IPCC Fifth Assessment Report (AR5) — The Legacy Anchor Still in Force
The IPCC Fifth Assessment Report (AR5), published across 2013 and 2014, is the report that most corporate greenhouse gas accounting was built on — and, for a large share of mandatory reporting, it is still the basis in force today. It is tempting to treat AR5 as simply “the old report” now that AR6 (2021) exists. That framing is wrong and produces real reporting errors. AR5 is not retired. It is the mandated Global Warming Potential basis for national inventory reporting under the Paris Agreement, for the US EPA Greenhouse Gas Reporting Program, and for the UK DEFRA conversion factors that thousands of organisations feed into their inventories every year.
This page is the definitive operational reference for AR5 as a GHG-accounting standard. It documents exactly which GWP values AR5 published, how they differ from AR4 before it and AR6 after it, the specific — and frequently mishandled — treatment of methane, where AR5 remains mandatory versus where it has been superseded, and how to migrate an inventory from AR5 to AR6 without breaking series comparability. The companion current-science reference is the AR6 standards page; the conceptual primer is the GWP glossary entry. Where a figure on this page is one that legacy calculators commonly get wrong, that error is called out explicitly.
1. What Is the IPCC AR5?
The Intergovernmental Panel on Climate Change (IPCC) is the United Nations body that synthesises the peer-reviewed climate science literature into consensus assessments approved by its member governments. It does not conduct original research; it assesses and integrates existing studies. Assessment Reports are the vehicle for that consensus, issued roughly every six to eight years.
The Fifth Assessment Report was the fifth in the series, produced in stages: Working Group I (The Physical Science Basis) was finalised in September 2013; Working Group II (Impacts, Adaptation and Vulnerability) and Working Group III (Mitigation of Climate Change) followed in March and April 2014; and the integrating Synthesis Report was published in November 2014. For GHG accounting the operative volume is Working Group I, and within it Chapter 8 (Anthropogenic and Natural Radiative Forcing), which publishes the Global Warming Potential values in Table 8.7 and the fuller Table 8.A.1 of its appendix.
AR5 mattered to corporate accounting for one dominant reason: for the better part of a decade it was the “latest IPCC assessment” that every reporting framework pointed to. When the GHG Protocol, CDP, ISO 14064-1, and national inventory guidelines said “use the latest IPCC GWP values,” they meant AR5. That is why AR5 is embedded so deeply in existing inventories, base years, historical target baselines, and — critically — in emission-factor libraries that have not yet been rebuilt on AR6.
Provider directory
Need help keeping this up all year?
See who does this work. Every listing names the standards it works to, and paid placements are labelled. Including Anthesis Group and Pangolin Associates.
Browse 6 carbon accounting & inventory providers →Do this work? A listing is US$390 a year. Get listed →
2. Why AR5 Still Matters — The Legacy-Anchor Map
The single most important thing to understand about AR5 in 2026 is that its status is split. For corporate disclosure, AR6 has largely taken over: IFRS S2 and ESRS call for the latest IPCC values, and the GHG Protocol and CDP recommend them. But for several major mandatory regimes, AR5 is not merely permitted — it is the specified basis. An organisation that assumes “everyone has moved to AR6” will misreport under those regimes.
AR6 is the current science. AR5 is the current law in a significant part of the compliance landscape. The two statements are both true simultaneously. Any organisation subject to both voluntary disclosure (CSRD/IFRS S2 → AR6) and a mandatory AR5 regime (UNFCCC/EPA/DEFRA) is running two GWP bases at once and must disclose which basis applies to which figure. This is the defining practical fact of AR5 today.
Under the Enhanced Transparency Framework, Parties report national inventories using AR5 GWP-100 values (without climate-carbon feedbacks). The decision applies to inventory submissions under the Paris Agreement transparency rules.
The EPA Greenhouse Gas Reporting Program (40 CFR Part 98, Table A-1) specifies AR5 100-year GWP values, adopted in a 2024 rulemaking that moved the program from AR4 to AR5. US-regulated facility reporting runs on AR5.
