AR5 GWP100 in maritime emissions accounting

The IPCC AR5 GWP100 values used in every maritime regulation currently in force are CO2 = 1, CH4 = 28, and N2O = 265, without climate-carbon feedback. These come from Table 8.7 of Working Group I Chapter 8, published in 2013. IMO MEPC.391(81), adopted 22 March 2024, and EU Regulation 2023/1805 (FuelEU Maritime) Annex II both specify these AR5 values explicitly.

Background: Global Warming Potential as a regulatory metric

The Global Warming Potential index was created in 1990 to give policy a single scalar for comparing carbon dioxide with shorter-lived gases. The IPCC First Assessment Report introduced GWP20, GWP100, and GWP500 to capture the time-dependence of radiative forcing. Methane breaks down in the atmosphere within roughly twelve years; nitrous oxide persists for about 109 years; carbon dioxide remains effective for centuries once emitted. A single number can’t describe all three lifetimes, so the IPCC settled on a family of horizons and left the choice to regulators.

GWP100 became the default in the UNFCCC reporting framework and propagated through every domestic emissions trading system that followed. The Kyoto Protocol locked GWP100 in for first-commitment-period reporting, and the Paris Agreement Enhanced Transparency Framework continued the practice. Maritime regulation inherited the choice through several routes. MEPC anchored its first lifecycle guidance to the GWP100 values current at the time of drafting, and the EU built FuelEU Maritime on the same foundation to keep dual-regime reporting consistent.

Three reasons drove the GWP100 choice over other horizons. The horizon roughly matches the residence time of CO2 in the atmosphere, giving the metric a physical anchor. Century-scale policy targets such as net-zero by 2050 align naturally with a 100-year forcing integral. And GWP100 produces conservative numbers for short-lived species, which simplifies negotiation between fossil-fuel exporters and climate-vulnerable states. The conservatism cuts both ways: GWP100 understates the near-term impact of methane, and that understatement is the single most important critique of the metric in the climate-science literature.

The maritime sector amplifies the methane question because the LNG dual-fuel engine family slips unburned methane through the cylinder during low-load operation. The slip rate at MEPC.391(81) default values runs at 3.1 percent of fuel input on a low-pressure Otto-cycle engine, and that slip is converted to CO2-equivalent using GWP100 = 28. Under GWP20, the same physical slip multiplies by three, transforming LNG from a transitional fuel into something only marginally better than VLSFO.

GWP100 definition: 100-year integrated radiative forcing

GWP100 measures the time-integrated radiative forcing produced by a unit mass pulse emission of a gas, expressed as a ratio against the same time-integrated forcing from a unit mass pulse of CO2:

The numerator integrates the radiative efficiency $a_g$ of gas $g$ multiplied by its decaying atmospheric concentration $C_g(t)$ over 100 years following a pulse emission. The denominator does the same for CO2. The ratio is dimensionless and represents how much warming one kilogram of gas $g$ delivers across a century compared with one kilogram of CO2.

Three physical quantities determine each GWP value: radiative efficiency (how much warming per molecule per square metre of atmosphere), atmospheric lifetime (how long the molecule persists), and indirect effects (chemical reactions that alter the lifetime of other GHGs). Methane has high radiative efficiency, short lifetime, and significant indirect effects through stratospheric water vapour and tropospheric ozone formation. Nitrous oxide has moderate efficiency, long lifetime (about 109 years), and indirect effects on stratospheric ozone chemistry.

The IPCC AR5 numerical values come from the Bern carbon cycle model coupled with MAGICC simple-climate-model emulators of full atmosphere-ocean general circulation simulations. The 100-year integration window was held fixed across AR4, AR5, and AR6, but the underlying physics estimates evolved as observations improved and feedback parameterizations were refined.

For maritime accounting, the practical consequence is that emission inventories report methane and nitrous oxide in CO2-equivalent units using a fixed multiplier supplied by the regulator. The shipping company doesn’t run the Bern model; it multiplies measured kilograms of CH4 by 28 and measured kilograms of N2O by 265 to get CO2eq under MEPC.391(81). The GFI attained calculator implements exactly this multiplication for IMO compliance; the FuelEU GHG intensity calculator does the same for EU compliance.

