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16 April 2020

Towards the IMO’s GHG Goals: A Critical Overview of the Perspectives and Challenges of the Main Options for Decarbonizing International Shipping

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DICAAR—Department of Civil and Environmental Engineering and Architecture, University of Cagliari, via Marengo 2, 09123 Cagliari, Italy
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Author to whom correspondence should be addressed.
Sustainability2020, 12(8), 3220;https://doi.org/10.3390/su12083220
This article belongs to the Special Issue Decarbonisation of International Shipping: How to Achieve the IMO’s GHG Goals?
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Abstract

The Initial Strategy on reduction of greenhouse gas (GHG) emissions from ships adopted by the International Maritime Organization (IMO) in 2018 commits the IMO to reduce total GHG emissions of shipping by at least 50% by 2050. Though the direction of the Strategy is clear, the path to implementation remains uncertain. The ambitious IMO’s target calls for widespread uptake of lower and zero-carbon fuels, in addition to other energy efficiency measures, including operational and market ones. Using a triangulated research approach, this paper provides a critical overview of the main measures and initiatives the shipping industry can adopt to try to cope with the new IMO’s requirements. The pros and cons of the most popular emission reduction options are investigated along with the main challenges and barriers to implementation and the potential facilitators that could foster a wider application. The framework that is outlined is complex and not without controversy. Research can play a key role as a facilitator of shipping’s decarbonization by providing its contribution to overcoming the existing controversies on various decarbonization options and by developing a wealth of knowledge that can encourage the implementation of low-carbon initiatives.

1. Introduction

Maritime transport is, by far, the most cost-effective way to move goods around the world, and despite a slight decline in 2018, it remains the backbone of international trade: More than four-fifths of global trade by volume is carried by sea [1]. Though shipping is widely known for its overall environmentally friendly performance compared to road and air transport, it remains characterized by several undesirable environmental impacts. Particularly, shipping emissions are coming under increasing attention and scrutiny as a result of growing awareness on climate change and environmental issues. Shipping emits various pollutants, causing a range of issues:
  • Carbon dioxide (CO2) is the most significant greenhouse gas (GHG) released by ships. The emission of GHGs is the main reason for global warming;
  • Sulfur oxides (SOx) and nitrogen oxides (NOx) contribute to the formation of acid rain and are highly undesirable, due to their effects on human health;
  • Carbon monoxide (CO), volatile organic compounds (VOC) and particulate matter (PM) affect human health. PM also includes black carbon (BC), which is not only particularly harmful to humans, but also the second most powerful climate forcer after CO2.
In this study, the focus is on, but not limited to, GHG emissions.
International shipping is considered one of the largest GHG emitting sectors of the global economy, and it is also expected to become one of the fastest-growing sectors concerning GHGs [2]. In 2014, the Third GHG Study by the International Maritime Organization (IMO) estimated that international shipping accounts for around 2.2% of global annual CO2 emissions and that emissions from international shipping could grow between 50% and 250% by 2050 mainly due to the growth of the world trade [3]. Moreover, IMO predictions for 2050 foresee that 15% of total CO2 emissions will be attributable to maritime transport. Further estimates of 2019 foresee a 39% demand growth for seaborne trade by 2050. Particularly, the deep-sea segment is estimated to account for more than 80% of world fleet CO2 emissions, thus making clear that it is particularly important to find technically feasible and cost-effective emission reduction solutions for this segment [4]. In May 2019, IMO’s Marine Environment Protection Committee (MEPC) announced the start of work for the Fourth IMO GHG Study, which will include, among other things, an inventory of global emissions of GHG emissions from international shipping from 2012 to 2018 and scenarios for future international shipping emissions in the period 2018–2050. The final report of the study is expected for Autumn 2020.
With the adoption of the Paris Agreement in December 2015 [5], 195 countries agreed to keep a global temperature rise this century well below 2 °C above pre-industrial levels and to pursue efforts to limit the temperature increase even further to 1.5 °C. Though shipping has not yet been included in any international climate agreement, it is called upon to make its fair contribution to global commitments on climate change by reducing its emissions. The IMO, as the body responsible for regulating maritime emissions, is expected to lead this process. Following years of internal debate on how and whether the shipping sector should align with the goals of the Paris Agreement, the IMO has finally developed a challenging roadmap for the decarbonization of the sector. A cornerstone of this roadmap is the adoption, in 2018, of the Initial IMO Strategy to achieve reductions in GHG emissions from shipping. With 2008 as a baseline year, the Strategy aims to at least halve total GHG emissions from shipping by 2050, and to reduce the average carbon intensity (CO2 per ton-mile) by a minimum 40% by 2030, and 70% before mid-century [6]. The Strategy represents the IMO’s initial contribution to the global goals of the Paris Agreement to respond to climate change.
To meet internationally agreed levels of mitigation, the shipping industry must undertake fundamental changes in its emissions pathway. Policymakers and stakeholders are called to make substantial efforts to find and implement solutions aimed at reducing the carbon footprint of shipping activities. The increasing interest in the environmental impact of shipping is also demonstrated by the growing attention the corresponding transport environmental literature is devoting to measures, policies, and initiatives aimed at mitigating it [7]. Several recent literature reviews with a focus on sustainability measures across the maritime industry are available. The study by Reference [8] uses bibliometric tools to offer a systemic mapping of the literature with some sustainability focus in the field of maritime transport published between 1975 and 2014. More recently, the paper by Reference [9] has proposed a literature review on sustainability in the field of maritime studies using text-mining techniques to provide future research directions concerning topics and co-authorship patterns. Some reviews are more focused on a specific problem, such as bunker consumption optimization methods [10], while others provide general overviews of emissions reduction measures [11,12]. A general overview of the measures with high CO2 reduction potential can be found in Reference [13], while a specific focus on emission reduction in the port area is in Reference [14]. A holistic assessment of the combined potential of fuels, technologies and policies to reduce GHG emissions from international shipping can be found in Reference [15]. Several quite recent special issues dealing with specific aspects of sustainability in the shipping and port industry are also available [16,17,18,19], further confirming the strong interest in the topic.
Notwithstanding this significant body of literature, it seems that the complex issue of decarbonizing the maritime industry would benefit from an overall summary framework designed to configure the key elements that are going to characterize the low-carbon transition, both for the industry and research agenda. Two different, but interconnected, perspectives should be considered when dealing with the decarbonization process of the shipping industry: The perspective of the policymakers, which focus on achieving the established decarbonization targets; and the perspective of the ship-owners and maritime operators, which are requested to face in the short-term decisions that will surely have long-term implications on their business. The 50% emission reduction target set by the IMO is ambitious and will likely call for widespread uptake of lower and zero-carbon fuels, in addition to other energy efficiency measures, including operational and market ones. In an attempt to contribute increasing the general understanding of the decarbonization challenge for the shipping industry, this paper provides a critical overview of the main measures available for cutting down shipping emissions and discusses the main organizational, technical, economic and political challenges and barriers to implementation along with the potential facilitators that could foster a wider application. The framework that is outlined is complex and brings to light some important controversies related to shipping’s decarbonization that exist in both the scientific and business communities.
The remainder of this paper is structured in eight sections. Following this introduction, Section 2 introduces five research questions and outlines the methodology used to give them an answer. Section 3 focuses on the identified drivers for the adoption of low-carbon practices, while Section 4 discusses the main measures and initiatives available to the shipping industry to comply with the new decarbonization and desulfurization targets set by the IMO. The main challenges and barriers to implementation are discussed in Section 5, while some considerations about the technical possibility to achieve the ambitious IMO’s GHG targets by 2050 are in Section 6. Section 7 discusses the potential enablers that could facilitate a quicker and successful low-carbon transition for the shipping industry. Finally, Section 8 summarizes the major findings of the study and points out the potential role of research as a facilitator of the decarbonization process.

