Decarbonization of International Shipping: Importance of Alternative ‘Green’ Fuels

by Joel Ong Jie Hao,* a Research Assistant at the Oceans Law and Policy Programme at the Centre for International Law (CIL), National University of Singapore (NUS), Singapore.

This publication is supported by the MPA-CIL Oceans Governance Research Programme 2023 funded by the Singapore Maritime Institute (SMI-2023-MA-03).

International shipping is a significant source of carbon emissions. It accounts for some 2.89% of global anthropogenic emissions and is the sixth largest emissions source by sector. Yet, it is indispensable to international trade, carrying over 80% of the world’s merchandise trade by volume. In 2023, the United Nations Conference on Trade and Development (UNCTAD) published a report finding carbon emissions from international shipping were 20% higher than 2013, with overall emissions increasing by 40% by 2050 due to the expected increase in global trade volume. With the impacts of climate change becoming more frequent and intense, curtailing global anthropogenic emissions is ever more important, and the implications of international shipping on climate change will necessarily have to be given more attention. What emerging ‘green solutions’ will shipping take, and what are the risks and benefits?

This blog post examines how the International Maritime Organization (IMO) – the United Nations specialized agency responsible for safe, secure and efficient shipping and the prevention of ship-source pollution – attempts to regulate shipping emissions, and why it is important to examine the implications of alternative fuels development to climate change and reduction of Greenhouse Gas (GHG) emissions from international shipping.

The Big Picture of Shipping and Climate Change

That said, international shipping is separate from the climate change law regime. Parties to the 1992 United Nations Framework Convention on Climate Change (UNFCCC) had expressly requested industrialised countries in the Kyoto Protocol pursue emission limitations from maritime transport through the IMO (Article 2.2). Further, international shipping was not part of the Paris Agreement. As the global regulator for international shipping, the IMO has been the key forum to address GHGs from shipping, while also reporting its progress regularly to UNFCCC. Given this context, the developments at the IMO with respect to decarbonization is crucial for successfully mitigating climate change.

“Colours” of Hydrogen-based Fuels – A Labelling Convention

This blog post will solely focus on two leading new fuels of choice: ammonia and methanol. Colours are attached to hydrogen-based fuel names to identify the process of producing such fuels and to facilitate assessment of how much carbon is emitted in its entire lifecycle. Green represents production using renewable energy feedstocks, whereas blue represents natural gas feedstock. For instance, blue ammonia is conventional ammoniaproduced from natural gas, with CCS (carbon capture and storage). Grey ammonia is ammonia produced from natural gas without any CCS, and brown ammoniais ammonia produced from coal without any CCS. However, in terms of end product, grey, green or blue methanol or ammonia are chemically indistinguishable, making transparency of their prefixes a significant issue.

Dangers of Transitioning to New Fuels Too Quickly

It is important that this energy transition and adoption of new fuels technology is carefully managed. For instance, while ammonia is commonly used in agriculture, the use of ammonia (and other new fuels) as bunker fuel is novel, and carries new risks which must be carefully considered.

First, there is a potential legal impact if the environmental aspects of pollutive alternative fuels are not adequately addressed or comprehensively substantiated. There should be a greater focus on the threats to the marine environment. Pure ammonia is highly toxic to marine environments, and is approximately 1161 times more lethal to fish than Heavy Fuel Oil (HFO). Further, there is a large liability gap where spills of these substances as bunkers do not fall under the existing liability and compensation regimes. Even in absence of a spill, environmental NGOs may raise legal challenges on emissions transparency, as they have on the use of liquid natural gas in Europe. For instance, grey ammonia and methanol are even more pollutive than conventional fuels such as Marine Gas Oil or HFO. Additionally, while grey ammonia does not produces carbon upon combustion, it contributes almost 1.5 times the GHG emissions of HFO on a lifecycle basis. With no chemical difference between green, blue or grey, attaching the wrong prefixes could undermine the transparency of lifecycle GHG emissions accounting and be counter-productive to climate change mitigation. While the IMO is working on draft lifecycle guidelines in this regard (‘LCA Guidelines’), more caution should be exercised in verifying the sustainable origins of new fuels, since it would significantly impact the amount of upstream emissions.

Second, while the IMO’s 2024 theme of “Navigating the future: safety first!” focuses on the human element, more attention should be given to the dangers posed to the safety of seafarers serving on board ships which run on highly toxic fuels. Methanol is highly flammable and burns invisibly in daylight. Ammonia “is a lethal toxin at low concentrations” when exposed to humans, yet shortfalls in safety standards due to crew fatigue and oversight are commonplace. From 1985 to 2019, there were approximately 71 accidents involving pure ammonia (the type envisaged for bunker fuel use) and the primary causes of deaths and injuries were identified as inhalation of the gas or fires. International Tanker Owners Pollution Federation Limited (‘ITOPF’), the world’s experts on ship-source spills, consequently noted that “the risks to human health with these new emerging fuels are widespread.”