The DEFRA/BEIS conversion-factor workbook pre-aggregates CO2e using AR5 GWP-100. Any organisation using DEFRA factors — the default for UK SECR — inherits AR5 for those line items, even inside an otherwise AR6 inventory.
Net-zero targets set on AR5 inventories carry AR5 CO2e. Moving those inventories to AR6 means restating the base year too, and the AR5 figures remain the reference point for the change.
Regulatory adoption changes over time. Always confirm the current mandated GWP version with the specific regulator for the reporting year in question; the rows above reflect the position as verified in mid-2026 and link through to the framework-by-framework detail in §7.
3. Key AR5 Outputs for Corporate Carbon Accounting
AR5 produced a very large scientific output. For a corporate inventory, four outputs are directly operational.
3.1 Global Warming Potential Values
The GWP values published in WGI Chapter 8 are the most consequential output. They convert each greenhouse gas mass into a CO2-equivalent figure over a chosen time horizon. AR5 publishes GWP at 20-year and 100-year horizons, both with and without climate-carbon cycle feedbacks. The GWP-100 values without feedbacks are the operative basis for corporate and national inventory reporting under AR5, consistent with how AR4 values were used before them. The full table, including AR4 and AR6 comparison columns, is in §4.
3.2 Atmospheric Lifetimes
AR5 publishes the atmospheric lifetime (or perturbation lifetime) of each gas — the mechanical reason GWP varies so strongly with time horizon. Methane’s roughly 12-year lifetime is why its GWP-20 (84 in AR5) is far higher than its GWP-100 (28): most of the pulse has decayed well before year 100. Long-lived gases such as SF6 (a lifetime measured in millennia) show almost no sensitivity to horizon choice within the 20–100 year window.
3.3 Radiative Forcing and Climate Sensitivity
AR5 updated radiative-efficiency estimates for the greenhouse gases and introduced the widely cited “likely” range for equilibrium climate sensitivity of 1.5 °C to 4.5 °C. For most reporters this is background, but the defensible point in verification is that AR5 GWP values derive from physical-science measurement, not policy negotiation.
3.4 Cumulative Carbon and the RCP Scenarios
AR5 formalised the near-linear relationship between cumulative CO2 emissions and warming, and introduced the Representative Concentration Pathways (RCP2.6, RCP4.5, RCP6.0, RCP8.5) that underpinned a generation of corporate scenario analysis and transition-risk disclosure before AR6’s SSP scenarios replaced them.
4. The AR5 GWP-100 Values — Verified Reference Table
The table below is the operative AR5 GWP-100 set (without climate-carbon feedbacks) as reproduced in the GHG Protocol’s AR5 reference and AR5 WGI Table 8.A.1, with AR4 and AR6 columns for comparison. These are the values to cite in an AR5-basis methodology statement. Rows where legacy calculators most commonly carry the wrong number are flagged.
| Gas | Formula | AR4 (2007) | AR5 (2013) | AR6 (2021) | AR5 lifetime | Typical source |
|---|---|---|---|---|---|---|
| Carbon dioxide | CO2 | 1 | 1 | 1 | Variable (multi-century) | Combustion, calcination |
| Methane | CH4 | 25 | 28 | 27.0* | 12.4 yr | Combustion slip, fugitives, landfill, livestock |
| Nitrous oxide | N2O | 298 | 265 | 273 | 121 yr | Fertiliser, wastewater, combustion |
| Sulphur hexafluoride | SF6 | 22,800 | 23,500 | 24,300 | 1,000 yr | Electrical switchgear, semiconductors |
| Nitrogen trifluoride | NF3 | 17,200 | 16,100 | 17,400 | 500 yr | Semiconductor manufacturing |
| HFC-134a | CH2FCF3 | 1,430 | 1,300 | 1,530 | 13.4 yr | Mobile AC, light commercial refrigeration |
| HFC-32 | CH2F2 | 675 | 677 | 771 | 5.2 yr | Lower-GWP AC refrigerant |
| HFC-23 | CHF3 | 14,800 | 12,400 | 14,600 | 222 yr | HCFC-22 production by-product |
| HFC-125 | CHF2CF3 | 3,500 | 3,170 | 3,740 | 28.2 yr | Refrigerant blends (R-410A, R-404A) |
| HFC-143a | CH3CF3 | 4,470 | 4,800 | 5,810 | 47.1 yr | Refrigerant blends |
| PFC-14 (CF4) | CF4 | 7,390 | 6,630 | 7,380 | 50,000 yr | Aluminium smelting, semiconductors |
| PFC-116 (C2F6) | C2F6 | 12,200 | 11,100 | 12,400 | 10,000 yr | Semiconductors |
All AR5 values are GWP-100 without climate-carbon feedbacks (AR5 WGI Table 8.A.1), the operative convention for corporate and national reporting. *AR6 introduced a fossil (29.8) / biogenic (27.0) methane split that AR5 does not use in its headline table; the AR6 figure shown is biogenic for like-for-like comparison. R-410A and R-404A are blends whose CO2e is computed from component GWPs, not a single tabulated value.
