International: Marine: GHG


In 2018, the International Maritime Organization (IMO) released the Initial IMO Strategy on Reduction of GHG Emissions from Ships. This strategy established specific greenhouse gas (GHG) reduction targets through 2050, as well as a longer-term goal to phase out GHG emissions as soon as possible this century. The strategy identified potential policy measures to meet these targets. In June 2021, the IMO adopted some mandatory measures designed to reduce the carbon intensity of international shipping. Their ultimate effectiveness will depend on their stringency, enforceability, and compliance. Further measures are expected to be adopted in the future.

Standard type

Greenhouse gas (GHG) emission limits

Varies by regulation: Some apply to ships 400 gross tonnes and above (e.g., EEDI), others to ships 5,000 gross tonnes and above (EEXI and CII), with various exemptions and correction factors


Note: For more information on the formation and history of the International Maritime Organization (IMO), see the International Marine Emissions page.

As acknowledged by the Kyoto Protocol, CO2 emissions from international shipping cannot be attributed to any one national economy due to shipping’s global activities and complex operation. Therefore, the IMO has been pursuing the limitation and reduction of greenhouse gas (GHG) emissions from international shipping, in recognition of the magnitude of the climate change challenge and the intense focus on this topic.

According to the Second IMO GHG Study 2009, the most comprehensive and authoritative assessment of the level of GHG emitted by ships to date, international shipping was estimated to have emitted 870 million tonnes, or about 2.7 percent of the global anthropogenic emissions of CO2 in 2007. As the United Nations organization responsible for reducing the intensity of growth in CO2 emissions from shipping, the IMO used this study to develop a regulatory framework on GHG emissions.

History of IMO work on GHG emissions

  • 1997: An International Conference of Parties to the MARPOL Convention adopted Resolution 8 on CO2 emissions from ships. This resolution invited the Marine Environment Protection Committee (MEPC) to consider what CO2 reduction strategies might be feasible in light of the relationship between CO2 and other atmospheric and marine pollutants. The resolution also invited IMO, in cooperation with the UNFCCC, to undertake a study of CO2 emissions from ships for the purpose of establishing the amount and relative percentage of CO2 emissions from ships as part of the global inventory of CO2 emissions.
  • 2000: The first IMO GHG Study on GHG emissions from ships was published, which estimated that ships engaged in international trade in 1996 contributed about 1.8 percent of the total global anthropogenic CO2 emissions.
  • 2003: The IMO Assembly adopted Resolution A.963(23) on IMO Policies and practices related to the reduction of GHG emissions from ships, which urged MEPC to identify and develop the mechanism(s) needed to achieve the limitation or reduction of GHG emissions from international shipping.
  • 2009: MEPC finalized a suite of specific technical and operational reduction measures.
  • 2010: MEPC started the consideration of making the technical and operational measures mandatory for all ships, irrespective of flag and ownership.
  • 2011: Technical measures for new ships and operational reduction measures for all ships were adopted, which was the first ever mandatory global GHG reduction regime for an entire industry sector. The adopted measures add to MARPOL Annex VI (Resolution MEPC.203(62)), a new chapter entitled Regulations on energy efficiency for ships, making the Energy Efficiency Design Index (EEDI) for new ships and the Ship Energy Efficiency Plan (SEEMP) mandatory for all ships. The EEDI is the first regulation to establish COstandards across the global sector.
  • 2013: EEDI and SEEMP entered into force and applied to all ships over 400 gross tonnage.
  • 2018: IMO adopted the 2018 Initial IMO Strategy on Reduction of GHG Emissions from Ships.
  • 2019: IMO adopted revised EEDI phase 3 standards which affected the technical carbon intensity of new ships in 2022 or 2025, depending on the ship type.
  • June 2021: IMO adopted new carbon intensity standards for existing ships, including a technical carbon intensity standard called the Energy Efficiency Existing Ship Index (EEXI) and an operational carbon intensity standard called the Carbon Intensity Indicator (CII). These amendments will enter into force in 2023.

Technical Standards

In 2011, MARPOL Annex VI, Chapter 4 introduced two mandatory mechanisms intended to ensure an energy efficiency standard for all ships:

  1. Energy Efficiency Design Index (EEDI): A performance-based mechanism that requires a certain minimum energy efficiency in new ships. Ship designers and builders are free to choose the technologies to satisfy the EEDI requirements in a specific ship design.
  2. Ship Energy Efficiency Management Plan (SEEMP): Establishes a mechanism for operators to improve the energy efficiency of ships.

