| name | calculate-ghg-inventory |
| description | Use when calculating the greenhouse gas (GHG) emissions of an activity, organization, product, or project — applying the GHG Protocol Corporate Standard or ISO 14064 to quantify Scope 1, 2, and 3 emissions in CO₂-equivalent units. |
| source | GHG Protocol Corporate Accounting and Reporting Standard (World Resources Institute, 2004); ISO 14064-1:2018; IPCC Sixth Assessment Report (AR6) Global Warming Potentials; EPA Emission Factors Hub |
| tags | ["carbon-footprint","ghg-emissions","climate","sustainability","scope-1-2-3","co2-equivalent","lifecycle-assessment"] |
Calculate GHG Inventory
Quantify greenhouse gas emissions using GHG Protocol methodology — inventorying Scope 1, 2, and 3 emission sources, applying correct global warming potential (GWP) factors, and expressing results in CO₂-equivalent (CO₂e) units with documented uncertainty.
Why This Is Best Practice
Why best: GHG Protocol + ISO 14064 give a standardized, auditable method for quantifying emissions across scopes — using current IPCC GWP factors and documented boundaries avoids both understating footprint (skipping Scope 3) and misstating it (stale GWP values), the two most common failure modes in carbon accounting.
Adopted by: GHG Protocol standards are used by >90% of Fortune 500 companies that report emissions and are required by CDP (Carbon Disclosure Project), TCFD (Task Force on Climate-related Financial Disclosures), and SEC climate disclosure rules (2024). ISO 14064-1 is the basis for third-party verification of corporate GHG inventories. The Science Based Targets initiative (SBTi) requires GHG Protocol-aligned inventories for target validation.
Impact: The IPCC AR6 (2021) quantified global warming potential (GWP₁₀₀) for all GHGs — the updated factors change total emissions estimates for industries using refrigerants, methane, or N₂O significantly vs. AR4 values. Organizations using wrong GWP values systematically misstate their footprint. Scope 3 emissions typically account for 70-90% of an organization's total impact — organizations reporting only Scope 1+2 miss the vast majority of their climate risk.
Steps
1. Define scope and organizational boundary
Establish what is included before calculating anything:
- Operational control approach: include all emission sources you have operational control over (preferred for regulatory compliance)
- Equity share approach: include sources proportional to your equity share
- Financial control approach: include sources you have financial control over
Document included and excluded sources and the rationale for any exclusions.
2. Inventory emission sources by scope
Scope 1 (direct emissions — sources you own or control):
- Combustion of fuels in owned equipment (boilers, vehicles, furnaces, generators)
- Process emissions (industrial processes: cement calcination, steel making)
- Fugitive emissions (refrigerants leaking from HVAC/refrigeration; SF₆ from electrical equipment; methane from wastewater)
Scope 2 (indirect — purchased energy):
- Electricity consumption: report both market-based (using supplier emissions factors or RECs) and location-based (grid average)
- Purchased steam, heat, cooling
Scope 3 (indirect — value chain):
- Upstream: purchased goods and services, capital goods, waste, business travel, employee commuting
- Downstream: use of sold products, end-of-life treatment, investments
For most organizations, prioritize Scope 3 categories by screening estimation first; focus detailed calculation on categories >1% of estimated total.
3. Collect activity data
Activity data = the physical quantity of emission-generating activity:
- Fuel: liters, m³, or GJ consumed
- Electricity: kWh consumed
- Refrigerants: kg charged/leaked/disposed
- Materials: tonnes purchased or produced
- Travel: passenger-km or freight-tonne-km
Sources: utility bills, fuel purchase records, production logs, supplier data, financial spend data (for estimation).
4. Apply emission factors and GWP100
Emission factor converts activity data to CO₂e:
CO₂e emissions = Activity data × Emission factor × GWP₁₀₀(gas)
Use current GWP₁₀₀ from IPCC AR6 (2021):
- CO₂: 1 (reference)
- CH₄ (methane): 27.9 (AR6; was 25 in AR4)
- N₂O: 273 (AR6; was 298 in AR4)
- HFC-134a (refrigerant): 1,526
- SF₆: 25,200
Emission factor sources (hierarchical preference):
- Supplier-specific factors (Scope 3 purchased goods)
- National government factors (EPA, DEFRA, IEA)
- Industry-average databases (ecoinvent, EcoInvent, SimaPro)
5. Sum and report by scope
Total Scope 1 = Σ(activity_i × EF_i × GWP_i) for all direct sources
Total Scope 2 = electricity consumption × grid EF (market-based or location-based)
Total Scope 3 = Σ(category subtotals)
Total footprint = Scope 1 + Scope 2 + Scope 3 (tCO₂e)
Report:
- Base year and reporting year
- Organizational and operational boundaries
- Emissions by scope (tCO₂e)
- Emission factor sources and GWP values used
- Estimation methods for data gaps
- Uncertainty assessment (typically ±10-30% for Scope 3)
6. Verify calculations with independent review
For external reporting: third-party verification against ISO 14064-3 limited or reasonable assurance standard. For internal use: cross-check Scope 1 against energy bills; Scope 2 against electricity bills; Scope 3 screening against spend-based estimates.
Common Mistakes
- Using outdated GWP values (AR4 instead of AR6): Methane GWP shifted from 25 to 27.9 — a 12% difference that matters at scale.
- Reporting only Scope 1 and 2: For most product companies, Scope 3 (especially sold products and purchased materials) represents >70% of the real footprint. Partial reporting misleads stakeholders.
- Double-counting in complex supply chains: Scope 3 of your upstream supplier may overlap with your Scope 3 purchased goods. Document scope carefully to avoid double-counting.
When NOT to Use
- Product-level lifecycle assessment (LCA): use ISO 14044 LCA methodology instead — it includes impact categories beyond GHG (water use, toxicity, eutrophication) and is required for Environmental Product Declarations (EPD).