HFC-134a is 1,300 under AR5, not 1,430. The value 1,430 is the AR4 figure. A calculator that labels itself “AR5” but uses 1,430 for R-134a is silently mixing report generations — a material error for any refrigerant-heavy inventory. Methane is 28 under AR5, not 25. The value 25 is AR4. Both errors point the same direction: an “AR5” implementation quietly running on two-generation-old AR4 numbers. See §6 and §9.
5. AR4 → AR5 → AR6 — Reading the Direction of Change
GWP values do not move in one direction across report generations, and this trips up practitioners who assume “newer report = higher number.” The changes reflect improved radiative-efficiency and lifetime measurements, and they can go either way. Two AR5 movements are counter-intuitive and worth internalising.
N2O dropped from AR4 to AR5, then partially recovered in AR6. An agriculture- or wastewater-heavy inventory that moved AR4→AR5 saw its N2O CO2e fall by ~11% for the same physical emissions.
Methane rose AR4→AR5 (mainly from including indirect ozone and stratospheric-water effects), then split into fossil/biogenic values in AR6. The oft-quoted “25×” is two generations stale.
Refined radiative efficiencies pulled several well-mixed HFCs down in AR5 before AR6 raised them again. This non-monotonic path is exactly why “AR5 = 1,430” is a giveaway of an AR4 mislabel.
Because AR5 lowered some gases and raised others relative to AR4, an AR4→AR5 base-year restatement could move a company’s reported total either up or down depending on gas mix. The same is true AR5→AR6. Never assume the sign of the change; recalculate the base year gas-by-gas and disclose the net effect.
6. The AR5 Methane Special Case — 25, 28, 30, and 34
No single figure causes more AR5 confusion than the methane GWP-100, because four different numbers circulate and each is “correct” for a different question. Getting this right is the clearest signal of a rigorous AR5 implementation.
| Value | What it is | When it applies |
|---|---|---|
| 25 | Not AR5. This is the AR4 (2007) GWP-100. | Only for AR4-basis reporting. Using it on an “AR5” inventory is an error. |
| 28 | The AR5 operative value. GWP-100, without climate-carbon feedbacks, AR4-consistent methodology. | The default for corporate and national AR5 reporting. This is the number to cite. |
| 30 | AR5 methane of fossil origin, with the CO2-oxidation product accounted separately (+2 vs the 28 base). | A refinement AR5 describes but does not place in its headline GWP table; most frameworks used 28 regardless of origin. |
| 34 | AR5 GWP-100 with climate-carbon feedbacks (Table 8.7). | Published for scientific completeness; not the corporate-reporting basis. |
It is a common misconception — including in some secondary references — that AR5 published a clean fossil/biogenic methane split like AR6’s 29.8 / 27.0. It did not. AR5’s headline, framework-adopted value is a single 28 for methane regardless of origin. The fossil “30” exists in AR5 only as a separately described oxidation adjustment that the standard GHG Protocol AR5 table explicitly does not show. The genuine, table-level fossil/biogenic split arrives with AR6. Treating AR5 as if it split methane the AR6 way is a factual error, not a simplification.
