-term economic viability resulting from improved efficiencies can offset these initial investments. Furthermore, companies that adopt green shipping practices tend to gain competitive advantages by capitalizing on the emerging market for environmentally friendly shipping services, as evidenced by studies identifying positive correlations between sustainability initiatives and customer loyalty [69,70].

The literature also indicates significant gaps in knowledge regarding the adoption of sustainable shipping practices, particularly in non-European contexts. Comparisons drawn between high-performing green shipping companies and their less sustainable counterparts reveal that cultural and operational differences can significantly impact adoption rates [62,71]. Initiatives aimed at increasing knowledge-sharing and resource exchange between different regions could enhance the overall capacity for the adoption of best practices, helping to level the playing field among global competitors [72].

Additionally, the role of education and awareness in achieving sustainability goals within the maritime industry is paramount. As noted by Olaniyi et al., developing environmental awareness among maritime educators and practitioners can foster a culture of sustainability within the industry [73]. Enhancing curricula to incorporate sustainability principles will equip future maritime professionals with the tools needed to navigate the industry’s evolving landscape, making education a vital component of the decarbonization agenda.

Understanding the social dimensions of sustainability is equally significant. The shipping industry is faced with the dual challenge of addressing environmental impacts while ensuring the well-being of the workforce involved in maritime operations [1,70]. Corporate social responsibility (CSR) is emphasized as a critical factor in establishing a sustainable maritime industry that minimizes environmental impacts while enhancing labor conditions and community relations [74]. Shipping firms that prioritize CSR are better positioned to cultivate trust and loyalty among consumers and stakeholders, which can lead to improved financial performance and market standing.

To summarise the discussion, the authors suggest that arriving at net zero by 2050 is a mammoth challenge, where all major players of the shipping industry, including but not limited to IMO, classification societies collectively as IACS (International Association of Classification Societies), various Port State Control and Flag State Control authorities, shipbuilders, ship owners, ship charterers and engine builders collaborate and address this issue of reduction in GHG on a common platform for the benefit of humanity.

7. Conclusions and Future Prospects

This study investigated the bibliometric analysis of articles that addressed the IMO’s ambitious target of reducing GHG emissions to net zero by 2050. The authors investigated this bibliometric analysis using three different software programs: VOSviewer, Bibliometrix, and Harzing’s Publish or Perish. This provides a reliable source of comparison of results using the different methods of evaluation and validation of results. The authors believe that the results and figures presented in the research should serve as good food for thought for researchers in the maritime field. The publications were collected using the Scopus database. The authors reviewed research papers published from 2008 onwards. By using Harzing’s Publish or Perish software, the articles were ranked based on their quality and citations. A comprehensive bibliometric literature review resulted in the identification of the top-ranked journal articles. These articles were reviewed, and insights were included based on the expert opinions of the authors, who have a combined experience of 68 years in the maritime domain. The study evaluated publications related to keywords, and results were tabulated and displayed in the word tree. Since the reduction in GHG from ships is a global phenomenon, the study considered the total strength of co-authorship of each country with other countries. The contribution of Scandinavian countries—including Norway, Sweden, Denmark, China, the United Kingdom, and the United States- ranks higher in their pursuit of reduction in GHG emissions from ships compared to other maritime nations. The maximum citation of articles coupled with the corresponding authors was identified. The countries’ contribution to IMO’s net zero initiative and their collaboration with other maritime nations were identified, and the results were tabulated and graphically displayed. Bradford’s law in Bibliometrix identified the top-ranking journals pertaining to this study. A three-field plot showed the relationship between the three parameters comprising authors, references, and keywords. Finally, the trending topics, like integer programming, case studies, and intensity indicators related to IMO’s ambitious goal of carbon net zero, were identified for future work. This study’s limitations include the reliance on a single database (Scopus) for article selection and the restriction to English-language publications, which may have excluded relevant non-English research.

The complexity of decarbonizing a sector as vast and globally interconnected as shipping necessitates coordinated efforts across various domains to create sustainable and scalable solutions. Looking forward, we suggest the following promising areas of research expected to drive the decarbonization agenda in the shipping industry:

• Advanced Fuel Technologies: As we transition to low-carbon and zero-emission fuels, further research into hydrogen fuel cells, ammonia combustion, and the development of synthetic fuels will be crucial. The efficiency of these fuels, coupled with innovations in storage and distribution infrastructure, will determine their viability for large-scale adoption.
• Digitalization and Smart Shipping: The increasing integration of digital technologies, such as IoT, artificial intelligence, and machine learning, holds great potential for optimizing shipping operations. Real-time monitoring systems for energy consumption, predictive maintenance, and automated routing could reduce fuel consumption, improve safety, and enhance the overall environmental performance of vessels.
• Regulatory and Policy Frameworks: Continued advancements in the regulation of emissions and the implementation of economic incentives (e.g., carbon tax subsidies for clean technologies) will play a pivotal role in driving industry-wide change. Future research should explore the impact of policy measures on the adoption of green technologies and assess the role of international collaborations in creating a unified global approach to maritime decarbonization.
• Lifecycle Analysis and Sustainability: Research on the lifecycle emissions of shipping technologies, including shipbuilding, operation, and decommissioning, will help ensure that sustainability is achieved at every stage of the industry’s supply chain. The development of metrics for evaluating the environmental impact of various solutions will guide decision-making and support the transition to truly sustainable maritime practices.
• Public–Private Partnerships and Investment: Collaborative efforts between governments, research institutions, and industry stakeholders will be essential in accelerating the development and implementation of decarbonization technologies. Strategic investments, particularly in infrastructure for clean fuels and retrofitting existing fleets, will be key to meeting the 2050 emissions target.