IPCC Sixth Assessment Report (AR6) — The Definitive Reference

1. What Is the IPCC AR6?

The Intergovernmental Panel on Climate Change (IPCC) is the United Nations body responsible for synthesising scientific knowledge on climate change. The IPCC does not conduct original research; it assesses and integrates the existing peer-reviewed literature, reaching consensus positions among 195 member governments. Assessment Reports are the mechanism through which that consensus is published.

The Sixth Assessment Report is the most recent in the series, produced in cycles across 2021 and 2022. Working Group I alone — the volume that publishes the Global Warming Potential values used in inventory work — involved 234 lead and contributing authors from 66 countries, drawing on approximately 14,000 cited studies. The level of scientific authority embodied in AR6 is one that no single institution can replicate: its conclusions represent the collective judgment of the global climate science community, expressed through a consensus process that is itself part of the document’s methodological credibility.

For corporate GHG accounting, AR6 matters for one specific reason: it is the source document for the GWP values that underpin every CO₂-equivalent calculation. When the GHG Protocol Corporate Standard, CDP, CSRD ESRS E1, ISO 14064-1, or SBTi reference “the latest IPCC assessment values,” they mean the values published in AR6 Working Group I, Chapter 7 Supplementary Material, Table 7.SM.7 (page 7-SM-47). No other source carries equivalent authority for this purpose, and no other source can substitute for it in a defensible methodology disclosure.

2. The Three Working Groups — Mapped to Accounting Use Cases

AR6 is structured across three Working Groups, each covering a distinct domain of climate science. Knowing which Working Group covers which topic is essential for sourcing the correct scientific basis for any specific aspect of an inventory or disclosure.

3. Key AR6 Outputs for Corporate Carbon Accounting

AR6 produces a substantial volume of scientific output. For a corporate GHG inventory, four outputs are directly operational and are documented below.

3.1 Global Warming Potential Values

The GWP values published in WGI Table 7.SM.7 are the most directly consequential output of AR6 for corporate reporting. They define how many units of CO₂ warming effect are equivalent to one unit of any other greenhouse gas, integrated over a specified time horizon. Every tCO₂e figure in every inventory ultimately traces back to this table. AR6 publishes GWP at 20-year, 100-year, and (for selected gases) 500-year horizons, both with and without climate-carbon cycle feedbacks. The GWP-100 values without feedbacks are the operative basis for corporate reporting, consistent with the practice that prevailed under AR5 and with current GHG Protocol guidance. The full dataset, including all gases, all horizons, and AR5 comparison values, is in the AR6 GWP reference table.

3.2 Atmospheric Lifetimes

AR6 publishes the atmospheric lifetime of each greenhouse gas — the average time a molecule of the gas persists in the atmosphere before chemical removal or decay. These lifetimes are not simply scientific background; they are the mechanical explanation for why GWP varies so substantially with the choice of time horizon. Methane’s GWP-20 (80.8) is far higher than its GWP-100 (29.8) because its 11.8-year perturbation lifetime means most of the molecule has decayed before year 50. Carbon dioxide, with a lifetime of centuries, has the same GWP at every time horizon by definition (1 at all horizons). Nitrous oxide’s 109-year lifetime gives it nearly identical GWP-20 (273) and GWP-100 (273), which is why the choice of time horizon is far more consequential for methane-intensive operations than for nitrogen-intensive ones.

3.3 Radiative Forcing Updates

AR6 refined the radiative efficiency estimates for all greenhouse gases — the quantification of how much additional energy each gas traps per unit of atmospheric concentration. More precise radiative-forcing measurements were the primary scientific driver of the GWP changes between AR5 and AR6. For most corporate reporters this is background science. For organisations in regulated sectors where the specific scientific basis of GWP values may be challenged in third-party verification, the key defensible point is that GWP changes derive from improved physical-science measurement rather than from policy decisions. AR6 GWP values are the result of better science, not negotiated outputs.