2. The Research Approach

This study aims to provide a critical overview of the main measures the shipping industry can apply to reduce its emissions. The overview tries to highlight the strengths and weaknesses of the most popular emission reduction options with reference to the potential contribution they can make to achieving the IMO’s targets. The specific aspects of interest have been formulated into five research questions (RQ):
  • RQ 1: What are the main drivers that are pushing the shipping industry to undertake major efforts to contain its emissions?
  • RQ 2: What are the pros and cons of the most popular emission reduction measures the shipping industry can adopt to try to cope with the new IMO’s requirements?
  • RQ 3: What are the main challenges and barriers to implementing decarbonization measures?
  • RQ 4: Is there an objective way to assess the technical possibility to achieve the ambitious IMO’s GHG targets?
  • RQ 5: Towards the IMO’s goals, who are the potential facilitators that could foster a wider and quicker decarbonization process of the shipping industry?
The overview has been built by following the triangulation research approach. In qualitative research, the triangulation approach refers typically to the use of multiple and diverse data sources to develop a comprehensive understanding of complex phenomena. In order to build an overall framework of the complex issue of shipping’s decarbonization that considers the different dimensions and perspectives of the phenomenon, this research has been developed on two fronts:
  • by reviewing the relevant academic work published on the topic;
  • by analyzing technical reports and updated bulletins of trusted sector experts.
In a preliminary phase of the study, purely numerical research was carried out using the Scopus database to get a view of the quantitative impact (in terms of the number of works published) the issue of decarbonization, and more generally of the environmental sustainability of shipping, has had in the last decades on the scientific literature. Scopus was chosen as one of the leading research databases, including about 20,000 journals with a focus on physical sciences, health sciences, life sciences, social sciences and humanities. The search on Scopus aimed at identifying maritime-related papers which contained in the title the following keywords:
  • IMO + strategy/GHG/Greenhouse;
  • GHG + shipping/maritime/port(s)/ship(s);
  • Decarbonization/decarbonize + shipping/maritime/port(s);
  • Low carbon + shipping/maritime/port(s);
  • Emission(s) + shipping/maritime/port(s)/ship(s);
  • Green + shipping/maritime/port(s)/ship(s);
  • Sustainable/sustainability + shipping/maritime/port(s)/ship(s);
  • Emission Control Areas (ECAs).
The keyword search was performed in October 2019, and updated in March 2020 to include the studies published in 2019. The search covered the 30 years from 1990 to 2019 resulting in 958 works, including journal articles, conference papers, books and book chapters. This general search is not meant to be complete or all-encompassing, as there may be several studies that the search by keywords in the title has not captured and not all journals and studies appear in the Scopus database. Nevertheless, it offers a quantitative indication of the rise of papers covering sustainability issues in the field of shipping and port industry. Looking at the distribution of the studies concerning the year of publication (Figure 1), the trend shows a significant increase in the number of papers published starting in 2009 (840 out of 958 studies were published after 2009). Although the environmental issue first emerged in the early sixties, it is especially in the last decade that it has imposed itself with greater force on the attention of institutions and the conscience of public opinion. Particularly, looking at the graph, it seems possible to identify four main growth stages in the annual number of published studies: 2009–2011 (+85%), 2011–2014 (+54%), 2014–2018 (+49%) and 2018–2019 (+29% in just one year).
Figure 1. Distribution of studies by year of publication.
The start dates of the four phases do not seem random as they coincide with four milestones of the IMO regulation:
  • 2009—publication of the Second GHG Study of the IMO [20];
  • 2011—mandatory measures to enhance energy efficiency for international shipping adopted by Parties to MARPOL Annexe VI [21];
  • 2014—publication of the Third GHG Study of the IMO [3];
  • 2016—setting of the limit for sulfur in fuel oil used onboard ships of 0.50% (mass by mass) from 1 January 2020 [22];
  • 2018—Initial IMO Strategy on reduction of GHG emissions from ships [6].
These and other regulatory aspects will be discussed in depth in Section 3.
Regarding the geographical origin of the studies analyzed, the largest number of them are related to China and the US. This is not surprising considering that the two countries are considered the two biggest emitters in the world and the need for emission reductions is acute for both [18]. The top 20 countries by the number of studies are listed in Table 1.
Table 1. Geographical origin of the studies analyzed—top 20 countries.
To answer the five RQs by capturing the most recent research developments, the analysis has focused on studies published after the Second IMO GHG Study (2009). The studies published as of 2009 and identified through the keyword search were preliminarily filtered by reading the titles and abstracts to understand if they could fall within the scope of the analysis. Only those deemed more adherent to the purpose were reviewed more in detail, giving priority to the most recent ones. Sixty-seven studies were selected through this process. In addition, 51 academic works that were not captured by the keyword search but considered relevant for the analysis were identified through the authors’ personal knowledge, suggestions from the editor, anonymous reviewers, and by identifying additional key publications from the reference lists of the papers already selected. In total, the review of the academic literature converged on 118 studies whose contents are used in this paper to answer the five RQs. Moreover, to offer an up-to-date framework of the context of decarbonization in shipping, the literature review was completed with the study of technical reports and bulletins of trusted sector experts. This was necessary to capture the latest developments in the sector and to include in the analysis of the contemporary factors that nowadays may influence operators’ priorities and actions.

