Scope 3 Category 1: moving from spend-based to material-based

Scope 3 Category 1 covers the cradle-to-gate emissions of everything you buy. For manufacturers it alone exceeds half of the total inventory. Yet in most first inventories the whole category is calculated by multiplying spend figures from an accounting report by sector-average emission factors.

Four calculation methods

The GHG Protocol Scope 3 Standard defines four methods and ranks them by data quality. Method choice follows from the data you have — but which data you collect is your choice.

• Spend-based — emission factor per unit of currency. Fastest, lowest quality. A price change moves your footprint.
• Average-data — sector-average factor per mass or unit. Price-independent, still generic.
• Hybrid — supplier data for critical lines, average data for the rest. The most common mature approach in practice.
• Supplier-specific — the supplier’s own product footprint or EPD. Highest quality, highest collection cost.

The structural flaw in the spend-based method

The spend-based method cannot serve as a management tool, because there is no link between the number it produces and the decisions you make: switching to a lower-emission supplier does not change your footprint if you pay the same amount. Conversely, buying the same goods from the same supplier at a lower price makes your footprint fall.

That is an acceptable starting point for a first inventory — it gives you the order of magnitude. But the moment you set an abatement target and start reporting progress you have to change method, because you cannot measure movement towards the target.

A staged transition strategy

Collecting primary data from every supplier is neither realistic nor necessary. The Pareto distribution is strong here: ten per cent of purchase lines account for more than eighty per cent of emissions. The strategy is to move that ten per cent onto supplier-specific data and leave the rest on average data.

• Year 1 — spend-based baseline inventory, produce a hotspot map
• End of year 1 — move the 20-30 highest-contribution lines to material-based factors
• Year 2 — request EPDs or product footprints from those lines’ suppliers
• Years 2-3 — move to a hybrid model: primary for critical lines, average for the rest
• Continuously — add an emissions data criterion to supplier pre-qualification

The base year recalculation trap

When you raise data quality the total changes — usually downward, because sector averages are conservative. That looks like progress but is not. The GHG Protocol requires base year recalculation for differences arising from methodology changes. Otherwise the reduction you report is really just a measurement improvement.