AR5 GWP100 values: CO2 = 1, CH4 = 28, N2O = 265

The AR5 GWP100 table for maritime-relevant species, from Chapter 8.7 Table 8.7:

The MEPC.391(81) Annex 1 default emission factors use the no-feedback values: CH4 = 28 and N2O = 265. This is the authoritative AR5 figure. The number 273 for N2O is the AR6 value from Table 7.15 of IPCC AR6 Working Group I, Chapter 7 (2021). It does not appear anywhere in AR5 Table 8.7.

A common error in regulatory commentary is attributing N2O = 273 to AR5. It isn’t an AR5 figure. The AR5 N2O value dropped from the AR4 value of 298 to 265, reflecting improved radiative efficiency estimates and refined atmospheric chemistry modeling for the 109-year lifetime of N2O. AR6 then pushed it back up to 273 with further refinement.

The CO2 value of 1 is by definition: GWP is normalized to carbon dioxide, so the metric anchors against the gas being abated. CO2eq totals reduce to physical CO2 mass plus the GWP-weighted contributions of the other species, giving the regulator a single number to assign quotas against.

For shipping operations the dominant non-CO2 contributor is methane on LNG-fuelled vessels and nitrous oxide on ammonia-fuelled vessels. CH4 dominates in the LNG case because slip rates of 1.7 to 3.1 percent of fuel input far exceed the trace N2O emissions from any combustion pathway. The LNG WtW article tabulates the resulting CO2eq penalty under both horizons. The ammonia WtW article shows that N2O leakage from ammonia engines, even at very low absolute mass rates, creates large CO2eq penalties because of the N2O multiplier.

With-feedback vs without-feedback values

AR5 produced two columns of GWP values: one with climate-carbon feedbacks included and one without. The distinction matters for both the science and the regulatory text.

Climate-carbon feedback captures the secondary warming produced when an initial GHG pulse warms the climate enough to alter natural carbon sinks. A warmer ocean absorbs less CO2; warmer permafrost releases methane; drier forests transition from sink to source. AR5 estimated the feedback contribution as a roughly 30 percent uplift to non-CO2 GWP values: CH4 climbs from 28 to 36 and N2O climbs from 265 to 298. The CO2 reference itself stays at 1 because the feedback is already embedded in the carbon cycle response that produced the original CO2 pathway.

Regulators have used the without-feedback values. MEPC.391(81) Annex 1, FuelEU Annex II, and the EU ETS amending directive all reference the without-feedback column. The reasoning is that climate-carbon feedback estimates carry larger uncertainty than the direct radiative forcing calculations, and incorporating feedback into a regulatory metric blurs the line between a fuel emission factor and a climate-system property. UNFCCC inventory reporting under the Enhanced Transparency Framework also uses the without-feedback column for the 2024 onward cycle.

The practical consequence for shipping is measurable. A 3 percent methane slip rate on LNG produces a CO2eq penalty calculated as $0.03 \times 28 = 0.84$ kg CO2eq per kg fuel under the without-feedback rule. Under the with-feedback rule the same slip produces $0.03 \times 36 = 1.08$ kg CO2eq per kg fuel, a 29 percent increase that would push some Otto-cycle LNG pathways above VLSFO equivalent. The choice of column therefore matters as much as the choice of horizon for fleet-level economics.

GWP20 alternative and methane sensitivity

GWP20 truncates the integration window to 20 years instead of 100, raising methane’s CO2-equivalent factor sharply because the integral captures most of methane’s atmospheric lifetime before any of CO2’s century-long forcing has accumulated. The AR5 GWP20 values are CH4 = 84 (no feedback), and N2O = 264. Methane’s factor approximately triples; nitrous oxide barely moves because its lifetime exceeds the integration window in both cases.

The argument for GWP20 rests on the urgency of the next two decades. Climate science establishes that limiting warming to 1.5 degrees Celsius requires aggressive near-term cuts in short-lived climate pollutants, and a 100-year horizon dilutes methane’s near-term forcing in regulatory accounting. A 1 percent methane slip on a 100,000 tonne fuel-year operation produces 1,000 tonnes CH4. Under GWP100 that converts to 28,000 tonnes CO2eq. Under GWP20 it converts to 84,000 tonnes CO2eq.

