Scope 3 is where greenhouse gas accounting programmes stall. Fifteen categories, most of the data sitting outside the organisation, and no single internal owner. The standard advice — "start with the material categories" — is correct but unhelpful: how do you know which categories are material before you have any data?
Screen roughly first, then go deep selectively
What works is a screening pass using data you already have: financial data. Take purchasing spend broken down by category, multiply by spend-based emission factors, and within two to three weeks you have a rough shape of where the mass sits.
That estimate is not accurate enough to publish. It is accurate enough to direct effort. In most cases we see, Category 1 — purchased goods and services — accounts for somewhere between 50% and 80% of total Scope 3, and the first thirty suppliers cover most of it.
Upgrade from spend-based to activity-based only where it pays
Once you know where the mass is, collect activity data or supplier-specific data for those categories only. The priority order we use:
- High share, concentrated supplier base — best return; go direct for primary data.
- High share, fragmented supplier base — invest in better industry averages rather than chasing hundreds of suppliers.
- Below the materiality threshold — leave it spend-based and document the reasoning clearly.
What a verifier will want to see
The most common source of findings is not a wrong number. It is an inability to explain why a method was chosen. Prepare three things:
- A screening record showing all fifteen categories were assessed and why any were excluded.
- Emission factor provenance — source, vintage, and why that factor suits your context.
- A traceable path from source record to reported figure that an outsider can follow without asking the preparer.
The third matters most. If a number lives in a spreadsheet only one person in the organisation can explain, it is a finding waiting to happen — whether or not the number is right.
The recurring mistake
Meticulously counting employee commuting to arrive at 0.4% of the total, while the raw materials category at 60% still runs on spend-based estimates. Precision belongs where it can change the conclusion.
This article is published for general information and does not constitute professional advice on any specific matter. To discuss your organisation’s situation, please contact our team.