calculate-ghg-inventory

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

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):

1. Supplier-specific factors (Scope 3 purchased goods)
2. National government factors (EPA, DEFRA, IEA)
3. 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).