3.4 Remaining Carbon Budget

AR6 WGI published updated estimates of the remaining carbon budget — the total cumulative future CO₂ emissions consistent with limiting warming to a specified level. As of the start of 2020, the remaining budget for a 50% probability of limiting warming to 1.5°C was approximately 500 GtCO₂; for a 67% probability, approximately 400 GtCO₂. These budget estimates are the scientific foundation on which the SBTi Corporate Net-Zero Standard constructs its target-setting methodology. The trajectories that define what a “science-based” 1.5°C-aligned reduction target means for any given company’s sector and base year emissions are derived directly from these AR6 budget figures combined with AR6 WGIII sectoral pathway analysis.

4. AR5 to AR6 — What Changed and What It Means

The shift from AR5 (2013) to AR6 (2021) changed GWP values for most non-CO₂ gases. These are not minor technical adjustments. For organisations with significant methane, N₂O, refrigerant, or F-gas exposure, the changes are material to reported totals and may require base-year recalculation under the GHG Protocol consistency principle. The magnitude and direction of each change is summarised below for the most material gases.

4.1 Full AR5 to AR6 Comparison

4.2 The Inventory Implication

Consider a manufacturer combusting 50 million m³ of natural gas annually. The CH₄ slip component of that combustion, at AR5 GWP-100 of 28, produces a specific CO₂e contribution. Recalculated at AR6 GWP-100 of 29.8, the same physical methane mass produces a CO₂e figure 6.4% higher with no change in actual operations. This is not an increase in emissions; it is a more precise measurement of the warming impact that was always occurring. For an organisation with a public net-zero target denominated in tCO₂e, the recalculation can materially affect the gap between base year and target trajectory.

4.3 Base Year Restatement

The GHG Protocol Corporate Standard requires base-year recalculation when methodology changes are material. A switch from AR5 to AR6 GWP values is unambiguously a methodology change. The correct treatment is to restate the base year emissions using AR6 values, preserving comparability across the reporting series. An inventory that uses AR5 for all historical years and AR6 from the current year forward — without restating the base year — produces an artificial apparent reduction (or apparent increase) that verifiers will flag as a comparability failure. Disclosure of the GWP basis applied to each year of the series is mandatory under ISO 14064-1 and ISO 14064-3 verification. See the GHG Protocol base year recalculation policy for the specific recalculation rules.

5. The AR6 → Calculator Pipeline

Understanding how AR6 values flow from the source document to a calculator output is essential for audit-trail documentation. The diagram below shows the complete chain from IPCC publication to tCO₂e result. Every node on the path is independently verifiable and links to the corresponding page or system.

6. Framework Adoption — Where Each Standard Stands

The transition from AR5 to AR6 is not occurring uniformly across reporting frameworks. Each major framework sits at a different point in the adoption cycle, and aligning correctly requires knowing exactly which version is in force for each disclosure obligation.

7. Methodological Nuances

7.1 Climate-Carbon Cycle Feedbacks — With and Without

AR6 introduced a refinement that most online GWP tables still do not document clearly: the explicit distinction between GWP values calculated with and without climate-carbon cycle feedbacks. When greenhouse gases warm the atmosphere, they trigger secondary effects — permafrost thaw releasing additional CO₂ and CH₄, ocean carbon uptake efficiency declining, vegetation responses shifting — that further amplify warming over time. These feedback loops are real, scientifically well-characterised in AR6, and quantitatively material.

Including the feedbacks produces slightly higher GWP values:

For corporate GHG reporting under the GHG Protocol, the GHG Protocol Secretariat’s August 2024 guidance, CSRD ESRS E1, CDP, SBTi, and most national inventory guidelines, the values without climate-carbon feedbacks are the operative basis. This preserves consistency with the AR5 reporting convention. The with-feedback values are published for scientific completeness and appear in some climate policy and research contexts but are not the basis for corporate inventory calculation. When a source quotes methane GWP-100 as 32.8, it is using the with-feedback value; when it quotes 29.8, it is the without-feedback value that applies to corporate reporting. Both are correct AR6 values answering different questions.

7.2 Why GWP-100 Is the Default Time Horizon

GWP is always defined over a specific time window: 20, 100, or 500 years. The choice of horizon is a policy decision, not a scientific one. GWP-100 became the global default because it was the time horizon adopted in the Kyoto Protocol negotiations of the 1990s, and every subsequent reporting framework was constructed on that foundation. GWP-100 is not scientifically superior to GWP-20; GWP-20 in fact provides a more accurate picture of near-term warming impacts for short-lived gases such as methane. But comparability across reporting systems requires a consistent default, and that default is 100 years.

