At the 80th session of the Marine Environment Protection Committee (MEPC 80), Member States of the International Maritime Organization (IMO) adopted the 2023 IMO Strategy on Reduction of GHG Emissions from Ships, updating the Initial GHG Strategy. The 2023 IMO GHG Strategy sets an enhanced ambition to reach net-zero GHG emissions from international shipping by or around 2050, with indicative checkpoints for 2030 and 2040. To support this objective, the Strategy envisages the development of mid-term GHG reduction measures building on previously adopted short-term measures.
At present, there are no mandatory requirements for monitoring or reporting methane (CH4) and nitrous oxide (N2O) emissions under the existing short-term measures, i.e. IMO EEDI/EEXI, SEEMP, DCS and CII. However, such requirements are expected to be introduced as part of future mid-term measures, including a GHG Fuel Intensity (GFI) metric, supported by IMO’s life cycle assessment (LCA) framework. The 2024 LCA Guidelines (resolution MEPC.391(81)) provide default emission factors for CH4 and N2O, including default methane slip by engine type, while also allowing the use of actual emission factors subject to the development of relevant procedures for verification and certification.
Although adoption of the mid-term measures was originally planned at an extraordinary session in 2025 (MEPC.ES/2), that meeting was adjourned for one year to allow additional time for deliberations. Consideration of the measures is expected to continue at MEPC 85 (December 2026), supported by further discussions at intersessional meetings of the Intersessional Working Group on Reduction of GHG Emissions from Ships (ISWG-GHG). Should mid-term measures be adopted under the tacit acceptance procedure, they would enter into force 16 months thereafter (i.e. at the earliest in 2028).
To calculate the GHG emissions as CO2-eq, the 2024 LCA Guidelines use the global warming potential (GWP) over a 100-year time-horizon, as given in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5).
GHG
|
GWP
|
CO2
|
1
|
CH4
|
28
|
N2O
|
265
|
Default methane slip factors under 2024 LCA Guidelines
Engine type
|
Methane slip (% of consumed fuel)
|
LNG Otto (dual fuel, medium speed)
|
3.5%
|
LNG Otto (dual fuel, slow speed)
|
1.7%
|
LNG Diesel (dual fuel, slow speed)
|
0.15%
|
LBSI (lean-burn spark ignition)
|
2.6%
|
Steam turbines and boilers
|
0.01%
|
The difference between default and actual methane slip values can be significant, particularly for medium-speed four-stroke dual-fuel engines, where the default factor of 3.5% may substantially exceed measured performance. Engine manufacturers have indicated that actual methane slip for the latest engine designs — incorporating technologies such as variable compression ratio (VCR) and intelligent combustion emission reduction (iCER) — can be 50–80% lower than default values. For high-pressure, diesel-cycle gas injection engines (e.g. two-stroke ME-GI type), the default slip of 0.15% is already relatively low, meaning the potential benefit of testing and certification is more limited.
At MEPC 84, the Committee agreed that, without prejudice to any future mandatory framework, actual Tank-to-Wake (TtW) emission values for CH4 and/or N2O may be quantified on a voluntary basis using the following approaches:
- Engine load monitoring, in accordance with the ELM Guidelines, in conjunction with the 2026 Guidelines for test-bed and onboard measurements of CH4 and/or N2O emissions from marine diesel engines; and/or
- A continuous emissions monitoring system (CEMS), in accordance with the CEMS Guidelines.
Test-bed or onboard measurements
At MEPC 83, the Guidelines adopted through resolution MEPC.402(83) established a protocol for measurements, calculation and reporting of CH4 and/or N2O emission values from marine diesel engines, including documentation and verification requirements.
At MEPC 84, the 2026 Guidelines were adopted through resolution MEPC.414(84), superseding MEPC.402(83). Further details on this revision are provided in the ABS MEPC 84 Brief.
Measurements, calculations and reporting for CH4 and/or N2O emission values should be carried out in accordance with the NOx Technical Code 2008 (NTC 2008), as amended, except as specifically provided in the protocol set out in appendix 1 to MEPC.414(84).
Emission measurements should be carried out at the mode points specified in the relevant test cycles under the NTC 2008. For test cycles E2 and E3, emissions at the 10% mode point or the lowest mode point at which gas fuel would be used should be measured and reported in addition to the existing mode points.