The practical rule for an AR5-basis inventory: use 28 for methane across sources, disclose “AR5 GWP-100, without climate-carbon feedbacks,” and do not import the AR6 fossil/biogenic logic into an AR5 dataset. If the reporting obligation is AR6, switch the whole methane treatment — value and the fossil/biogenic distinction — together. See the methane glossary entry for the full source-classification framework under each report.
7. Framework-by-Framework — Where AR5 Stands Today
The adoption picture is genuinely mixed, and the mix is the point: a single multinational can face AR5 and AR6 obligations simultaneously. The table records where each framework sits as verified in mid-2026, with the direction of travel noted.
| Framework | GWP basis | AR5 status | Notes |
|---|---|---|---|
| UNFCCC / Paris Agreement | AR5 (mandated) | AR5 required | National inventories under the Enhanced Transparency Framework use AR5 GWP-100 without feedbacks. Distinct from corporate frameworks; governs national inventory comparability. |
| US EPA GHGRP | AR5 | AR5 required | 40 CFR Part 98, Table A-1 specifies AR5 GWP-100, following the rulemaking that moved the program from AR4 to AR5. Applies to US-regulated facilities. |
| UK DEFRA / SECR | AR5 (embedded in factors) | AR5 retained | DEFRA conversion factors pre-aggregate CO2e on AR5. Users inherit AR5 for those line items. DEFRA AR5/AR6 treatment → |
| GHG Protocol Corporate Standard | AR5 (text) → AR6 (guidance) | Transitioning | The original Standard text references AR5; the Secretariat’s August 2024 GWP guidance confirms AR6 for new inventories. Full details → |
| CDP Climate Questionnaire | AR6 recommended | Either, named | CDP’s 2024 guidance recommends AR6 and lists AR5 among the GWP references a company can select. Name the one you used. |
| CSRD / ESRS E1 | AR6 (de facto) | Superseded | ESRS E1 references “the latest IPCC AR.” AR6 is the operative basis; assurance providers flag AR5 usage as an observation. |
| SBTi Corporate Net-Zero | Not stated | Check with validator | The SBTi criteria and Net-Zero Standard we read name no GWP set. Their inventories follow the GHG Protocol, which recommends the most recent IPCC values. |
| ISO 14064-1 (2018) | Latest IPCC | Version-neutral | Requires “the latest IPCC GWP values” (now AR6) with mandatory disclosure of the version and time horizon applied. Permits AR5 where a downstream regime mandates it, if disclosed. |
For a new voluntary inventory, use AR6. But do not delete AR5 from your factor library: if you report to UNFCCC-linked national schemes, the US EPA GHGRP, or you consume DEFRA factors, you are still on AR5 for those figures. The defensible pattern for a mixed-regime organisation is dual calculation — AR5 where mandated, AR6 for voluntary disclosure — with the basis stated explicitly on each output.
8. Migrating AR5 → AR6 Without Breaking Comparability
The most common serious error in the AR5→AR6 transition is not the arithmetic — it is applying the new basis prospectively while leaving history on the old basis, which manufactures an artificial “reduction” or “increase” that verifiers reject. The GHG Protocol consistency principle requires that a material methodology change be applied across the whole series.
8.1 Worked Example — What the Restatement Actually Moves
Consider a site emitting 1,000 t of methane and 50 t of N2O in the base year. On AR5, that is (1,000 × 28) + (50 × 265) = 28,000 + 13,250 = 41,250 tCO2e. On AR6 (biogenic methane 27.0, N2O 273): (1,000 × 27.0) + (50 × 273) = 27,000 + 13,650 = 40,650 tCO2e — a modest net fall, because AR6’s biogenic methane value is lower than AR5’s single 28 and the N2O rise does not offset it. Swap the methane to fossil (AR6 29.8) and the same physical emissions become (1,000 × 29.8) + 13,650 = 43,450 tCO2e. The physical activity never changed; only the characterisation did. This is exactly the point that must be communicated in any year-on-year comparison crossing the transition.
