Carbon Dioxide Equivalent

What CO₂e Means and Why It Exists

Greenhouse gases are not interchangeable. One kilogram of methane has approximately 30 times the 100-year warming impact of one kilogram of CO₂. One kilogram of nitrous oxide has 273 times the warming impact. One kilogram of sulphur hexafluoride (SF₆) has more than 25,000 times the warming impact. Adding raw mass figures across different gases produces a number that is mathematically meaningless — there is no shared denominator.

CO₂e (carbon dioxide equivalent) is the shared denominator. It expresses the mass of any greenhouse gas in terms of the mass of CO₂ that would cause the same warming effect over a chosen time horizon — almost always 100 years for corporate reporting. CO₂ itself is the reference gas, defined as having a Global Warming Potential of 1 at every horizon by convention. Every other gas is expressed as a multiple of CO₂.

CO₂e is the unit of every corporate inventory total, every GHG Protocol Scope 1, 2, and 3 line, every CSRD ESRS E1-6 gross emissions disclosure, every CDP submission, every SBTi baseline. A company that reports its emissions in tonnes of “CO₂” without converting other gases to their CO₂e equivalent is reporting a partial inventory, not a complete one.

The Seven Kyoto Basket Gases and Their AR6 GWP-100 Values

The GHG Protocol Corporate Standard requires corporate inventories to cover the seven greenhouse gases listed in the Kyoto Protocol’s Annex A, expressed in CO₂e using the most recent IPCC GWP-100 values. From the 2023 reporting cycle, that means IPCC AR6 values — adopted by CDP from its 2023 questionnaire and by SBTi in its Net-Zero Standard v1.1 (2023). The table below lists every gas covered, with current AR6 GWP-100 values rendered live from the GreenCalculus MasterBrain.

★ = dedicated GreenCalculus glossary entry available. Source: IPCC AR6 WGI, Table 7.SM.7 (2021). Stable until IPCC AR7 (~2028). See the complete AR6 dataset →

The Core Formula and How to Apply It

The conversion from any greenhouse gas mass to CO₂e is a single multiplication. Multiply the mass of the gas by its GWP-100 value to get CO₂e in the same mass unit. GWP-100 is dimensionless — kilograms in, kilograms out; tonnes in, tonnes out. The conversion does not change the unit of mass, only its meaning.

The worked examples below use AR6 GWP-100 values and represent fixed snapshots — the values shown are the values that applied at the time of publication and will not auto-update with future IPCC revisions. (For the always-current AR6 values, use the live reference table in Section 2.)

All GWP-100 values from IPCC AR6 WGI Table 7.SM.7. Worked example values hardcoded — see header comment for shortcode policy.

GWP Time Horizons — GWP-20, GWP-100, and GWP-500

GWP is defined over a chosen time horizon, because greenhouse gases have different atmospheric lifetimes. Methane breaks down within about 12 years; N₂O persists for 109 years; SF₆ lasts approximately 3,200 years; CO₂ has a complex multi-reservoir profile that effectively means parts of an emitted CO₂ pulse stay in the atmosphere indefinitely. Choosing a time horizon means choosing how long a window to integrate the warming effect over.

The corporate reporting default — and the only horizon currently accepted by GHG Protocol, CSRD, CDP, and SBTi — is GWP-100. GWP-20 is sometimes used in short-term climate action assessments because it amplifies the impact of short-lived climate pollutants (methane in particular). GWP-500 is used in scientific analysis for very persistent gases (SF₆, PFCs) that continue to accumulate warming over multi-century horizons. Neither GWP-20 nor GWP-500 is currently mandated for any major corporate disclosure framework.

GWP-20 and GWP-500 values hardcoded — not yet available as MasterBrain shortcode keys (see Editorial Standards §9d). Source: IPCC AR6 WGI Table 7.SM.7 and Table 7.15 (CH₄ with oxidation effect).

AR5 vs AR6 — Why the GWP Basis Matters

IPCC Assessment Reports periodically revise GWP values as the underlying climate science improves. The current standard is AR6 (Working Group I, 2021). It superseded AR5 (2014), which superseded AR4 (2007). Each revision changes specific gases by different amounts — the methane revision from AR5 to AR6 was one of the largest in the cycle, while CO₂ remains 1 by definition across every report.

The AR6 changes that matter most for corporate inventories are concentrated in three places: methane (fossil CH₄ rose from 28.0 to 29.8, a 6.4% increase), nitrous oxide (rose from 265 to 273, a 3.0% increase), and the fluorinated gases (most rose 5–14%). Two HFC blends — HFC-404A and HFC-410A — show no AR6 change because their underlying constituent values offset.

AR5 column hardcoded — historical record per Editorial Standards §9b. AR6 column rendered live from MasterBrain v2025.4. All values verified against IPCC AR6 WGI Table 7.SM.7.

CO₂e in Corporate GHG Inventories

Every line in a corporate GHG inventory ends in CO₂e. The arithmetic that gets it there differs by scope: Scope 1 typically calculates per-gas mass first and applies GWP-100 to each; Scope 2 uses pre-aggregated grid factors that have already done the GWP arithmetic; Scope 3 is a mix of both, depending on the category and the underlying dataset. Three contexts cover the operational reality.

Scope 1 — Direct emissions, per-gas arithmetic

Scope 1 covers direct emissions from sources owned or controlled by the reporting company — combustion of fuels in boilers and vehicles, fugitive releases of refrigerants and SF₆, process emissions from industrial chemistry. These typically arrive as per-gas mass figures (kg of CO₂, kg of CH₄, kg of N₂O, kg of HFC-410A, etc.), and each gas is multiplied by its AR6 GWP-100 individually to produce a CO₂e contribution. The contributions are summed to give the total Scope 1 CO₂e. The Scope 1 Combustion Calculator applies AR6 conversions across all fuel types and surfaces the per-gas breakdown in the audit trail.

