AR5 vs AR6 GWPs: What Changes in Your Methane Number...

A coal mine reports 50,000 tonnes of fugitive methane this year. Under NGER, that's 1.4 million tCO2-e. Under AASB S2, the same number is 1.49 million tCO2-e. The methane didn't change. The framework did.

This is the GWP gap. It now sits inside every Scope 1 number produced by an Australian methane-heavy reporter, and most reporting teams still don't have a clean answer when their auditor asks which figure goes where.

Why two numbers exist

Global Warming Potential is a ratio. It expresses how much heat a gas traps over 100 years compared to the same mass of CO2. The IPCC updates these ratios with each Assessment Report as the underlying atmospheric science improves.

AR5 was published in 2013. AR6 followed in 2021. AR7 is scheduled for somewhere between 2029 and 2031. Each cycle refines two things: the radiative forcing measurements themselves, and the treatment of climate-carbon feedback (the secondary warming caused by a gas weakening natural carbon sinks).

AR6 was the first headline assessment to bake climate-carbon feedback into the published GWP100 values. It also split methane into two categories because the chemistry differs.

Gas	AR5 GWP100 (2013)	AR6 GWP100 (2021)
CO2	1	1
CH4 (fossil)	28	29.8
CH4 (non-fossil / biogenic)	28	27.0
N2O	265	273

The split matters. Fossil methane (from coal seams, oil and gas operations) eventually oxidises to fossil CO2, adding a small extra forcing kick. Biogenic methane (from livestock, landfill, wastewater) oxidises to CO2 that was already part of the short-term biological carbon cycle, so it doesn't get the same uplift.

Net result for an Australian reporter: AR6 raises the warming impact attributed to fossil methane by about 6.4% versus AR5, and lowers biogenic methane by about 3.6%.

What NGER uses today

The National Greenhouse and Energy Reporting (Measurement) Determination 2008 is the legal instrument that defines how every NGER reporter calculates emissions. Its tables embed AR5 GWP values, and the Clean Energy Regulator has not signalled an immediate move to AR6.

There's a reason for the lag. NGER underpins the Safeguard Mechanism, ACCU baseline calculations, the Renewable Energy Target, and a chain of contractual arrangements built on those numbers. Switching GWP sets mid-stream would recalibrate baselines, change ACCU surrender obligations, and force every reporter to restate historical data. The CER's public position is alignment with international convention when the timing makes sense, not before.

The 2025 edition of the National Greenhouse Accounts Factors workbook from DCCEEW continues to publish all factors on the AR5 basis. If you're filing an NGER report on 31 October 2026, you're using AR5. Full stop.

What AASB S2 expects

AASB S2 paragraph B19 directs reporters to use GWPs from "the latest version" of the IPCC assessment available. As of 2026, that's AR6. The standard does not give you the option of staying on AR5 simply because NGER does.

The AASB recognised the tension this creates. AASB S2025-1, issued 15 December 2025, granted limited jurisdictional relief: an NGER reporter can use AR5 GWPs in their AASB S2 disclosure for the portion of their emissions that's covered by NGER, without recalculation. The relief is narrow. It applies only to NGER-covered portions, only to the AASB S2 disclosure (not other reporting), and does not extend to operations outside NGER's scope.

This is helpful, but it doesn't make the problem disappear. Most reporters have NGER-covered facilities and non-NGER operations sitting in the same consolidated entity. International subsidiaries, recently acquired sites that haven't been brought into NGER yet, and joint ventures where you're not the operator all fall outside the relief. For those portions, AR6 applies in your AASB S2 number.

The reconciliation problem in numbers

Take a hypothetical integrated gas producer with 100,000 tonnes of fugitive methane across its Australian operations.