Measurement Location
|
Individual Engine
|
Engine Group represented by the Parent Engine
|
Engine Family represented by the Parent Engine
|
Test-bed
|
Accepted
|
Accepted
|
Accepted
|
Onboard
|
Accepted
|
Accepted
|
Not accepted without further justification
|
Required content of the CH4 and/or N2O file
The applicant for the establishment of the emission values should prepare a CH4 and/or N2O file containing the items specified in Section 2 of MEPC.414(84), which are summarized below.
Resolution MEPC.414(84) refers to the "Applicant" as the party that may establish the CH₄ Technical File; therefore, it does not have to be the OEM. In practice, however, the Applicant would need to work closely with the engine OEM, since data from the NOₓ testing and the engine's NOₓ Technical File — which are typically considered the OEM's intellectual property — would be required. Arranging separate onboard testing without OEM cooperation would likely be very costly.
Use of existing measurement data
Measurements conducted prior to the adoption of these Guidelines may be accepted, provided that they were carried out in accordance with the NTC 2008.
Where CH₄ measurements are not available, emission values for total hydrocarbons (HC), as set out in the engine's NOₓ technical file, may be used as a proxy.
- The CH₄ emission values may be established based on the weighted percentage of methane in the gas fuel.
- Where the fuel composition is not available in the engine's NOₓ technical file, total HC emissions should conservatively be assumed to consist of 100% CH₄.
- Where existing E2 data lack the 10% load point (or lowest gas-fuel mode point), D2 data from the same Parent Engine may be considered representative, provided the Parent Engine is certified for both test cycles and operates with identical settings.
Verification of the CH4 and/or N2O emission values
To confirm that the emission values have been established in accordance with these Guidelines, the CH4 and/or N2O file should be submitted to the Administration for verification. Upon satisfactory review, a Statement of emission values for CH4 and/or N2O will be issued.
Engine load monitoring
The Guidelines adopted by resolution MEPC.415(84) specify a method for engine load monitoring (ELM) to derive emission factors reflecting the actual operation of the engine.
ELM may be used in place of pre-defined weighting factors specified in section 3.2 of the NTC 2008. It should be combined with verified CH4 and/or N2O emission measurements to derive representative emission values. The Guidelines provide a procedure for calculating the methane slip value for the entire year.
Emission values at different engine loads
Emission values at different engine loads are derived from the verified CH4 and/or N2O emission values established in accordance with resolution MEPC.415(84), as follows:
- Between measured points, the emission values should be calculated by linear interpolation.
- For engine loads below the lowest measured load point, the emission value at the 10% load point (or the lowest gas-fuel mode point) should be used.
- For loads above the highest measured load point, the emission value at the highest measured load point should be used.
Load monitoring
Engine load should be continuously monitored and may be:
- Measured from torque flange, shaft strain gauge or calculated from the generator output; or
- Estimated indirectly (e.g. cylinder pressure or injection duration), subject to Administration approval.
Measurement equipment should meet the requirements of appendix IV (tables 3 and 4) of the NTC 2008 and be calibrated per manufacturer's recommendations.
Engine load should be continuously monitored and recorded at a frequency of not less than 0.0035 Hz. Load should be averaged over 30-minute intervals, or another period approved by the Administration. Zero-load and liquid-fuel-only operation must be excluded from the averaging.
The monitoring method, data-collection procedure, annual load monitoring data and corresponding emission calculations should be approved by the Administration.
Calculations
Two options are provided in the ELM Guidelines for calculating annual emission values (g/g fuel):
Option A: If a fuel flow measurement device is installed (example shown below).
Option B: If no fuel flow measurement device is installed (example shown below).
Engine Load Monitoring (ELM) is emerging as a practical solution because it allows measured methane emissions to be weighted against the vessel's actual operating profile rather than relying solely on standard test-cycle assumptions. This is particularly relevant for vessels with variable load profiles, such as LNG carriers and cruise vessels.
Approval and verification
The monitoring method, data collection procedures and annual load monitoring data and corresponding emission calculations should be approved by the Administration or an RO.
Continuous emission monitoring system
The Guidelines adopted by resolution MEPC.416(84) establish a framework for the onboard measurement and mass-based quantification of CH4 and/or N2O emissions using continuous emissions monitoring systems (CEMS).
The Guidelines also include:
- Requirements for data recording and storage.
- Documentation requirements, including:
- An Administration approved CEMS file detailing the system and the operating procedures to be followed together with related requirements covering the functionality of the CEMS and verification that the data generated is reliable.
- A CEMS record book detailing maintenance, servicing and fault resolution activities.
A framework for the certification of a CEMS by the Administration, including initial and annual survey requirements.
.png){kind=icon}
.png){kind=icon}
.png){kind=icon}