The argument against GWP20 rests on policy stability and metric continuity. UNFCCC, the GHG Protocol, ISO 14064, ISO 14067, and every domestic emissions trading system already align on GWP100. Switching maritime regulation to GWP20 would create cross-sector inconsistency and complicate the conversion between vessel-level reports and corporate Scope 3 disclosure. Investors using TCFD frameworks would find LNG carrier emissions reporting incompatible with parent-company GHG accounting unless the TCFD framework also moved to GWP20.

The maritime climate-policy community has produced compromise proposals. A dual-reporting requirement (GWP100 for compliance, GWP20 for disclosure) appeared in early drafts of the IMO Net-Zero Framework but was dropped to keep the regulation tractable. The 2028 review cycle is expected to revisit the question, and the EU FuelEU Maritime review clause permits an upgrade to AR6 GWP values without statutory amendment.

IMO MEPC.391(81): GWP100 in the 2024 LCA Guidelines

MEPC.391(81), adopted on 22 March 2024 at MEPC 81, is the 2024 Guidelines on Life Cycle GHG Intensity of Marine Fuels. It replaced the earlier LCA guidelines that had been adopted at MEPC 80 (July 2023) and introduced updated default emission factors, revised appendix templates for well-to-tank and tank-to-wake submissions, and an expanded scope of fuel pathways. Annex 1 of MEPC.391(81) specifies that all non-CO2 GHG emissions shall be converted to CO2-equivalent using AR5 100-year Global Warming Potential values without climate-carbon feedback: CO2 = 1, CH4 = 28, N2O = 265.

The drafting history behind the GWP metric choice is instructive. The IMO Intersessional Working Group on Reduction of GHG Emissions from Ships debated the GWP horizon at length during 2022 and 2023. Submissions from Tuvalu, the Marshall Islands, the Solomon Islands, and Vanuatu argued for GWP20 to penalize LNG more sharply. Submissions from Japan, Norway, Singapore, and LNG-producer interests argued for GWP100 on grounds of cross-sector consistency. The European Commission supported GWP100 because FuelEU Maritime had already been drafted on that basis and a divergence would have created EU-IMO double-counting problems.

The committee adopted GWP100 at MEPC 80 and confirmed the specific AR5 values when MEPC.391(81) was finalized at MEPC 81. The IMO Net-Zero Framework draft amendments approved at MEPC 83 in April 2025 carry the same metric forward into a new Chapter 5 of MARPOL Annex VI. Their formal adoption was adjourned at the October 2025 extraordinary session (MEPC/ES.2) and is set to be reconsidered in 2026, with implementation targeted for 2027. Once adopted, Chapter 5 would lock AR5 GWP100 into treaty-level law subject to tacit-acceptance entry-into-force procedures.

The political compromise behind the GWP100 choice also shaped the methane-slip default factors. The 3.1 percent slip default for low-pressure Otto-cycle engines in MEPC.391(81) Annex 1 is more conservative than vendor-claimed performance, which partially offsets the lenience of the GWP100 metric and prevents LNG from accumulating an unrealistically clean lifecycle score.

EU FuelEU Maritime Annex II: AR5 GWP100 alignment

Regulation (EU) 2023/1805 (FuelEU Maritime) Annex II tabulates default well-to-wake CO2-equivalent emission factors for every fuel category. The footnote to the Annex II table specifies that the CO2eq calculation uses AR5 GWP100 without climate-carbon feedback: CH4 = 28, N2O = 265. The footnote explicitly excludes AR6 values until a future implementing act updates the table.

The implementing regulation issued under Article 23 of FuelEU Maritime sets out the verification rules. Verifiers shall apply AR5 GWP100 unless the operator submits a certified well-to-wake actual value that has itself used AR5 GWP100. Operators using AR6 values in their actual-value calculation must convert to AR5 equivalents before submission. The conversion is straightforward for the methane component: the CH4 factor moves only from 28 (AR5) to 29.8 (AR6), less than 7 percent. For N2O the shift is from 265 (AR5) to 273 (AR6), about 3 percent. Neither gap is large enough to affect compliance decisions materially, but audit-trail consistency requires using the specified metric.