3. RQ1: What Are the Main Drivers that Are Pushing the Shipping Industry to Undertake Major Efforts to Contain Its Emissions?

In the age of climate change and sustainable development, shipping has been forced to become more environmentally friendly by increasingly stricter regulation and external pressures which are directly derived from environmental concerns. Furthermore, considering that fuel is by far the single largest cost to the sector, there can also be clear economic incentives to improve energy efficiency in shipping and invest in cleaner technology. Although the reasons behind implementing emission reduction measures may be manifold, it seems possible to identify at least three crucial factors that push shipping to lower its environmental impact:
  • Regulatory and institutional pressures;
  • Market factors and resource availability issues;
  • Social pressures and ecological awareness and responsiveness.
The order follows somehow the importance they play in the operators’ choice.

3.1. Regulatory and Institutional Pressures

To meet internationally agreed levels of mitigation, the shipping industry has to play its part in reducing its carbon emissions, and to do so, it must undertake fundamental changes in its emissions pathway [23]. The regulatory preserve of international shipping resides mainly with the IMO, which is the United Nations specialized agency responsible for the prevention of marine pollution by ships and the safety and security of shipping activities. The IMO has traditionally addressed the environmental impact of maritime activities by introducing international conventions and laws to regulate them. In 1997, it was designated by the UNFCCC (United Nations Framework Conference on Climate Change) as the body responsible for regulating maritime emissions. The main measure implemented by the IMO is the International Convention for the Prevention of Pollution from Ships (MARPOL), which came into force in 1983 intending to prevent and minimize pollution caused by ships for both operational and accidental reasons. The MARPOL convention includes six technical Annexes, of which Annexe VI, regulates air pollution generated by ships [21]. Specifically, Annexe VI establishes the limits of SOx, NOx and PM global emissions and introduces Emission Control Areas (ECAs) in which more stringent emission policies apply. Annexe VI seeks a progressive reduction of sulfur content in marine fuel oils to achieve the target value of 0.5% by weight by 2020 [22]; it also includes progressively restrictive policies regarding NOx applicable only to ships built after January 2016. IMO established also certain SO2 Emission Control Areas (SECAs) in which the sulfur limit is 0.01 since 2015 (Baltic Sea, North Sea, North American, United States Caribbean Sea). Furthermore, in January 2019, amendments to MARPOL Annexe VI designate the North Sea and the Baltic Sea as ECAs for NOX, both will take effect on 1 January 2021.
At the European level, the European Union (EU) has adopted directives for the abatement of shipping emissions which further limit the maximum sulfur content in marine fuels to 0.1% by weight for ships at berth in several EU ports [24]. Member States are also required to build liquefied natural gas (LNG) refueling points in all ports and install infrastructures for shore-side electricity by the end of 2025 [25].
In April 2018, the IMO agreed on the Initial IMO Strategy to reduce GHG emissions in the shipping sector to meet the Paris Agreement goals [6]. Though the Strategy suggests an indicative framework of measures to be implemented in the short- (2018–2023), medium- (2023–2030) and long-term (after 2030), a final plan is not expected until 2023. The Strategy includes a target to “reduce the total annual GHG emissions by at least 50 per cent by 2050 from 2008 levels whilst pursuing efforts towards phasing them out”. This suggests the IMO will probably implement further and more stringent emissions regulations in the years to come.
Being able to meet the ambitious decarbonization and desulfurization IMO’s targets is probably the major challenge the maritime industry has to face in decades.