Last, there is a danger that ports in the Global South and Less Developed Countries may have fewer opportunities to implement new technologies due to high costs and  inaccessible technologies. ITOPF remarked that accidents could lead to a drastic loss of life and further economic damage. That said, IMO has included equity as a guiding principle in its 2023 Strategy.

Climate Ambitions for International Shipping at the IMO

Faced with increased environmental pressure, the IMO has adopted a strategic document in 2018 (2018 Strategy)setting out quantitative targets to tackle ship-source GHG emissions. Its second iteration, the 2023 IMO Strategy on Reduction of GHG Emissions from Ships (2023 Strategy, or “Revised Strategy”), was unanimously adopted in 2023. Although the 2023 Strategy is technically non-legally binding, the IMO Assembly adopted Resolution A.1173(33) which detailed the IMO’s Strategic Plan for 2024-2029. It stated that the IMO will develop appropriate implementation measures to respond to climate change, and that measures to facilitate the transition to new fuels will be integral to achieving the ambitions of the 2023 Strategy.

The 2023 Strategy is a significant enhancement of its predecessor, the 2018 Initial Strategy, accelerating the move for shipping to explore and adopt alternative fuel pathways to meet the IMO targets. The 2023 Revised Strategy set the quantitative target reduction in carbon intensity by “at least 40% by 2030,” the same as the 2018 Initial Strategy. However, the key changes were:

  1. The 2050 ambition was doubled from “at least 50% by 2050” to net-zero “by or around” 2050, with newly added interim targets of at least 20% by 2030 and 70% by 2040;
  2. A new fuels criterion (“uptake of zero or near-zero GHG emission technologies, fuels and/or energy sources to represent at least 5% of the energy used by international shipping by 2030”) was added.

On implementation, the IMO laid out its 3-stage Road Map timeline of short (2018-2023), mid (2025-2030), and long-term (beyond 2030) measures. Short-term energy-efficiency measures (e.g., IMO 2020, Energy Efficiency Existing Ship Index (EEXI), Carbon Intensity Indicator (CII), Ship Energy Efficiency Management Plan (SEEMP)) have already been implemented and will be enhanced. Alternative fuels technology is an essential part of the portfolio of mid-term measures expected to be finalized by IMO by 2025. Other possible mid-term measures include onboard carbon-capture and storage.

While there has been progress in substituting fuels for smaller, short-range vessels with electrification of harbour crafts, international shipping is considered a “hard-to-abate” sector because it is technically unfeasible to completely eliminate the use of fuels for large ocean-going vessels on long voyages at sea. Transition is costly, so shipowners are more likely to retain their existing fossil-engine ships and not update their ageing fleets, given uncertainties over the yet-enacted IMO measures and best alternative fuel pathway. UNCTAD reported  that om 2023, the average age of global fleet is 22.2 years old, with about 54% of vessels being over 15 years old. Hence, supporting the uptake of alternative fuels is a key focus of the IMO for the next six years.

The Case for Alternative Fuels: National Interests

In the long-term, States with abundant natural gas or oil resources tend to support blue ammonia as a way to reduce their carbon contributions, since natural gas through steam methane reforming could be used to produce the hydrogen required for blue ammonia. States who face constraints on adopting renewable power sources, such as Japan, would also find these fuels an attractive substitute for fossil fuels for their power and electricity generation needs. Notably, in 2020, the Saudi firm Sabic worked with Japan’s Institute of Energy Economics to ship 40t of blue ammonia from Saudi Arabia to Japan to test the concept of importing ammonia for power generation. Production of methanol requires carbon dioxide – which can be reused from point-source capture of CO2 from hard-to-abate industrial activities such as steel or cement-making.

For green ammonia or methanol, zero-carbon fuels such as e-ammonia or e-methanol can be produced from green electricity, which is attractive to States with abundant wind, hydro, or solar power. For instance, Yara’s anticipated ammonia plant in Norway would run on power from offshore wind farms in the Netherlands, or hydropower from Norway. Bio-methanol, which requires biomass or biowaste as feedstock, could also be favoured by States with large agricultural industries or imports. That said, many port States, such as Singapore, envisage a multi-fueled future, unless – and until – there is one clear, decisive technological winner.


According to the IMO, the eventual substitution of fossil-based bunker fuels for alternative low or zero-carbon fuels is a necessity to mitigate shipping’s contributions to the impacts of climate change. At the same time, the goal of minimizing climate change from ships has to be balanced against the goals of protecting the marine environment and human safety. This blog post thus sought to give a primer into the different aspects of such fuels, and illustrate some potential risks from adopting them for commercial shipping.

*The views expressed in this publication are those of the author and do not necessarily reflect the position of the NUS Center for International Law or any organization with which the author is affiliated.



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