The alignment between MEPC.391(81) and FuelEU Annex II is deliberate and tightly coordinated. The European Commission DG Climate Action and the IMO Secretariat ran parallel drafting tracks during 2022 to 2024, with explicit cross-references in the EU FuelEU recital language to the then-pending MEPC guidelines. A vessel reporting CO2eq under the FuelEU intensity formula and CO2eq under the IMO GFS attained intensity calculation produces the same numerator, with the two regimes differing only in their targets, compliance boundary, and penalty structure.

The EU ETS-FuelEU double regulation article catalogues every place where the two regimes intersect. GWP100 alignment is a foundational compatibility, and any divergence on the metric would explode compliance complexity for EU-trading vessels.

EU ETS: AR5 GWP100 consistency

The EU Emissions Trading System Directive 2003/87/EC, as extended to maritime by Directive (EU) 2023/959, calculates surrenderable allowances using AR5 GWP100 for CH4 and N2O. The maritime extension covers CO2 from 2024, CH4 and N2O from 2026, and the same metric applies across all three gases.

Three things follow from the EU ETS use of AR5 GWP100. First, the verifier-accepted MRV emissions report under Regulation (EU) 2015/757 (THETIS-MRV) supplies CO2eq values that the EU ETS administering authority uses without re-conversion. Second, allowance prices in the Emissions Union Transaction Log clear at a CO2eq tonne basis, so the carbon market itself prices methane and nitrous oxide at their GWP100-weighted contribution. Third, the EU ETS link to FuelEU compliance through Article 25a of the Directive uses CO2eq totals on the same metric basis, preventing arbitrage between the two regimes.

The European Commission has flagged the GWP horizon as a candidate for review in the 2028 EU ETS revision package. Any move to GWP20 or to AR6 GWP100 would propagate simultaneously through EU ETS, FuelEU Maritime, the EU effort-sharing decision, and the EU industrial-emissions framework. The cross-cutting nature of the metric makes unilateral upward revision politically difficult, which favors metric stability over scientific currency.

Practical impact: LNG methane slip case study

The LNG well-to-wake calculation is the clearest illustration of how the GWP100 choice cascades into regulatory scoring. The MEPC.391(81) Annex 1 default factors for low-pressure Otto-cycle dual-fuel engines are CO2 = 2.750 kg per kg fuel (combustion), CH4 slip = 0.0310 kg per kg fuel, and N2O = 0.000110 kg per kg fuel.

CO2-equivalent at AR5 GWP100 (no feedback), using CH4 = 28 and N2O = 265:

CO2-equivalent at AR5 GWP20, using CH4 = 84 and N2O = 264:

The GWP20 calculation produces a tank-to-wake total roughly 48 percent higher than the GWP100 calculation. When the upstream well-to-tank component (LNG production, liquefaction, transport) is added, the GWP100 LNG WtW intensity sits around 76 to 92 gCO2eq/MJ depending on supply chain origin; the GWP20 figure rises to 110 to 130 gCO2eq/MJ, which is at or above parity with VLSFO at 91 to 95 gCO2eq/MJ. LNG’s perceived CO2eq advantage over conventional marine fuels disappears under GWP20.

The high-pressure Diesel-cycle dual-fuel engine has a methane slip default of 0.0019 kg per kg fuel, roughly 16 times lower than the Otto-cycle figure. The Diesel-cycle WtW intensity is 76 gCO2eq/MJ at GWP100 and 79 gCO2eq/MJ at GWP20. The horizon choice barely matters for this engine because slip is small. The GWP100 vs GWP20 controversy is therefore really an Otto-cycle controversy: the Otto-cycle technology family is what GWP100 effectively subsidizes in the regulatory accounting.

The bio-LNG article shows the same arithmetic for biomethane pathways. Biomethane combustion produces the same physical methane slip as fossil LNG, so the GWP100 vs GWP20 divergence applies equally to renewable LNG.