9. Common AR5 Reporting Errors
10. GreenCalculus Implementation — AR5 as a Live Parallel Basis
Because AR5 remains mandatory in several regimes, GreenCalculus stores AR5 values as a live, maintained basis alongside AR6 rather than archiving them. Every AR5 figure is traceable to its source table.
IPCC Fifth Assessment Report, Working Group I — The Physical Science Basis (2013), Chapter 8 (Anthropogenic and Natural Radiative Forcing), Table 8.7 and Table 8.A.1. Cross-checked against the GHG Protocol AR5 reference set and the US EPA GHG Inventory Annex 6 AR4/AR5/AR6 crosswalk. ipcc.ch/report/ar5/wg1/
Each gas stores both ar5_100 and ar6_100 fields. The v2025.2 release corrected CH4_fossil.ar5_100 and CH4_biogenic.ar5_100 from an erroneous 25 (the AR4 value) to the correct 28, and confirmed HFC134a.ar5_100 = 1300 against the AR4 1,430. Values are GWP-100 without climate-carbon feedbacks.
Calculators expose an explicit AR5/AR6 basis toggle. The engine reads window.gcMasterBrain.gwp.<gas>.ar5_100 or .ar6_100 per the selected basis — never a fixed value — so mandated-AR5 outputs and voluntary-AR6 outputs are produced from the same source of truth.
Every result carries a gwp_basis field stating the report generation, horizon, and feedback treatment — e.g. "IPCC AR5 GWP-100, no feedbacks (CH4=28, N2O=265, SF6=23,500)" — plus the MasterBrain version. This makes a dual-basis inventory self-documenting for assurance.
The AR5 and AR6 datasets are exposed at /wp-json/greencalculus/v1/gwp-values, returning both bases, GWP-20 where published, atmospheric lifetimes, and AR4/AR5/AR6 deltas with full source citation.
Every AR5 value used across GreenCalculus is independently verifiable against AR5 WGI Table 8.A.1 and the AR4/AR5/AR6 crosswalk. The complete machine-readable dataset — both bases, all horizons, lifetimes, and full deltas — is published at the GWP values reference table (which carries AR5 columns alongside AR6).
11. Audit and Assurance Implications
For ISO 14064-3 verification and assurance under ISAE 3410, an AR5-basis inventory must satisfy documentation requirements that frequently surface as findings — several of them specific to AR5’s mixed-mandate status.
| Requirement | What auditors look for | Common finding if absent |
|---|---|---|
| GWP basis disclosure | Explicit “AR5 GWP-100, without climate-carbon feedbacks” statement | Basis ambiguous; cannot confirm 28 vs 25 vs 34 |
| Source table citation | “AR5 WGI Table 8.A.1” specifically, not generic “IPCC AR5” | Citation insufficient for assurance file |
| Report-generation integrity | Methane = 28 and HFC-134a = 1,300 (AR5), not the AR4 25 / 1,430 | Undetected AR4 values inside an “AR5” inventory |
| Mixed-regime documentation | Where AR5 (mandated) and AR6 (voluntary) coexist, each output labelled | Undisclosed mixed basis across disclosures |
| DEFRA inheritance | Where DEFRA (AR5) factors sit inside an AR6 inventory, the split disclosed | Internal methodology inconsistency unflagged |
| Base-year consistency | Whole series on one basis; transition step-change explained | Comparability failure (high severity) |
12. Frequently Asked Questions
Both, depending on the regime. AR6 (2021) is the current IPCC science: IFRS S2 and ESRS call for the latest IPCC values, and the GHG Protocol and CDP recommend them. But AR5 remains the mandated GWP basis for national inventory reporting under the UNFCCC/Paris Agreement, for the US EPA Greenhouse Gas Reporting Program (40 CFR Part 98), and it is embedded in the UK DEFRA conversion factors. For a new voluntary inventory, use AR6; for those mandatory regimes, AR5 is still in force. Organisations subject to both should run dual bases and disclose which applies to each figure.