3.2. Market Factors and Resource Availability Issues

Market factors have traditionally been the main drivers of innovation in shipping. In particular, the constant fluctuation of fuel prices, due to market forces and the cost of crude oil, has always led to explore alternative energy options or operational practices, such as slow steaming. The global consumption of ship fuel today is estimated in around 400 million tons of oil equivalent [26]. The crucial role assumed by financial aspects becomes evident when considering that energy costs represent on average between 50% and 70% of operating costs of a vessel [27]. This percentage is expected to increase further as heavy fuel oil costs increase, making attractive the adoption of alternative fuels.
As for fuel availability and related costs, precise information concerning the location and amount of global fuel reserves are difficult to obtain, and the available estimates are often controversial or difficult to verify [28]. Moreover, the political instability of several regions holding important oil reserves raises important concerns about security and availability of fuel resources leading several countries to explore and invest in the development of alternative fuels. Diversification of energy alternatives seems to be the key in the future of shipping.

3.3. Social Pressures and Ecological Awareness (Be Green)

The environmental and climate issue is every day in the spotlight and the agendas of governments all over the world. Pressures to adopt “greener” behavior constantly come from various stakeholders, including institutions, customers, citizens, investors and others. In the shipping industry, customers’ and investors’ demands may be strong drivers for the adoption of more environmentally friendly practices as companies need their approval and legitimacy to stay in business [29,30]. The study by Reference [31] stresses that actors in a maritime supply chain should adhere to customers’ expectations and identifies four main customer requirements, including competitive costs, pollution reduction, efficient use of fuel, and health and safety. Nowadays, it is particularly evident how citizen groups, NGOs and other environmental organizations can also influence this change through local activism. By disregarding from what is socially accepted, a company risks losing customers, and thus profits. It is not a case if an ever-increasing number of shipping companies and port operators are progressively investing in communication campaigns and initiatives aimed at promoting their green image to increase their environmental legitimacy. It can also happen that the threat of strictest regulatory actions can become a driving factor for operators to be proactive by taking pre-emptive green moves aimed at increasing their advantage to competitors [29]. A proactive environmental strategy that exceeds regulations is also believed to produce some benefits in terms of recognition from communities and institutions [32]. Sometimes, although not frequently, it can also happen that the implementation of energy efficiency measures is directly influenced by the company’s environmental values and moral commitments to adopt “greener” measures [33].

5. RQ3: What Are the Main Challenges and Barriers to Implementing Decarbonization Measures?

Continuous and widespread deployment of emission reduction measures is reported not to happen at the required speed [13]. The reasons for this slow implementation can be different—and may concern obstacles relating to both the complex combination of factors that characterize the decision-making process in the shipping industry and the maturity of the measures. For example, measures that may first appear very promising in practice could need to overcome a number of barriers to fully exploit their emission reduction potential on a large scale [112].
Moreover, several surveys have shown that although some reduction measures may seem cost-effective, their level of implementation remains low and what can be called an “efficiency gap” exists between the actual level of implementation and the higher level which would be expected based on techno-economic analysis [27,113]. Cost considerations, which typically represent the main part of the decision process, are thus not the only elements at stake and additional technical, operational and market factors must necessarily be considered. Fragmented and diverse ambitions of stakeholders in the sector are reported to further contributing to slowing-down the implementation process of emission reduction [114].
References [33] and [115] categorize barriers to energy efficiency into three broad categories: economic, organizational, and behavioral. By extending and further expanding the existing classifications, this section discusses the main implementation barriers and obstacles to wide diffusion and adoption of emission reduction measures for a transition toward a lower-carbon (and lower-sulfur) shipping industry.

5.1. Economic Barriers

According to industry experts estimates, the new IMO regulations will cost the shipping industry about $60 billion USD per annum [116]. MGO is currently around 60% more expensive than conventional HFO [117]. Considering that bunker costs represent around 47% of the vessel’s operating costs [77], IMO regulations will yield significantly higher costs for the shipping industry. This higher cost will result in an increase in transportation rates estimated at 10% per TEU for customers and the question regarding who will bear these higher costs is not small [118]. However, the cost of fuel is not the only significant cost—there are also capital and operating costs to be considered when evaluating alternative energy options.
Restricted access to the capital market is widely recognized as a barrier to investing in emission reduction solutions [27]. Most shipping companies are relatively small and with limited resources to invest in scrubbers or retrofitting and the payback time of the various alternatives is a crucial factor in the decision-making of ship-owners [52]. Furthermore, ships have a second-hand value that does not reflect investments in energy efficiency equipment [12]. Scrubbers appear to be a more attractive option for new builds compared to retrofits, and old ships seem not suitable for scrubber installations when their remaining lifespan is less than four years [119].
For those interested in using LNG as a marine fuel, the transition to LNG not only needs relevant investments to build the required refueling points, but it also involves the purchase of new powered LNG ships that on average cost 10%–30% more than equivalent diesel-fueled ships conventional ships [59]. Retrofitting can also be extremely expensive, as it requires much space to install bigger fuel tanks [120]. The cost of converting conventional engines so they can be fueled with methanol is estimated to be lower [121].
Full-electric solutions are also unlikely to pay back investments alone. Although battery costs are reported to be in sharp decline [45], their installation and replacement cost (an expected lifetime of 10 years is currently the marine industry standard) is significantly higher than for traditional engines [4]. Moreover, investments in essential shore-based charging facilities are far from negligible. The study by Reference [41] has investigated the economic feasibility of shore side facilities investment in ports founding that the overall investment can be economically viable only when the port could make a net profit by selling electricity. A multi-objective model for strategic planning regarding whether and when to retrofit ships to use shore side electricity has been provided by Reference [122].
Likewise, fuel-cell powered ships are currently much more expensive than comparable diesel ships. Capital costs for a newbuild fuel-cell powered ship are estimated to be 1.5 to 3.5 times higher than a comparable diesel vessel while operating costs are up to eight times higher [123].
A financial evaluation of alternative approaches to the abatement of NOx and SOx emissions can be found in Reference [124]. The study considers a sample of real ships that operate within the ECA of Northern Europe and accounts for revenue that might be generated from emissions trading within a cap-and-trade market. The use of LNG is found to be the most financially attractive measure for reducing SOx emissions; alternative distillates do not appear to be an economical solution to meeting regulatory requirements, while scrubbers and selective catalytic reduction systems can constitute financially attractive abatement options.