For organisations with significant methane exposure — natural gas utilities, oil and gas producers, livestock operations, large landfill operators — the difference is consequential: methane GWP-20 is 80.8 versus GWP-100 of 29.8. An operation that reports at GWP-20 will show emissions roughly 2.7 times higher than the same operation reporting at GWP-100, with no physical change in operations. The time horizon must therefore always be disclosed alongside the GWP basis, and switching time horizons within a multi-year reporting series constitutes a methodology change requiring base-year restatement under the GHG Protocol consistency principle. The full time-horizon analysis, including GWP-20 use cases, is in the GWP glossary entry.

7.3 The Fossil / Biogenic Methane Split

AR6 introduced an important refinement absent from AR5: separate GWP values for fossil methane and biogenic methane. The physics of the molecule is identical, but the carbon balance differs. Fossil methane releases new CO₂ to the atmosphere when it oxidises (the CO₂ released has been geologically sequestered for millions of years); biogenic methane releases CO₂ that was recently in the atmospheric carbon cycle and would have returned there regardless. AR6 quantifies this difference:

• Fossil methane: GWP-100 = 29.8 (use for natural gas combustion slip, oil & gas fugitives, coal mine methane, fossil-derived industrial CH₄)
• Biogenic methane: GWP-100 = 27.9 (use for landfill gas, livestock enteric fermentation, manure management, rice cultivation, anaerobic digestion losses, wastewater)

The difference is small but creates a non-trivial methodology question for inventories with mixed sources. Practitioners should default to fossil GWP for combustion sources where carbon origin is unverified, and apply biogenic GWP only where the biological origin is documented and traceable. A natural gas combustion source is fossil regardless of whether the supply contains a biomethane blend — the supplier’s biomethane content informs the underlying CO₂ treatment (biogenic CO₂ reported separately, out of scope), not the methane slip GWP. See the methane glossary entry for the full source-classification framework.

7.4 IPCC Calibrated Uncertainty Language

IPCC uses a calibrated vocabulary for expressing scientific confidence. Every finding in AR6 carries a calibrated likelihood term tied to a specific probability range:

For the core GWP values (CO₂, CH₄, N₂O), AR6 assigns “very likely” to “extremely likely” confidence to the central estimates published in Table 7.SM.7. This means the published GWP-100 value is appropriate to use as-is in inventory calculations without uncertainty adjustment. For some longer-lived F-gases, where radiative-efficiency measurement is less well-constrained, the uncertainty range is wider. The practical implication for verifiers and assurance providers: if a company uses the central-estimate GWP values from AR6 Table 7.SM.7 and discloses that basis explicitly, the methodology is scientifically defensible. The IPCC central estimates are the universally accepted convention for inventory reporting; this is established consensus, not unresolved uncertainty.

8. AR6 Sectoral Context — Where Emissions Originate

AR6 WGIII published an updated breakdown of global GHG emissions by sector. These figures matter for corporate reporting in two ways: they identify which Scope 3 categories are most material for any given industry, and they provide the baseline against which science-based targets are calibrated.

Source: IPCC AR6 WGIII Summary for Policymakers (2022). Figures are approximate shares of total global GHG emissions including land use change. Sectoral boundaries differ from GHG Protocol scope definitions; the same physical emissions may map to different scopes depending on the reporting entity.

9. Common AR6 Reporting Errors

10. GreenCalculus Implementation — Provenance Chain

GreenCalculus is built on a principle of complete data provenance: every CO₂e figure produced by any calculator on this platform must be traceable to a named source document and a specific table reference. For GWP values, the chain is documented explicitly below and verifiable end-to-end.

11. Audit and Assurance Implications

For ISO 14064-3 verification, limited or reasonable assurance under ISAE 3410, and CSRD-mandated assurance, the AR6 implementation must satisfy several documentation requirements that frequently appear as findings.

12. Frequently Asked Questions

Related GreenCalculus References