se. 3. Factor Metadata Awareness: Many emission factors (e.g., grid electricity, refined fuels) are already expressed in tCO₂e per unit of activity. The resolver checks unit metadata to avoid double-multiplication. 4. Methodology Versioning: Every calculation output carries a methodologyVersion tag, enabling audit trails and retrospective recalculations.

Implementation

// types/carbon-engine.types.ts
export type GasCategory = 'CO2' | 'CH4_FOSSIL' | 'CH4_BIOGENIC' | 'N2O' | 'SF6';
export type EmissionUnit = 'kg' | 'MWh' | 'liters' | 'tonnes';
export type MethodologyVersion = 'AR5' | 'AR6';

export interface GWPRegistry {
  version: MethodologyVersion;
  multipliers: Record<GasCategory, number>;
}

export interface EmissionFactor {
  value: number;
  unit: EmissionUnit;
  isPreMultiplied: boolean; // True if factor already outputs tCO2e
  source: string;
}

export interface ActivityInput {
  gasCategory: GasCategory;
  quantity: number;
  unit: EmissionUnit;
  factor: EmissionFactor;
}

export interface CalculationResult {
  co2eTonnes: number;
  methodology: MethodologyVersion;
  gasCategory: GasCategory;
  rawActivity: number;
  appliedGWP: number;
}

// engine/gwp-registry.ts
export class GWPRegistryManager {
  private static registry: Record<MethodologyVersion, GWPRegistry> = {
    AR5: {
      version: 'AR5',
      multipliers: {
        CO2: 1,
        CH4_FOSSIL: 25,
        CH4_BIOGENIC: 25,
        N2O: 265,
        SF6: 23500
      }
    },
    AR6: {
      version: 'AR6',
      multipliers: {
        CO2: 1,
        CH4_FOSSIL: 29.8,
        CH4_BIOGENIC: 27.9,
        N2O: 273,
        SF6: 25200
      }
    }
  };

  static getMultiplier(version: MethodologyVersion, gas: GasCategory): number {
    const registry = this.registry[version];
    if (!registry) throw new Error(`Unsupported methodology: ${version}`);
    return registry.multipliers[gas];
  }
}

// engine/carbon-ledger.ts
export class CarbonLedger {
  private methodology: MethodologyVersion;

  constructor(methodology: MethodologyVersion = 'AR6') {
    this.methodology = methodology;
  }

  public calculate(input: ActivityInput): CalculationResult {
    const { gasCategory, quantity, factor } = input;

    // Validate gas routing for methane
    if (gasCategory === 'CH4_FOSSIL' || gasCategory === 'CH4_BIOGENIC') {
      if (factor.isPreMultiplied) {
        throw new Error(`Methane factors must not be pre-multiplied for ${gasCategory}. Use raw gas quantities.`);
      }
    }

    const gwp = GWPRegistryManager.getMultiplier(this.methodology, gasCategory);
    let co2eTonnes: number;

    if (factor.isPreMultiplied) {
      // Factor already outputs tCO2e/unit; only quantity scaling applies
      co2eTonnes = quantity * factor.value;
    } else {
      // Raw gas: Activity × EF × GWP
      co2eTonnes = quantity * factor.value * gwp;
    }

    return {
      co2eTonnes: Number(co2eTonnes.toFixed(6)),
      methodology: this.methodology,
      gasCategory,
      rawActivity: quantity,
      appliedGWP: gwp
    };
  }

  public recalculate(results: CalculationResult[], targetVersion: MethodologyVersion): CalculationResult[] {
    return results.map(res => {
      const newGWP = GWPRegistryManager.getMultiplier(targetVersion, res.gasCategory);
      const adjusted = res.co2eTonnes * (newGWP / res.appliedGWP);
      return {
        ...res,
        co2eTonnes: Number(adjusted.toFixed(6)),
        methodology: targetVersion,
        appliedGWP: newGWP
      };
    });
  }
}

Why This Structure Works

• Type Safety: The GasCategory union prevents accidental routing of biogenic methane through fossil multipliers.
• Pre-Multiplied Guard: The isPreMultiplied flag stops the most common calculation inflation error. Grid electricity and refined fuel factors are typically already converted to CO₂e equivalents; multiplying by GWP again creates phantom emissions.
• Recalculation Pipeline: The recalculate method enables retrospective methodology swaps without losing raw activity data. This is critical for audit responses and regulatory transitions.
• Zero Runtime Dependencies: The engine operates purely on configuration and arithmetic, making it trivial to embed in CI/CD validation steps, data pipelines, or frontend calculators.

Pitfall Guide

1. Monolithic Methane Multiplier

Explanation: Applying a single CH₄ value (e.g., 25 or 29.8) to all methane sources ignores the carbon cycle distinction between fossil extraction and biological decomposition. Fix: Enforce strict routing at the data ingestion layer. Tag every methane activity record with origin: 'fossil' | 'biogenic' and map to separate GWP constants.

2. Double-Multiplying Pre-Multiplied Factors

Explanation: Many emission factors (DESNZ, EPA, Ecoinvent) are published in tCO₂e/MWh or kgCO₂e/liter. Multiplying these by GWP inflates results by 25–30x. Fix: Require isPreMultiplied metadata on every factor record. Implement a runtime guard that throws if GWP is applied to pre-converted units.