5.2. Technological Barriers

Alternative energy options are not available in large quantities today, and LNG seems currently the only green fuel that is scalable commercially and globally for the deep-sea segment in the short-term. However, and despite its benefits for reducing SOx emissions, numerous scholars agree that, as it cannot enable the required IMO’s GHG reductions, its role as a marine fuel will be only transitory. Over the period it will remain in demand, the main limitation to the shift from conventional fuels to LNG is identified in the lack of availability of bunkering facilities at ports. This barrier is a typical “chicken-and-egg” problem: On the one hand, bunker suppliers are unwilling to invest in bunkering points until there is sufficient demand for LNG as a marine fuel, on the other hand, ship-owners are unwilling to invest in LNG vessels if LNG refueling opportunities are not easy to obtain [55].
Storage capacity is another technological barrier to several alternative fuels. As an example, LNG fuel tanks require two to three times the volume of fuel-oil tanks with the same energy content. In the comparison between liquid and gaseous fuels, the former require storage tanks that are more easily integrable onboard. Conversely, storage tanks for gas fuels are typically more costly, space-consuming and challenging to integrate onboard.
Similarly, the electrification of modern ships brings a number of challenges concerning the need for more shore-based facilities for battery charging and more. Despite major technological advances in battery capacity and efficiency, batteries must still become considerably more efficient and less heavy to meet the needs of large ocean-going ships. While several mature applications of battery-electric propulsion can be found in the short-sea-shipping segment (the first battery-electric short-sea ferry headed out in 2015) [125,126], much work is still needed before similar applications in the deep-sea market. This does not exclude the successful application of batteries on large ocean-going ships for specific purposes [127]. As an example, several shipping companies are pioneering the use of hybrid marine propulsion systems in ECAs [128].
As regards clean alternatives (biofuels, methanol, hydrogen, etc.), despite their high potential to reduce CO2 emissions, they are not expected to become feasible on a large scale within a short time because of several technical, economic and safety challenges. The same major concerns exist in using nuclear energy for ship propulsion.

5.3. Time Barriers

Implementing green initiatives needs time and planning. The survey by Reference [81] finds out that the reasons for the lack of implementation of energy efficiency measures are related to time constraints in decision making, absence of planning and few incentives. By way of example, it takes time to convert ships to LNG or to build new LNG-burning vessels, and it also takes time to install scrubbers. Ship-owners may also have their agenda and objectives, and these can conflict with IMO ones.

5.4. Barriers Related to Unclear and Unfair Regulatory Frameworks

Environmental standards and regulations across the world have to be agreed and adopted in order not to distort competition in international shipping [129]. A level playing field across the EU and possibly at the international level must be guaranteed. If a country adopts unilaterally more strict measures, the competition is distorted [130]. As an example, in February 2019, the EU tabled a proposal to the IMO requesting a harmonization of rules relating to scrubber discharge. This comes in the course of concerns on open-loop scrubbers and regional restrictions imposed on their use. It goes without saying that despite strict policies and regulations need to be taken on an international level, complementary action on the regional and national level can still facilitate the low-carbon transition through ad hoc initiatives [131].

5.5. Side Effects

There are some concerns, according to which some regulations aimed at reducing some problems can exacerbate others [130]. The introduction of low-sulfur fuels would result in increased CO2 emissions and a potential modal shift to land transport [132]. Likewise, the implementation of some measures that are successful from a solely environmental perspective may result in a reduction in the profitability of maritime transport bringing several undesirable side-effects, such as cargo shifts to other modes or reduction of trade [133]. According to some estimates, the designation of the Mediterranean Sea as an ECA will cause an increase in transport costs of about 6.95 €/ton and will result in a modal shift of 5.2% in favor of the road-only mode [134,135]. Similarly, the higher transport costs that will likely result from the implementation of CO2 mitigation measures is believed to potentially impact not only the modal share of international transport, but also global trade patterns [114].

5.6. Obstacles Related to Contractual Clauses and Split Incentives

Especially on the spot market, they can be mainly related to the type of charter contracts that may limit the implementation of technical and operational measures. Contracts can contain clauses on speed and bunker consumption or regulations of delays that can make unattractive some practices, such as slow steaming. Another obstacle is related to split incentives that arise when two parties engaged in a contract have different goals. In shipping, they typically occur between ship-owners and charterers, due to divided responsibility for fuel costs [27]. Ship-owners who make investments to improve the environmental performance of their fleet cannot be able to recover their investments unless they directly operate their ships or have specific long-term agreements with charterers.