Scope 2 — Purchased energy, pre-aggregated factors

Scope 2 covers emissions from purchased electricity, heat, steam, and cooling. Grid emission factors are published by national authorities (DEFRA for the UK, EPA eGRID for the US, IEA for international) as a single number per unit of energy consumed — for example, the DEFRA 2025 UK grid factor is published in kg CO₂e/kWh, with the CO₂, CH₄, and N₂O components from generation already converted to CO₂e and summed by the publishing authority. The reporting company multiplies activity (kWh consumed) by the factor and gets CO₂e directly — no per-gas arithmetic required.

Scope 3 — Value chain, mixed methodology

Scope 3 spans 15 categories per the GHG Protocol Scope 3 Standard, and the CO₂e arithmetic varies by category and by dataset. Spend-based and activity-based factors for purchased goods (Cat 1) typically arrive pre-aggregated as kg CO₂e per unit; supplier-specific data may arrive as per-gas figures requiring GWP application. Wastewater (Cat 5) uses pre-aggregated DEFRA factors. Capital goods (Cat 2) and use of sold products (Cat 11) often involve embodied-carbon datasets that need careful inspection of what is and is not already in CO₂e form.

How Pre-Aggregated Emission Factors Embed CO₂e

Most emission factors that practitioners use day-to-day — DEFRA for the UK, EPA for the US, IEA for cross-border — are pre-aggregated CO₂e factors. The publishing authority has already taken the per-gas component figures (kg CO₂ per unit, kg CH₄ per unit, kg N₂O per unit), applied the GWP conversion, and summed them into a single CO₂e factor per unit of activity. Practitioners multiply activity by the factor and get CO₂e directly. The GWP arithmetic is invisible.

The DEFRA 2025 natural gas factor illustrates the structure clearly. The headline factor is 0.18296 kg CO₂e/kWh (gross calorific value basis). Behind that single number sits a CO₂ component (the dominant contributor, around 0.18229 kg CO₂/kWh), a methane slip component (kg CH₄/kWh × GWP-100), and an N₂O component (kg N₂O/kWh × GWP-100). The three are summed by DEFRA at publication. A reporter using the headline 0.18296 figure has already applied AR5 GWP-100 (DEFRA’s stated basis) to the methane and N₂O components without realising it.

Per-gas raw mass fraction datasets — used by some methodology approaches and by some scientific studies — publish kg CO₂, kg CH₄, and kg N₂O separately for each fuel. In that workflow, the practitioner is responsible for applying GWP-100 to each gas and summing. This produces the same result as a pre-aggregated factor only if the same GWP basis is used at the same precision; mixing pre-aggregated with raw fraction inputs is the most common source of double-counting in corporate inventories.

Five Common CO₂e Mistakes in GHG Inventories

1. Using AR4 or AR5 GWP values instead of AR6. AR4 (2007) methane GWP-100 was 25; AR5 (2014) was 28; the AR6 value is now 29.8. A methane-intensive inventory still using AR4 values understates methane CO₂e by 19%. CDP flags AR5 usage as methodology inconsistency from its 2023 questionnaire onwards; AR4 usage is simply wrong by every current standard. Base-year restatement to AR6 is the correct response — not silently keeping AR5 values for trend-line continuity.
2. Mixing GWP time horizons within a single inventory total. Applying GWP-20 to methane (to emphasise short-term warming impact) and GWP-100 to nitrous oxide within the same CO₂e total produces a number that is neither a GWP-20 view nor a GWP-100 view — it is an inconsistent hybrid that cannot be compared to any standard regulatory threshold or science-based target baseline. Pick one horizon, apply it across every gas in scope, and disclose it explicitly.
3. Double-counting by applying GWP on top of a pre-aggregated DEFRA CO₂e factor. DEFRA 2025 emission factors are already in CO₂e — the GWP multiplication has been performed by DEFRA at publication. Adding a separate CH₄ × 29.8 or N₂O × 273 calculation on top inflates the total. The reverse error — applying a pre-aggregated factor to a raw mass fraction dataset — under-counts. Verify upfront which type of factor you have.
4. Confusing CO₂ and CO₂e in published disclosures. CO₂ is the carbon dioxide molecule — one specific gas. CO₂e is a unit of warming impact across all greenhouse gases expressed as CO₂ equivalent. A company that publishes “50,000 tonnes of CO₂” when it means “50,000 tonnes of CO₂e (all gases)” is either under-reporting or mislabelling, and verifiers will flag it. Under GHG Protocol, CO₂ alone and total CO₂e must both be reported in the per-gas breakdown, not collapsed into one figure.
5. Applying fossil methane GWP to biogenic methane sources. Fossil and biogenic methane have separate AR6 GWP-100 values: fossil 29.8 and biogenic 27. The 1.9-unit difference reflects the fossil-CO₂ tail of fossil methane oxidation that is absent for biogenic methane (where the carbon was already in the biological cycle). For landfill, livestock, rice cultivation, and anaerobic digestion, use the biogenic value. For natural gas combustion, fugitive gas, and coal mine methane, use the fossil value. Note also: biogenic CO₂ has GWP = 1 by GHG Protocol convention but is reported “outside scopes” in the carbon balance — biogenic methane, by contrast, is fully in scope at GWP-100 = 27.

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