The regulations apply to all ships of and above 400 gross tonnage, and entered into force in 2013. Flexibilities existed in the initial period of up to six and a half years after the entry into force, when the IMO could waive the requirement to comply with the EEDI for certain new ships, such as those that were already under construction.

In 2018, the Initial IMO Strategy on Reduction of GHG Emissions from Ships outlined the following specific emissions reductions goals:

  • Reduce the carbon intensity of international shipping at least 40% by 2030 compared to 2008
  • Pursue a 70% reduction in carbon intensity of international shipping by 2050 compared to 2008
  • Peak GHG emissions from international shipping as soon as possible and reduce the total annual GHG emissions by at least 50% by 2050 compared to 2008, while pursuing efforts towards phasing them out consistent with the Paris Agreement temperature goals

This strategy also outlined potential short-, medium-, and long-term plans to achieve these goals, shown in the table below.

Short-, medium-, and long-term plans in the IMO Strategy
Type Years Measure Target Current status
Short-term 2018–2023 Improve energy efficiency framework, focusing on EEDI and SEEMP New vessels EEDI requirements set through 2025
Develop new operational and technical efficiency measures In-service vessels SEEMP planning required
Establish an Existing Fleet Improvement Program In-service vessels EEXI and CII adopted at MEPC 76 in 2021
Consider speed reduction and optimization In-service vessels Indirectly encouraged through EEXI and CII
Consider measures to address methane and VOC emissions Engines and fugitive emissions Under consideration at MEPC
Encourage National Action Plans to address GHG emissions In-service vessels, port emissions, and fuels Member states have begun submitting their NAPs to IMO
Continue cooperation under Integrated Technical Cooperation Programme Developing countries Ongoing
Encourage port developments and global activities to facilitate GHG reductions Port emissions and fuels Ongoing
Establish International Maritime Research Board to oversee R&D to improve ship energy efficiency R&D funds Under consideration at MEPC
Develop incentives for first movers to develop and adopt new technologies Incentives Under consideration at MEPC
Develop lifecycle GHG/carbon intensity guidelines for all types of fuels Fuels Under development at MEPC
Promote the work of IMO to the international community on reaching the Sustainable Development Goals Public relations Ongoing
Undertake additional GHG studies and other studies to inform policy decisions Research Fourth IMO GHG Study published in 2020
Mid-term 2023–2030 Establish an alternative low-carbon and zero-carbon fuels implementation program Fuels/new and in-service vessels Not started
Further operational efficiency measures (e.g. SEEMP, operational efficiency standard) In-service vessels SEEMP planning required
Introduce market-based Measures (MBMs) to incentivize GHG reduction In-service vessels/fuels Under consideration at MEPC
Continue and further cooperation under ITCP Developing countries Ongoing
Develop feedback mechanism to collect information to enable sharing of best practices Information sharing Ongoing
Long-term 2030+ Development and provision of zero-carbon or fossil-free fuels Fuels/new and in-service vessels Ongoing
Facilitate uptake of other emissions reductions technologies New and in-service vessels Ongoing


The EEDI for new ships encourages the use of more energy efficient equipment and engines. The EEDI requires a minimum energy efficiency level per unit of transport work (deadweight tonne-nautical mile or gross-tonne nautical mile, depending on the ship type) for different ship type and size segments. The regulation came into force in 2013 and was followed by an initial two-year phase before new ship design needed to meet the reference level for their ship type. The level was to be strengthened incrementally over time. The EEDI is performance-based; it is up to the manufacturer to choose a technology to use in a specific ship design. As long as the required energy efficiency level is attained, ship designers and builders are free to use the most cost-efficient solutions for the ship to comply with the regulations. The EEDI is expressed in grams of carbon dioxide (g CO2) per ship’s capacity-mile (the smaller the EEDI the more energy efficient ship design) and is calculated by a formula based on the technical design parameters for a given ship. The CO2 reduction level (g CO2 per tonne mile) is currently divided into three phases with more to come in the future, with each phase requiring a different reduction level depending on the type and size of ship. The EEDI is in place for the largest and most energy-intensive ships around the world and will affect the following ship types: tankers, bulk carriers, gas carriers, general cargo, container ships, refrigerated cargo, combination carriers, LNG carriers, vehicle carriers, ro-ro cargo ships, ro-ro passenger ships, and cruise passenger ships with non-conventional propulsion. For types not covered by the current EEDI formula, updated formulas will be developed that will focus on the largest emitters first.