5.7. Barriers Related to Incomplete and Non-Transparent Information

Informational problems are relevant obstacles faced by shipping operators to the implementation of emission reduction solutions. More than a few operators complain about the lack of reliable and trusted information concerning costs and efficiency of the measures from independent third parties [12]. The literature shows several attempts to calculate the techno-economic potential of emission reduction measures for shipping [136]. The available evaluation methods are mainly based on the use of marginal abatement cost curves (MACCs) that provide the marginal cost of emission abatement for varying amounts of emission reduction [69,137]. The paper by Reference [119] examines the costs and benefits of different reduction measures in connection with the IMO sulfur regulations by integrating the private abatement costs for ship-owners and the social-environmental benefits from emission control. Study results show that the MGO solution is most appealing from a cost perspective, whereas, scrubbers are more efficient in reducing sulfur and particle emissions. Although MACCs can provide some indications about the likelihood of investment, they have several shortcomings concerning the assumptions on which they are based and the impossibility of fully representing the associated costs and risks, including hidden costs [27]. While the cost-effectiveness of low-carbon measures is usually evaluated on an individual basis, an attempt to evaluate the cost-effectiveness of correlated measures can be found in Reference [138]. As regards the financing issues related to the implementation of different emission reduction technologies, they are only partially discussed in the literature. A selection criterion for low-carbon technologies in shipping based on the impact of financing concepts on the overall net present value can be found in Reference [139].

5.8. Barriers Traditionally Inherent to the Sector: Inertia and Risk Appetite

Ship-owners and port operators are typically reported to be conservative and to make resistance to innovation. They generally do not welcome major changes and may be reluctant to implement solutions other than the standard ones. Shipping operators may be skeptical about the implementation of new solutions as a result of the needed large capital investments and the risk of being locked-in in unsuccessful technologies. The perception of risk, both perceived and real, is a traditional barrier to the implementation of innovation in the maritime industry. The risk can be classified into three dimensions—business, technical and external [115]. Business risk mainly concerns financing risk, due to investments repayments; technical risk concerns unreliability of the measures implemented and their performance; and external risk concerns economic trends, fuel prices, policy and regulations. Particularly, the latter is highly representative of what shipping operators are facing in these years and explains why many shipping operators are adopting a “wait-and-see” strategy as they consider it risky investing when additional regulations are still being developed.

5.9. Political Barriers

Especially regarding the adoption of MBMs, political obstacles (due to unnecessary fragmentation and complexity of the international scene, combined with factors that relate only to the political sphere) are believed to slow down the decarbonization process of shipping. Removing these obstacles is considered essential to push the decarbonization process forward [69].

6. RQ4: Is There an Objective Way to Assess the Technical Possibility to Achieve the Ambitious IMO’s GHG Targets?

Reducing shipping emissions implies being able to measure them. The issue of measurability is considered one of the most underrated problems when dealing with maritime emissions [74]: Is there a reliable way to measure emissions?
Shipping emissions are not measured directly—all the existing data are estimates produced by applying specific methods [140]. Available methods can be grouped into three main categories depending on the specific estimation approach they use [103,141]:
  • Fuel-based or top-down approaches;
  • Activity-based or bottom-up approaches;
  • Hybrid methods that combine fuel-based and top-down approaches.
The first approach combines data on marine fuel sales and fuel-related emission factors, but does not consider actual maritime activities. The second approach uses more detailed information on ship characteristics, as well as operational data for both port and marine activities, combined with emissions and load factors. The third approach is a mix of the first two.
Giving a reliable estimate of the emission reduction potential of the various measures is not easy. The existing body of literature includes broad estimates of emission reduction potential for several measures. In particular, References [13] and [114], based on 150 and around 70 studies, respectively, summarize a large portion of the literature on the potential emissions reductions for several green measures. Both reviews highlight a large range in the GHG emission reduction potentials per measure as derived from the available studies, and both suggest a combination of measures would result in larger reduction potentials. Table 2 reports and compares the GHG reduction potential ranges as derived from the two review papers analyzed. Looking at the numbers, it not only emerges a large range in the emission reduction potentials reported by each study, but also important differences among the potential reported by the two studies for the same measure. The reasons of such differences may be manifold, e.g., the specific application case, how the trial was conducted, the traffic segment (deep-sea or short-sea shipping) and the type of ship considered. However, if on the one hand such variability can be explained by differences in assumptions and baselines across the selected studies, on the other hand, it also indicates limited agreement across studies and estimation methodologies, and thus, significant uncertainty about the reported reduction potentials. These elements make clear the need for more transparent research in the field.
Table 2. Greenhouse gas (GHG) Reduction potentials as derived from the literature. Source: Author’s reworking.
The measurability issue also makes difficult assessing the technical possibility to achieve the ambitious IMO’s goals [142]. Though several scholars seem to agree that halving shipping emissions by 2050 will require a consistent switch to non-fossil fuel sources [56], very few studies focus on the quantitative assessment of the technical possibility to reach the IMO’s GHG targets by 2050. Some interesting attempts in this regard can be found in References [143] and [144], which examine the technical possibility of decarbonizing international shipping considering a time horizon up to 2035 and 2050, respectively. Particularly interesting is the work by Reference [114] which examines the technical possibility of decarbonizing maritime transport by 2035 by investigating four different decarbonization pathways. According to the results of the study, a maximum deployment of today’s technologies could make it possible to achieve almost full decarbonization by 2035. The authors also stress the importance of policy measures to support such a process. The study by Reference [13], points out that while no single measure is enough to achieve meaningful GHG reductions, a combination of current technologies can lead to a reduction in GHG emissions by more than 75% by 2050. According to [3], although the global reduction of air emissions from ships has been quite slow, energy efficiency measures alone could potentially reduce the CO2 emissions from shipping by 40 to 60%. An allometric model based on the relationship between the global fleet size and GHG emissions from international shipping is proposed in Reference [145] to assess the theoretical GHG emissions from shipping to 2050. The model is used to figure out the gap between the theoretical emissions and the IMO’s 2050 target in order to provide the authorities with a quantitative basis for promulgating reduction policies.
Both the EU and the IMO are now collecting emission data related to the shipping industry with the target to measure and potentially reduce its GHG/CO2 emissions:
  • The EU has its Monitoring, Reporting and Verification Regulation (EU MRV) for ships larger than 5000 gross tonnage of all flags calling at EU ports and ports within the EU Free Trade Area [146]. Data collection started on 1 January 2018;
  • The IMO has its Data Collection System on fuel consumption (IMO DCS), whose data collection started on 1 January 2019.
Both EU MRV and IMO DCS requirements are mandatory. While the EU MRV covers CO2 emissions from shipping activities to, from and within the EU area, the IMO DCS covers emissions from shipping globally. The two regimes are quite different, and at this point in time, it is not clear whether, how, and when they will converge.

