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Tier 4 — Audited

What it is

Annual third-party assurance over annual aggregates of tier-2 and tier-3 evidence, performed under ISAE 3000 (Limited Assurance Engagements Other than Audits or Reviews of Historical Financial Information). The assurance partner reviews the methodology document, calculation lineage, calibration results, pedigree-score assignments, and version-pinned emission-factor sources, and issues an opinion.

Tier 4 is not produced for a single query — it is the year-end opinion attached to annual aggregates. For a single query, "tier 4" effectively means "the tier-3 number, with a named assurance partner attached at year-end."

What the auditor reviews

The annual evidence pack provided to our assurance partner contains:

  1. Methodology document at all versions in scope of the period (the version-pinning ensures reproducibility)
  2. Per-receipt records for the period, structured as a JSON-LD calculation lineage
  3. Calibration runs — quarterly tier-3 measurements that calibrated tier-2 intervals
  4. Pedigree-score worksheets — every emission factor used, with its Weidema score and source
  5. Version-pinned emission factor sources — ecoinvent v3.10, ADEME 2024, IPCC AR6 GWP100, JRC NEEFE 2024
  6. Sub-processor change log — any changes affecting boundary assumptions
  7. Assumption log — every modelled assumption with rationale (PUE, embodied amortisation lifetime, batch-size estimation procedure)

Scope of the assurance opinion

ISAE 3000 limited assurance is, by design, a negative opinion: "nothing has come to our attention that causes us to believe..." This is the level of assurance that fits the maturity of the methodology. The alternative (ISAE 3000 reasonable assurance, a positive opinion) is significantly more expensive and not yet warranted given the iterative methodology improvements still occurring.

The opinion covers:

  • Climate (gCO₂e) annual aggregates per region per model family
  • Water consumption annual aggregates
  • Resource depletion annual aggregates (where reported)
  • Methodology version compliance
  • Calculation lineage completeness

The opinion does not cover:

  • Per-query receipts (those are the responsibility of the methodology, not the auditor)
  • Forward-looking projections
  • Selma's consumer-product Public Impact Ledger (separately attested by a notary; a different scope)

Customer-side use

If you are a CSRD reporter under active assurance, your assurance partner will typically want:

  1. The annual evidence pack for receipts attributable to your account
  2. Our annual ISAE 3000 opinion at the firm level
  3. The methodology document at the version your aggregate cites

We provide all three on request, NDA where the firm-level opinion has not yet been published.

Cadence

  • Q1: Annual financial audit (statutory, by BDO Stockholm)
  • Q2: Annual sustainability assurance (ISAE 3000, candidate Big-4 partner)
  • Q3: Annual governance review (Purpose Foundation Berlin)
  • Quarterly: Public Ledger attestation (rotating notary)
  • Quarterly: Methodology Council review (5 independent advisors)

The Q2 sustainability assurance is what produces the tier-4 opinion. The first formal opinion is targeted for FY2027 close (Q1 2028 publication).

Where this is implemented

methodology/tier4/evidence_pack.py

Citations

  • IAASB. ISAE 3000 (Revised) Assurance Engagements Other than Audits or Reviews of Historical Financial Information.
  • IAASB. ISAE 3410 Assurance Engagements on Greenhouse Gas Statements (related and informative).
  • EFRAG. European Sustainability Reporting Standards (ESRS) — set 1. As simplified mid-2026 under Omnibus I.