The Ship Energy Efficiency Management Plan (SEEMP) aims to improve the energy efficiency of a ship as cost-effectively as possible and provides a method for shipping companies to focus on ship and fleet efficiency. The Energy Efficiency Operational Indicator (EEOI) is an example of a monitoring tool used to assist owners and operators. The EEOI provides owners and operators with a method of measuring the fuel efficiency of a ship in operation and to estimate the result of any changes, such as improved voyage planning or more frequent propeller cleaning. The SEEMP encourages the owner and operator to analyze new technologies and practices throughout the life of the plan.


The Energy Efficiency Existing Ship Index (EEXI) will be a technical measure similar to the EEDI. Unlike the EEDI, which applies only to new ships, the EEXI will apply to all ships. Coming into force in 2023, all ships will be required to calculate their EEXI and compare it to the required EEXI for their ship type and size, which is a function of engine power and ship speed. Ships not in compliance will need to limit their engine or shaft power or install energy saving retrofits such as wind-assisted propulsion to achieve their required EEXI.


The Carbon Intensity Indicator (CII) is an operations-based measure that will come into effect in 2023. The CII annually measures GHG emissions per deadweight tonnage and nautical miles traveled. The CII will correspond to a ship rating between A and E, where A is the best and C is the minimum threshold for compliance. Through 2030, the corresponding CII to each rating will become increasingly stringent, encouraging adoption of energy efficiency technologies. Ships rated D or E for three consecutive years will be required to submit a corrective action plan detailing how the ship will be brought into compliance. The CII will be recorded in the SEEMP. IMO is encouraging ports to provide incentives to ships with a CII rating of A or B to encourage emissions and fuel consumption reductions during ship operation.


Technologies and operations strategies

In 2011, the ICCT collaborated on a major study and identified 53 different ship types to which efficiency technologies could be applied and for each evaluated the potential benefits of 22 existing technical and operational measures that could be deployed immediately or in the near future and had sufficient operational data to analyze. The measures that were considered are grouped into 15 general categories and are listed below.

Technologies and operations strategies to reduce GHG emissions from ships
Propeller polishing Hull cleaning Speed reduction
Autopilot upgrade Air lubrication Main engine retrofits
Water flow optimization Hull coating Speed controlled pumps and fans
Weather routing Wind power High-efficiency lighting
Propeller upgrade Waste heat reduction Solar panels

In the Fourth IMO GHG Study, marginal abatement cost curves were made taking into the account additional abatement technologies. These technologies were divided into four broad categories: energy-saving, renewable energy, alternative fuels, and speed reduction. The table of abatement technologies from the study is reproduced below.


Groups of 28 abatement technologies and use of alternative fuel
  Gr. no. Abatement technologies and use of alternative fuels and renewable energy
(1) Energy-saving technologies Group 1 main engine improvements Main engine tuning
Electronic engine control
Group 2 auxiliary systems Frequency converters
Speed control of pumps and fans
Group 3 steam plant improvements Steam plant operations improvements
Group 4 waste heat recovery Waste heat recovery
Exhaust gas boilers on auxiliary engines
Group 5 propeller improvements Propeller-rudder upgrade
Propeller upgrade (nozzle, tip winglet)
Propeller boss cap fins
Contra-rotating propeller
Group 6 propeller maintenance Propeller performance monitoring
Propeller polishing
Group 7 air lubrication Air lubrication
Group 8 hull coating Low-friction hull coating
Group 9 hull maintenance Hull performance monitoring
Hull brushingHull hydro-blasting
Dry-dock full blast
Group 10 optimization of water flow hull openings Optimization of water flow hull openings
Group 11 super light ship Super light ship
(2) Use of renewable energy Group 12 reduced auxiliary power demand Reduced auxiliary power demand (low energy lighting etc.)
Group 13 wind power Towing kite
Wind power (fixed sails or wings)
Wind engine (Flettner rotor)
Group 14 solar panels Solar panels
(3) Use of alternative fuels Group 15A use of alternative fuels with carbon LNG + ICE or FC
Methanol + ICE
Ethanol + ICE
Group 15B use of alternative fuels without carbon Hydrogen + ICE or FC
Ammonia + ICE or FC
Synthetic methane + ICE or FC
Biomass methane + ICE or FC
Synthetic methanol + ICE
Biomass methanol + ICE
Synthetic ethanol + ICE
Biomass ethanol + ICE
(4) Speed reduction Group 16 speed reduction Speed reduction by 10%

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