7. RQ5: Towards the IMO’s Goals, Who Are the Potential Facilitators that Could Foster a Wider and Quicker Decarbonization Process of the Shipping Industry?

It is shared the opinion that regulations alone will not be enough to reach the ambitious decarbonization targets unless they are accompanied by incentives and supporting policies that can contribute to accelerating the commercial viability and technical feasibility of low-carbon measures. In this regard, governments and research can have a crucial role as potential facilitators of the decarbonization process through an integrated package of actions which also involves the civil society, according to what is suggested by the Quadruple Helix model.

7.1. Institutions and Governments

To encourage their adoption, any regulation and new target that is imposed on the shipping industry should be accompanied by supporting institutional interventions. As potential enablers of the decarbonization process, international and local governments are required to develop a structured plan to push the adoption of lower- and zero-carbon initiatives. Supporting actions implemented by institutional bodies may include:
  • Providing financial incentives, such as taxes exemption or reduction, to reduce the price gap between conventional and more sustainable energy options not yet economically competitive;
  • Providing subsidies for the implementation of decarbonization initiatives;
  • Introducing supporting policies to encourage the large-scale production of zero-carbon fuels;
  • Investing in not-yet-competitive technologies which offer the best hedge for adopting lower- or zero-GHG alternatives;
  • Collaborating with financial institutions to create tailored financial instruments with low-interest loans for low-carbon shipping;
  • Aiding developing countries to face the decarbonization process;
  • Supporting research and development to develop not only the technical, but also the commercial viability of the most promising decarbonization measures.
According to the paper by Reference [57], which surveyed 249 initiatives that target the reduction of shipping emissions implemented globally in the period from 2008 to 2018, economic incentives are the most used measures (48%). However, particular attention has to be devoted to the provision of subsidies in the shipping sector as they have not always been successful [147], and need to be constantly monitored to be able to introduce the necessary corrections when conditions change, or the expected results are not achieved [15,148].
All the measures listed above are intended to be proactive rather than punitive or restrictive. They act mainly by encouraging the implementation of greener initiatives rather than punishing the less green ones. As previously mentioned, although restrictive approaches can have a theoretical efficiency, they also have the potential risk to cause some negative side effects, such as the shift from maritime to higher-carbon transport modes.
If it is true that environmental standards and regulations must be agreed and adopted at a global level in order not to distort competition, it is also true that facilitator measures work well also at a local level and port authorities can themselves play a key role in this regard.

7.2. Research

Research can have a pivotal role in the transition to decarbonized maritime transport. It can contribute to develop the technical and commercial viability of the most promising low-carbon measures and create an environment that facilitates the implementation of decarbonization initiatives. Though the literature concerning the environmental sustainability of shipping may appear substantive, there is an argument that it still gives an insufficient foundation for decision making, probably because only a few papers are based on empirical findings [149]. The key role of research as an enabler of the decarbonization process is well recognized also by the shipping industry. As an example, in December 2019, shipping associations representing more than 90% of the world fleet submitted a proposal to IMO’s MEPC calling for the establishment of an international non-governmental R&D board to accelerate the development of commercially viable zero-carbon emission vessels. According to the proposal, the board will be financed by shipping companies via a mandatory R&D contribution of $2 per ton of marine fuel [150].
This review has highlighted several areas in which much scholarly attention and more transparent research would be desirable to provide both practitioners and institutions with useful recommendations on the effective evaluation of the various decarbonization (and desulfurization) alternatives at hand, particularly:
  • Although it is universally recognized that some emission reduction measures can have some negative side-effects, these effects would need to be deeply and quantitatively investigated;
  • To date, there are significant disputes regarding the actual validity of some emission reduction measures, such as MBMs, LNG, speed management, etc. Further research is required to assess and develop their viability, both from a commercial and environmental perspective;
  • Reducing emissions implies to be able to measure them but a common, reliable and recognized estimation method for emissions has yet to be developed. Ensuring the availability of accurate and consistent information about emission trajectories is essential to assess what strategies have been implemented, which have been effective, and which have not;
  • Transparent cost-benefit assessment models are needed to support ship-owners to appraise the most effective emission reduction strategy for their business, considering both short- and long-term scenarios.

7.3. Civil Society

Finally, an important facilitating role in the decarbonization process can be played by civil society. Nowadays it is practically impossible to find someone who has never heard of Greta Thunberg and the “Fridays for Future” movement of school students from all over the world who take time off from class on Fridays to demand political leaders to take action to prevent climate change and switch to renewable energy. Driven by such growing societal pressures, several governments are now building a shared international agenda to implement strategies to face climate change. Furthermore, more and more governments are prioritizing greater public involvement in green processes [151]. In this regard, several studies agree that to cope successfully with important societal challenges, such as climate change, strong cooperation between government, industry, academy and society is required [152]. The most desirable approach seems to be the Quadruple Helix where society, research, industry and government are all core players to boost environmentally responsible growth [153]. The Quadruple Helix is an update of the Triple Helix model in which civil society joins academy, industry and government in the development of any innovation strategy. In this sense, even in the shipping sector, public involvement and participatory processes can contribute to legitimize research trajectories and producing more sustainable initiatives by re-orienting development toward public preference [154].