NGER submission (AR5, GWP 28): 2.80 million tCO2-e Scope 1
AASB S2 with relief applied (AR5 for NGER-covered, AR6 for the rest): somewhere between 2.80 and 2.98 million tCO2-e, depending on the NGER-covered share
AASB S2 if relief not applied or for non-NGER operations (AR6 fossil, GWP 29.8): 2.98 million tCO2-e Scope 1

The gap between the NGER and the strictest AASB S2 figure is 180,000 tCO2-e. That's a real number that will appear in your financial-grade climate disclosure, will not match your NGER submission, and will draw an auditor question every single time.

The methodology note has to do the heavy lifting. Reporters need to explicitly state which GWP set was applied to which scope and source, why, and how the totals reconcile across frameworks. The sentence "we use IPCC Global Warming Potentials" is no longer sufficient under ASIC RG 280 scrutiny.

The Safeguard Mechanism wrinkle

The Safeguard Mechanism baselines for the 219 covered facilities are calculated on NGER methodology. AR5 is locked in for compliance.

A facility with a baseline of 500,000 tCO2-e that emits 510,000 tCO2-e under NGER is 10,000 tonnes over. At an ACCU spot price somewhere in the $35-$40 band, that's roughly $350,000-$400,000 to cover. The decline rate is currently 4.9% annually, so the headroom shrinks each year.

Now run the same facility on AR6 fossil methane. If 80% of the Scope 1 number is fugitive coal-seam gas, the AR6 figure is materially higher. The same 510,000 tCO2-e under AR5 becomes around 532,000 under AR6, putting the AASB S2 disclosure 32,000 tonnes over what is functionally the same "baseline" if measured on the newer science.

The risk: investors and stakeholders reading the AASB S2 disclosure see one performance number. The CER, holding the Safeguard ledger, sees another. The two diverge predictably, and the reporting team is the one that has to explain it. Strategy decisions about ACCU surrender, on-site abatement investment, and methane-capture project priority should be driven by the NGER number for compliance and the AR6 number for the AASB S2 narrative. Mixing them creates confusion, particularly around target-setting language under AASB S2 paragraphs 33-36.

Where this hurts most

The AR5/AR6 question is academic for an electricity-heavy office portfolio. For methane-dominated Scope 1 inventories, it's a multi-million-dollar disclosure decision.

Coal mining. Fugitive emissions from underground operations (especially longwall faces) and ventilation air methane are the biggest single component of Scope 1 for the major coal producers. A 25% gain in implied warming impact on a 5 million tCO2-e fugitive number is a 1.25 million tCO2-e gap. Method 1 phase-out for open-cut mines from 1 July 2026 is already forcing site-specific measurement; the GWP question rides on top.

Oil and gas. Well, gathering, processing, and transmission losses across the supply chain. Even with leak detection and repair programs, fugitive intensities of 0.2-0.6% of throughput are typical. AR6 raises the reported climate impact by the same 6.4% that applies to all fossil methane.

Landfills and wastewater treatment. Anaerobic decomposition produces biogenic methane. AR6 actually reduces the reported number by 3.6% — the rare case where the upgrade works in the reporter's favour. Landfill operators using the FOD model under NGER Chapter 5 should run parallel calculations to quantify the gap.

Agriculture. Enteric fermentation from cattle and sheep is biogenic methane (AR6 lower). Manure management is mixed. Nitrous oxide from fertiliser and manure is AR6 higher (273 vs 265). The net direction depends on the herd-to-fertiliser ratio.

Mixed operations. An integrated dairy with boiler fugitives (fossil) and enteric fermentation (biogenic) needs to segregate the two methane streams to apply AR6 properly. That's a data architecture decision, not a calculation tweak.

The fossil vs biogenic methane split, in practice

AR6 distinguishes between methane that ends up adding "new" fossil CO2 to the atmosphere when it oxidises, and methane that's part of the short-term biological cycle. The split matters for any reporter with both sources.