The operative AR5 GWP-100 for methane is 28, without climate-carbon feedbacks. This is the value to use and cite for AR5-basis reporting. Three other numbers circulate: 25 is the older AR4 value (not AR5); 34 is the AR5 value with climate-carbon feedbacks (for scientific completeness, not corporate reporting); and 30 is AR5 fossil-origin methane with the CO2-oxidation product accounted separately, which AR5 describes but does not place in its headline GWP table. For standard corporate and national reporting under AR5, use 28.
Because one of them is mislabelled. The AR5 GWP-100 for HFC-134a is 1,300. The value 1,430 is the earlier AR4 figure. Several HFCs actually fell from AR4 to AR5 as radiative efficiencies were refined, so an “AR5” table showing 1,430 for R-134a is almost certainly an unconverted AR4 table. For refrigerant-heavy inventories this is a material error, since fugitive refrigerant emissions are often a large share of Scope 1.
Not in the way AR6 does. AR5’s headline, framework-adopted methane GWP is a single value of 28 regardless of source. AR5 does describe a separate fossil-origin adjustment (yielding 30) to account for the CO2 produced when fossil methane oxidises, but this is not shown in the standard AR5 GWP table and most frameworks applied 28 across the board. The clean table-level fossil (29.8) / biogenic (27.0) split is an AR6 feature. Do not apply AR6 fossil/biogenic logic to an AR5 dataset.
N2O fell from 298 (AR4) to 265 (AR5) as its radiative efficiency and lifetime estimates were refined; AR6 later moved it back up to 273. GWP values do not only rise across report generations — they track improved measurement in either direction. For agriculture, wastewater, and combustion inventories with material N2O, an AR4→AR5 restatement lowered reported CO2e for the same physical emissions by roughly 11% on that gas. This is why the direction of a restatement must always be checked gas-by-gas rather than assumed.
Treat it as a material methodology change and restate the whole series, not just the current year. Recalculate the base year and every intervening year on AR6, keep the time horizon constant (GWP-100 to GWP-100), re-base any net-zero target set on AR5, and disclose the GWP version and horizon for each year. Crucially, communicate that any step-change at the transition reflects improved measurement precision, not a change in physical operations. And keep AR5 available in parallel for any UNFCCC, EPA GHGRP, or DEFRA-based figures that remain on AR5. The full checklist is in §8.
Under AR5 GWP-100 (without feedbacks): sulphur hexafluoride (SF6) is 23,500; nitrogen trifluoride (NF3) is 16,100; PFC-14 (CF4) is 6,630; and HFC-23 is 12,400. These matter most for utilities and electrical infrastructure (SF6), semiconductor manufacturing (NF3, CF4), and aluminium smelting (CF4). Note that AR6 raised most of these — SF6 to 24,300, NF3 to 17,400, CF4 to 7,380 — so a facility switching basis will see these line items rise. The full set is in the reference table.
Cite AR5 Working Group I, Chapter 8, Table 8.A.1 for the full GWP-100 set without climate-carbon feedbacks (Table 8.7 is the shorter in-chapter table that also shows the with-feedback values). Specify the time horizon (GWP-100) and the feedback treatment (without feedbacks) explicitly. A generic “IPCC AR5” citation is a routine finding in ISO 14064-3 assurance because it does not let a verifier confirm which of the several AR5 value sets you used.
Run a calculation on the AR5 basis
The Scope 1 Combustion Calculator supports an explicit AR5/AR6 basis toggle and declares the GWP basis on every result.
Related GreenCalculus References
Current-science counterpart: IPCC AR6 (2021) — the report that superseded AR5 for voluntary disclosure.
Foundational standards: GHG Protocol Corporate Standard · Scope 3 Standard · ISO 14064-1 · CSRD ESRS E1
Factor sources that depend on AR5: UK DEFRA (AR5) · US EPA GHGRP · US EPA eGRID
Glossary: Global Warming Potential · Methane (CH4)
Apply this standard: Scope 1 Combustion · Scope 2 Electricity · Refrigerant Leak · All calculators