8. Conclusions and Key Findings

To achieve the ambitious IMO’s 2050 targets, shipping is required to make a fundamental change in its emission pathway. Using a triangulation research approach, this paper has offered an overall and critical overview of the most common and promising measures and initiatives the shipping industry can adopt to try to cope with the new IMO’s requirements. The main challenges and barriers to implementing decarbonization options have been discussed, along with the potential facilitators that could foster a wider application. Although the proposed overview is not meant to be complete or all-encompassing, as there may be aspects that the performed literature review has not captured, it can provide the stakeholders who are called to take major steps towards lower-carbon shipping with a wealth of information that can support an informed decision-making process. The review has been structured to answer five main Research Questions (RQ) concerning:
  • The main drivers for the adoption of emission reduction measures in shipping (RQ1);
  • The pros and cons of the most popular emission reduction measures the shipping industry can adopt to try to cope with the new IMO’s requirements (RQ2);
  • The main challenges and barriers to implementing decarbonization measures (RQ3);
  • The methods available to assess the technical possibility of achieving the ambitious IMO’s GHG goals (RQ4);
  • The role of governments, research and civil society as potential enablers of shipping’s decarbonization (RQ5).
RQ1: The threat of climate change and environmental issues are of growing concern for the shipping industry, due to ever stronger financial, regulatory, and societal pressures that are driving shipping towards a more sustainable operating way. The 50% GHG emission reduction target set by the IMO is very ambitious and will require the shipping industry to implement substantial changes in fuels, technologies and operations. Being able to meet the new internationally agreed IMO’s targets is probably the main challenge the shipping industry has to face in decades.
RQ2: Possible options to decarbonize shipping are various and include fuel switching, technological interventions, operational initiatives and market-based measures. Not all options have reached full maturity yet. Some of the analyzed measures are already technologically advanced, while others are not; some are easy to implement in the short-term, while others require relevant capital investments and longer timescales. Besides, some measures that in the first instance may appear very promising for reducing GHG emissions, in practice need to overcome a number of barriers to fully exploit their potential on a large scale. In the short-term there exist several measures that can be applied and combined to contribute to reducing CO2 emissions, but in the long-term full decarbonization will require a consistent switch from fossil fuels to alternative energy options. Any measure or action poses both risks and opportunities, and it is almost impossible to anticipate which solutions will prove most valid. Furthermore, important disputes exist in both the scientific and shipping community regarding the adoption of some popular measures, such as speed reduction, EEDI, MBMs, and LNG as a marine fuel.
RQ3: Continuous and widespread deployment of emission reduction measures is reported not to happen at the required speed, meaning that their reduction potential has not yet been fully realized. Furthermore, although the apparent cost-effectiveness some of these measures may have, what the literature calls “efficiency gap” exists between the actual level of implementation and the higher level which would be expected based on purely technical analysis. The reason for this has to be found in a number of barriers that impede a wider diffusion and adoption of emission reduction practices in the shipping industry. Such barriers concern not only the traditional resistance of the sector towards change, but also investment opportunities and risk, uncertainty on future regulatory steps, information and time constraints, technological limits, market issues and political obstacles. The real challenge for the future is to succeed in effectively integrating environmental sustainability with economic sustainability and shipping needs.
RQ4: Shipping emissions are not measured directly but estimated using various methods. The existing body of literature shows a large range in the reported GHG emission reduction potentials per measure, which indicates limited agreement across studies and estimation methodologies. Despite the measurability obstacle, several studies have tried to assess the technical possibility of decarbonizing international shipping and achieving the ambitious IMO’s goals. Studies seem to mostly agree that GHG emission reduction in line with the IMO’s targets is possible but extremely challenging. While it is a shared opinion that no single measure is enough to achieve meaningful GHG reductions globally, specific combinations of current technologies are believed capable of achieving the IMO’s target by 2050 (the most optimistic study predicts that maximum deployment of today’s technologies could make it possible to achieve almost full decarbonization by 2035). All studies stress the importance of policy measures to support such a process.
RQ5: Governments and research play a crucial role as potential facilitators of shipping’s decarbonization and can contribute push decarbonization initiatives through an integrated package of actions which also include the involvement of civil society. Supporting actions by institutional bodies may concern financial incentives, subsidies, investments in not-yet ready, but promising, low-carbon options and other facilitating policies. For its part, research can contribute to overcoming some controversies that are slowing down the implementation of decarbonization measures in shipping, thus creating an environment that facilitates and encourages the achievement of the IMO’s goals. The literature in the field may appear remarkable—however, this review has highlighted several areas where much scholarly attention, and more transparent research, would be expedient to provide both practitioners and institutions with valuable knowledge and quantitative tools. In turn, this would support the effective evaluation of the various decarbonization options in terms of their cost-effectiveness in the short- and medium-term, their actual benefits and side-effects, and their real mitigation potential.

Author Contributions

Conceptualization, P.S.; methodology, P.S.; validation, P.S. and G.F.; data curation, P.S. and G.F.; writing—original draft preparation, P.S.; writing—review and editing, P.S. and G. F. All authors have read and agree to the published version of the manuscript.

Funding

This research received no external funding.

Conflicts of Interest

The authors declare no conflict of interest.

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