Consider an integrated meat processor. Onsite gas-fired boilers leak methane (fossil, GWP 29.8). The animals upstream produce enteric methane that the company also reports under its expanded Scope 3 or Category 1 purchased goods (biogenic, GWP 27). The wastewater lagoon generates biogenic methane (GWP 27). The refrigeration plant's natural-gas-driven absorption chillers leak fossil methane (GWP 29.8).

You can't apply a blended factor. The defensible approach is gas-level inventory: capture CH4 in kilograms with its source classified as fossil or biogenic, then apply the appropriate GWP at the reporting layer. Systems that pre-aggregate to CO2-e at ingestion can't do this without re-extracting the source data.

What goes in your methodology note

The methodology disclosure required under AASB S2 and assurance review now has to state, explicitly:

The GWP set applied (AR5 or AR6, with reference to the IPCC report)
Whether the AR6 fossil/biogenic split was applied, and the classification logic used
Treatment of climate-carbon feedback (included in AR6 headline values; not in AR5)
Where AR5 was used under AASB S2025-1 jurisdictional relief, and the scope of that election
The reconciliation between the NGER submission and the AASB S2 number, with the GWP-set difference broken out

A defensible note will also include a sensitivity calculation: what the Scope 1 total would be under the alternative GWP set. Auditors are asking for this number, and the absence of it now reads as either ignorance or evasion.

The N2O question

The same logic applies to nitrous oxide, just with smaller absolute numbers. AR5 puts N2O at 265; AR6 at 273. A 3% uplift.

For agriculture (manure, synthetic fertiliser application), wastewater treatment, and combustion processes generating nitrous oxide as a side-product, the AR6 number is higher. A 1,000 tonne N2O inventory shifts from 265,000 tCO2-e under AR5 to 273,000 tCO2-e under AR6 — an 8,000 tonne difference.

Smaller than the methane gap, but it accumulates across categories and shows up in the consolidated total.

What this means for the data system

A reporting platform that handles both NGER and AASB S2 disclosure on the same source data has to do three things:

Store emissions at the gas level (CH4 fossil, CH4 biogenic, N2O, CO2 separately), not pre-aggregated to CO2-e
Apply different GWP sets in parallel for different reports off the same underlying gas inventory
Maintain an audit trail showing which GWP set was applied to which calculation, with version pinning so historical reports remain reproducible

Carbonly supports both AR5 and AR6 GWP sets running in parallel on the same source data, with per-gas storage, so a single ingestion of an emissions docket can feed an AR5-based NGER submission and an AR6-based AASB S2 disclosure without re-extraction. The reconciliation note between the two falls out of the data model rather than requiring a separate workbook.

The point isn't that you need a system to do this. The point is that the GWP question is a data architecture problem before it's a methodology problem. Pre-aggregated CO2-e is a one-way calculation. Reversing it to apply a different GWP set requires the source gas quantities, and many spreadsheet inventories don't retain them.

The forward look

AR7 is due 2029-2031. The trajectory is for headline GWP values to continue refining as the atmospheric science improves. The pressure on regulators to align with the latest assessment is now coming from two directions: international financial reporting frameworks (ISSB, EFRAG) and national-level methane-reduction policy (Australia signed the Glasgow Methane Pledge in 2021 and is a participant in the Global Methane Initiative).

The CER is not going to switch to AR6 quietly. When it does happen, it will involve a baseline restatement consultation, NGER methodology amendments, and likely a transition window. Reporters who've built their inventories at the gas level, with parallel GWP applicability, will handle that transition on a flag flip. Reporters who've pre-aggregated to CO2-e under AR5 will be re-extracting source data from invoice archives.

The right time to fix the data architecture is before the regulatory change forces it. Same kilogram of methane, different number in two reports, today. That's the signal.

For methodology questions on dual NGER and AASB S2 disclosure, email hello@carbonly.ai or join the waitlist.

AR5 vs AR6 GWPs: What Changes in Your Methane Number When AASB S2 Demands the Upgrade