Pacific Island States advocacy for GWP20

The Alliance of Small Island States (AOSIS), the Pacific Islands Forum, and the individual delegations of Tuvalu, the Marshall Islands, the Solomon Islands, Vanuatu, Kiribati, and Fiji have advocated GWP20 at IMO MEPC since the 2018 Initial GHG Strategy debate. The position is anchored in the climate-vulnerable framing: 1.5-degree pathways require near-term methane cuts, GWP100 understates near-term methane forcing, and therefore GWP100 can’t deliver a 1.5-aligned maritime regime.

The Pacific argument extends beyond the metric itself to the choice of transition fuel. Pacific delegations have repeatedly characterized LNG as “not a credible transition fuel” precisely because the methane-slip burden under realistic GWP20 accounting eliminates LNG’s intensity advantage over conventional fuels. A GWP20 regulatory regime would push the maritime sector toward methanol, ammonia, or direct electrification rather than LNG, reducing the lock-in risk of multi-decade LNG bunkering infrastructure investment.

The advocacy hasn’t prevailed at MEPC, but it has shaped the architecture of the IMO Net-Zero Framework in three ways. First, the GFS reduction trajectory accelerates from 2030 onward, which compensates partially for GWP100 lenience by tightening the absolute target. Second, the methane-slip default factors in Annex 1 are set high to limit Otto-cycle LNG’s calculated advantage. Third, the framework includes an explicit five-yearly review clause that allows GWP horizon revision without a full treaty amendment.

The Pacific position has stronger purchase in the EU FuelEU Maritime review than at IMO. The European Parliament rapporteur for the FuelEU 2028 review has signaled openness to a dual-horizon reporting requirement, retaining GWP100 for compliance and adding GWP20 for transparency. Whether that survives Council negotiation is open.

Climate science split: SR1.5, AR6 commentary

The climate science community isn’t unanimous on GWP100. The IPCC Special Report on 1.5 degrees Celsius (SR1.5), published in 2018, devoted a full chapter to short-lived climate pollutants and noted that the GWP100 metric “may underestimate the climate effects of short-lived species over policy-relevant near-term horizons.” The report stopped short of recommending a horizon change but flagged GWP* (a modified metric proposed by Cain and colleagues) as a possible alternative for short-lived gases.

GWP* (pronounced “GWP-star”) expresses methane emissions as the change in emission rate rather than the absolute emission rate, multiplied by a constant calibrated against CO2 forcing. The result is that a constant methane emission rate produces zero additional warming under GWP* (because the steady-state atmospheric burden has stabilized), while an increasing rate produces sharply increasing warming. GWP* gives a more accurate temperature outcome over multi-decadal horizons but is administratively complex because it requires baseline emission rates and rate-of-change calculations rather than simple multiplication.

AR6 Working Group I Chapter 7 expanded the discussion and revised the AR5 GWP100 values. CH4 (fossil) moves from 28 to 29.8; N2O moves from 265 to 273. The revision reflects updated radiative-efficiency observations, refined climate-carbon feedback parameterization, and a new fossil-vs-biogenic distinction for methane not present in AR5. AR6 also presents GWP20, GWP100, and GWP500 values plus GTP (Global Temperature Potential) values, leaving the policy choice to regulators.

The split inside the climate community runs between physical scientists (who tend to favor GWP* or GTP for accurate temperature outcomes) and integrated-assessment modelers (who tend to favor GWP100 for tractable cost-benefit analysis). Maritime regulators have aligned with the second camp.

AR6 revised values and the N2O distinction

AR6 Table 7.15 supplies updated GWP100 values that maritime regulators track for eventual transition:

The fossil/non-fossil distinction is new in AR6. Fossil methane oxidation produces CO2 that is itself a fossil increment, while biogenic methane oxidation produces CO2 that was already part of the active carbon cycle. Including this secondary CO2 in the methane GWP raises the fossil methane figure from 28 to 29.8. AR6 reports a composite value of 27.9 for the unspecified case, which often appears in policy commentary as “the AR6 GWP100 for methane.”

The AR6 N2O value of 273 does not “coincide” with MEPC.391(81), as some commentary claims. MEPC.391(81) specifies N2O = 265, the correct AR5 value. A future regulatory shift to AR6 would tighten N2O scoring by about 3 percent, which has a modest impact on ammonia pathways (where N2O dominates) and a more significant impact on fossil LNG pathways (where the fossil methane figure rises by about 6 percent). Neither change is large enough to alter compliance categories at current trajectory targets, but the cumulative effect on fuel lifecycle scoring is non-trivial at scale.