3. Hardcoding Methodology Versions

Explanation: Embedding AR6 values directly in business logic creates deployment friction when IPCC AR7 releases. It also breaks audit reproducibility. Fix: Externalize GWP tables to a versioned configuration store. Tag every calculation output with methodologyVersion and retain raw activity data for retrospective scaling.

4. Ignoring Reporting Standard Divergence

Explanation: GHG Protocol, ISO 14064, and CSRD have different transition timelines and allowed GWP sets. Assuming universal compliance leads to disclosure rejections. Fix: Implement a ReportingStandard enum that dictates which GWP registry is valid. Validate inputs against the standard's allowed methodology window before calculation.

5. Applying GWP to Spend-Based Scope 3 Data

Explanation: Economic intensity factors (kgCO₂e/$) already embed lifecycle emissions. Multiplying by GWP treats monetary units as physical gas quantities. Fix: Separate calculation pipelines for physical activity data (Scope 1/2) and economic spend data (Scope 3). Only apply GWP multipliers to physical gas or fuel inputs.

6. Assuming GWP-100 Captures Short-Term Dynamics

Explanation: GWP-100 integrates radiative forcing over a century. Methane's short atmospheric lifetime (~12 years) means GWP-100 understates near-term warming impact. Fix: Use GWP-100 for regulatory compliance. Document limitations in methodology notes. For internal climate risk modeling, consider GWP* or absolute temperature change potentials, but keep them separate from audited inventories.

7. Omitting Uncertainty Bounds

Explanation: IPCC GWP values are estimates with confidence intervals. Reporting single-point values hides methodological uncertainty and complicates assurance reviews. Fix: Store lower/upper bounds alongside central values. Propagate uncertainty through Monte Carlo or interval arithmetic when precision thresholds are breached.

Production Bundle

Action Checklist

• Audit existing calculation pipelines for hardcoded GWP constants and replace with versioned configuration
• Implement gas origin routing to separate fossil and biogenic methane multipliers
• Add isPreMultiplied metadata validation to prevent double-counting emission factors
• Tag all calculation outputs with methodologyVersion and retain raw activity data
• Map internal reporting standards to allowed GWP sets (GHG Protocol, CSRD, ISO 14064)
• Build a retrospective recalculation endpoint for methodology transitions
• Document uncertainty ranges and calculation assumptions in methodology notes
• Run regression tests against AR5/AR6 delta benchmarks before deployment

Decision Matrix

Scenario	Recommended Approach	Why	Cost Impact
Legacy tool with AR5 defaults	Implement configuration swap + recalculation pipeline	Avoids full rebuild; maintains audit trail	Low (config + testing)
New carbon platform build	Native AR6 registry with gas origin routing	Prevents technical debt; ensures compliance from day one	Medium (architecture overhead)
CSRD/SEC disclosure preparation	AR6 with uncertainty bounds + methodology versioning	Meets assurance requirements; supports third-party audit	Medium-High (documentation + validation)
Scope 3 spend-based reporting	Economic intensity factors only; no GWP application	Aligns with GHG Protocol Scope 3 guidance	Low (pipeline separation)
Internal climate risk modeling	GWP* or temperature potential metrics alongside GWP-100	Captures short-lived pollutant dynamics	High (separate modeling layer)

Configuration Template

{
  "methodology": {
    "version": "AR6",
    "allowedStandards": ["GHG_PROTOCOL", "CSRD", "ISO_14064"],
    "transitionDeadline": "2025-12-31"
  },
  "gwpRegistry": {
    "AR5": {
      "CO2": 1.0,
      "CH4_FOSSIL": 25.0,
      "CH4_BIOGENIC": 25.0,
      "N2O": 265.0,
      "SF6": 23500.0
    },
    "AR6": {
      "CO2": 1.0,
      "CH4_FOSSIL": 29.8,
      "CH4_BIOGENIC": 27.9,
      "N2O": 273.0,
      "SF6": 25200.0
    }
  },
  "emissionFactors": {
    "natural_gas_combustion": {
      "value": 0.0561,
      "unit": "kgCH4/MWh",
      "isPreMultiplied": false,
      "source": "IPCC_2006",
      "gasCategory": "CH4_FOSSIL"
    },
    "grid_electricity_us": {
      "value": 0.385,
      "unit": "tCO2e/MWh",
      "isPreMultiplied": true,
      "source": "EPA_EGRID",
      "gasCategory": "CO2"
    }
  }
}

Quick Start Guide

1. Initialize the registry: Load the configuration template into your application's configuration store. Set methodology.version to AR6 and verify allowedStandards match your reporting requirements.
2. Validate emission factors: Audit your factor database. Ensure every record includes isPreMultiplied and gasCategory. Reject any factor missing these fields.
3. Deploy the ledger: Instantiate CarbonLedger with your target methodology. Route incoming activity data through the calculate method. Verify outputs include methodologyVersion and appliedGWP.
4. Run delta tests: Execute a batch calculation using identical inputs against both AR5 and AR6 registries. Confirm methane outputs diverge by ~19.2% (fossil) or ~11.6% (biogenic). Fail CI if deltas fall outside ±0.5%.
5. Enable recalculation: Expose the recalculate endpoint for audit responses. Store raw activity data indefinitely to support future IPCC methodology transitions without data loss.