The IMO Secretariat has flagged AR6 adoption as a candidate item for the 2028 review of MEPC.391(81). Adoption would require an MEPC resolution amending Annex 1 and isn’t expected to require treaty-level amendment since the lifecycle guidelines are administrative rather than legal instruments. The EU FuelEU review process operates under a similar mechanism, allowing a Commission implementing act to update the Annex II values without parliament-and-council-level amendment.

IMO Net-Zero Framework Chapter 5: GWP100 in treaty law

The IMO Net-Zero Framework introduces a new Chapter 5 into MARPOL Annex VI, with associated regulations governing the GHG Fuel Standard, the Net-Zero Fund, and the lifecycle accounting boundary. The Chapter 5 regulations require every applicable vessel to calculate an attained GHG fuel intensity using the AR5 GWP100 metric specified in MEPC.391(81) Annex 1.

The framework’s targets are expressed in CO2eq per megajoule of fuel energy. The reference value of 93.3 gCO2eq/MJ (the baseline MARPOL Annex VI fleet intensity) is itself a GWP100-weighted figure, and every reduction milestone (4 percent in 2028, 17 percent in 2030, 65 percent in 2040, net zero by 2050) is anchored to that GWP100 baseline. Switching the metric mid-decade would invalidate the baseline and require a wholesale reconstruction of the trajectory.

The Chapter 5 Annex includes a five-yearly review clause that the IMO Secretariat will use to revisit metric choice, default emission factors, and trajectory milestones. The first review cycle aligns with the 2028 stocktake under the Paris Agreement; the second aligns with the 2033 stocktake. Industry observers expect the 2028 review to retain AR5 GWP100 and the 2033 review to consider AR6 GWP100.

The IMO Net-Zero Framework article and the GHG Fuel Standard methodology article cover the trajectory and the compliance surface. The metric discussion in those articles defers to this article for the underlying GWP definitions.

Post-2025 review pressure: AR6 adoption path

Pressure to update the GWP values from AR5 to AR6 comes from several quarters. The UNFCCC Secretariat’s Common Reporting Tables under the Enhanced Transparency Framework explicitly use AR5 GWP100 for the 2024 inventory cycle but flag AR6 as a candidate for the 2030 cycle. The European Commission’s 2024 Climate Communication signaled an intent to revisit AR6 across all EU climate instruments by 2028. The IMO Secretariat has flagged the 2028 review of MEPC.391(81) as the natural moment for an AR5-to-AR6 transition.

Three factors will drive the actual decision. Cross-sector consistency requires maritime, aviation, energy, and industrial sectors to move together on metric updates. Computational tractability requires that any new metric be compatible with existing inventory methodologies. Political acceptability requires that the metric change doesn’t produce sudden scoring shifts that disrupt long-term contractual arrangements (such as multi-year LNG bunkering supply agreements priced against specific GWP assumptions).

A coordinated AR5-to-AR6 transition across IMO, EU, and UNFCCC frameworks could happen at the 2030 to 2032 boundary. A unilateral IMO move ahead of EU and UNFCCC is unlikely because of the cross-counting risk for EU-trading vessels. A unilateral EU move ahead of IMO is possible through a Commission implementing act but is politically constrained by the same cross-counting concern.

Industry should plan for AR5 GWP100 to remain the regulatory metric through at least the 2028 GFS milestone. The 2030 milestone is a natural transition point if all major frameworks coordinate; the 2033 to 2035 window is the more conservative expectation.

TCFD and ISSB: GWP20 in climate-finance disclosure

The Task Force on Climate-related Financial Disclosures (TCFD) recommendations and their successor International Sustainability Standards Board (ISSB) IFRS S2 standard require disclosure of GHG emissions in CO2-equivalent terms but allow reporters discretion on the GWP horizon. Some climate-finance frameworks recommend GWP20 reporting alongside GWP100 to capture near-term physical risk that GWP100 understates.

The Climate Bonds Initiative’s Climate Bonds Standard for shipping, the Glasgow Financial Alliance for Net Zero (GFANZ) net-zero alignment criteria, and the Principles for Responsible Investment carbon footprint guidance all permit GWP20 supplementary disclosure. Several major shipping investors, including Poseidon Principles signatory banks, have begun requesting GWP20 figures as part of due-diligence questionnaires, particularly for LNG-fuelled fleets where the metric choice materially affects the climate risk profile.

The implication for shipping operators is that compliance reporting (under MEPC.391(81) and FuelEU) will use GWP100, while investor and lender reporting may increasingly use a dual-horizon format. The GWP-conversion arithmetic is straightforward: the operator computes the methane mass once and applies the relevant factor depending on the report. A vessel reporting 100 tonnes of methane slip per year reports 2,800 tonnes CO2eq under GWP100 and 8,400 tonnes CO2eq under GWP20.

The reputational risk of disclosing a GWP20 figure that triples the regulatory CO2eq is real. LNG carrier operators have argued that GWP20 disclosure misrepresents the regulatory situation by mixing a non-binding metric with a binding regime. The counter-argument is that climate-finance disclosure is about going beyond compliance to show the true underlying risk profile.

Commercial implications: LNG advantage under GWP100 vs GWP20

The commercial significance of the GWP horizon choice can be summarized in a comparison table for LNG dual-fuel Otto-cycle propulsion against VLSFO:

Under GWP100, LNG retains a 9 percent advantage over VLSFO that supports LNG’s framing as a transitional bridge fuel. Under GWP20, LNG is 29 percent worse than VLSFO and the bridge-fuel narrative collapses. The capital-investment implications are large: LNG bunkering infrastructure, dual-fuel engine installations, and Type C cryogenic tank investments rest on the regulatory metric choice as much as on technical performance.

The high-pressure Diesel-cycle LNG pathway is far less sensitive to the horizon choice because methane slip is much smaller. A GWP20 transition would re-rank Diesel-cycle LNG as the only credible LNG technology, with Otto-cycle LNG effectively deprecated. The ME-GI engine (high-pressure Diesel cycle) and the X-DF2.0 engine (improved low-pressure Otto cycle) would diverge in market position, with ME-GI gaining share.

Methanol, ammonia, and biomethane pathways are less sensitive to the horizon choice than LNG because their non-CO2 emission profiles differ. The methanol WtW article shows methanol with negligible CH4 slip and modest N2O emissions. The ammonia WtW article shows ammonia with negligible CH4 and significant N2O. A horizon change leaves methanol roughly unchanged and only modestly alters ammonia, since N2O’s GWP moves from 265 (AR5) to 264 (GWP20, AR5) under the 20-year horizon.

ISO 14067 and GHG Protocol alignment

ISO 14067:2018 specifies the methodology for product carbon footprint quantification and reporting. The standard defaults to GWP100 from the most recent IPCC assessment report unless a specific sectoral reporting requirement says otherwise. Marine fuels reported under ISO 14067 therefore default to AR6 GWP100 (CH4 fossil = 29.8, N2O = 273) for general supply-chain accounting, while regulatory submissions to MEPC.391(81) and FuelEU use AR5 GWP100 (CH4 = 28, N2O = 265).

The discrepancy between AR5 and AR6 is small enough that ISO 14067 reports and regulatory reports can be reconciled through a simple multiplier. Operators producing both report types should document the metric used and apply a consistent boundary, preferably matching the regulatory boundary (well-to-wake) for both. The bio-LNG article discusses ISO 14067 alignment for renewable methane pathways where the GWP horizon and the biogenic-vs-fossil methane distinction interact.

The GHG Protocol Corporate Standard, which underpins most Scope 1/2/3 corporate emissions reporting, has tracked the IPCC assessment cycle with a one-cycle lag. AR4 GWP100 was the GHG Protocol default until 2017; AR5 GWP100 became the default in 2018; AR6 GWP100 will become the default once the GHG Protocol Methane Standard finalization lands. Maritime operators will need to maintain triple metric capability: AR5 for IMO and EU, AR6 for ISO 14067 and GHG Protocol, and GWP20 for TCFD supplementary disclosure.

Common errors in GWP100 application

The most damaging error is applying N2O = 273 to AR5-governed submissions. That value is the AR6 figure. MEPC.391(81) and FuelEU Annex II both specify N2O = 265. Submitting 273 overstates the N2O CO2eq contribution by about 3 percent. It won’t materially affect compliance outcomes for most vessels (N2O contributes a small fraction of total CO2eq on conventional fuels), but it creates an audit-trail inconsistency that verifiers are required to flag.

The second common error is applying with-feedback values where the regulator specifies without-feedback. Using CH4 = 36 (with feedback) instead of 28 (without) overstates methane CO2eq by 29 percent and will produce a compliance score well outside the verified range. The third is applying AR4 values (CH4 = 25, N2O = 298) where AR5 is required, which understates methane CO2eq by 11 percent and overstates N2O CO2eq substantially.

The verifier-side check under FuelEU implementing rules and the IMO Document of Compliance verification under the GFS regime both screen for GWP100 consistency. Operators submitting actual values are required to disclose the GWP source explicitly in the methodology statement. An unexplained deviation from AR5 without-feedback values in a MEPC.391(81) or FuelEU submission is grounds for rejection.

The CO2-equivalent formula

The GWP-weighted CO2-equivalent total for a fuel-year is:

where $m_g$ is the mass of gas $g$ emitted in kilograms, and the GWP100 factors are 1, 28, and 265 respectively under MEPC.391(81) Annex 1 and FuelEU Annex II conventions.

For a fuel-specific intensity in gCO2eq per MJ, the per-MJ form is:

where $EF_g$ is the species-specific emission factor in kg per kg fuel, $LHV$ is the lower heating value in MJ per kg fuel, and the factor of 1,000 converts from kg/MJ to g/MJ.

A worked example for a vessel burning 30,000 tonnes of LNG per year on a low-pressure Otto-cycle dual-fuel engine, using MEPC.391(81) Annex 1 defaults:

• CO2: $30\,000 \times 2.750 = 82\,500$ tonnes
• CH4: $30\,000 \times 0.0310 = 930$ tonnes
• N2O: $30\,000 \times 0.000110 = 3.3$ tonnes

CO2-equivalent at AR5 GWP100 (CH4 = 28, N2O = 265):

Same vessel under GWP20 (CH4 = 84, N2O = 264):

The GWP20 total is 47.6 percent higher. The methane component drives almost the entire difference; the nitrous oxide contribution is near-identical because N2O’s GWP changes only slightly between the two horizons and its mass is small.

The LNG methane slip calculator implements the GWP100 and GWP20 comparison for any engine type and slip rate. The N2O emissions calculator handles the nitrous oxide component separately for ammonia and conventional fuel pathways.

See also

• MEPC.391(81) lifecycle guidelines
• Marine GHG Fuel Standard methodology
• FuelEU intensity formula breakdown
• Per-fuel WtW LNG (Otto vs Diesel cycle)
• Per-fuel WtW ammonia grades
• Per-fuel WtW bio-LNG
• EU ETS and FuelEU double regulation
• IMO GHG strategy
• IMO Net-Zero Framework
• GFI attained calculator
• FuelEU GHG intensity calculator
• LNG methane slip GWP calculator
• N2O emissions calculator
• GFI compliance calculator

References

The IPCC Fifth Assessment Report Working Group I Chapter 8.7 supplies the AR5 GWP100 source values (CO2 = 1, CH4 = 28, N2O = 265 without climate-carbon feedback) and the radiative-forcing methodology. The IPCC Sixth Assessment Report Working Group I Chapter 7 supplies the AR6 revisions (CH4 fossil = 29.8, N2O = 273) and the GWP* commentary. IMO MEPC.391(81) Annex 1, adopted 22 March 2024 at MEPC 81, and Regulation (EU) 2023/1805 Annex II carry the regulatory implementation. Directive 2003/87/EC and Directive (EU) 2023/959 carry the EU ETS treatment. The IPCC Special Report on 1.5 degrees (SR1.5) supplies the scientific commentary on horizon choice. The UNFCCC Common Reporting Tables, ISO 14067:2018, and the GHG Protocol Corporate Standard supply cross-sector context. Citations are listed in the frontmatter.