The honest starting point for most Scope 3 programs is this: the vast majority of suppliers, when first asked for their emissions data, do not have it. Not because they're hiding it — because nobody has ever asked them to measure it, and measuring it is genuinely work.
That gap is why most first-year Scope 3 inventories lean almost entirely on spend-based estimation — multiplying money spent with a supplier by an industry-average emission factor per dollar. It's fast, it's defensible as a starting point, and it is also close to useless for actually reducing anything, because it can't tell a genuinely low-carbon supplier from a high-carbon one in the same category. Spend more with either, and the estimated emissions go up identically.
The data hierarchy
The GHG Protocol ranks data quality roughly like this, from weakest to strongest:
- Spend-based — dollars × industry-average emission factor. Fast, cheap, directionally useless for supplier comparison.
- Average-data — physical quantity (tonnes, units, kWh) × industry-average emission factor. Better, still not supplier-specific.
- Hybrid — average-data methods supplemented with some supplier-specific inputs.
- Supplier-specific (primary data) — the supplier's own measured or calculated emissions, allocated to your purchases. The only tier that actually rewards a supplier for decarbonizing.
The jump from tier 2 to tier 4 is where almost all the real engagement work sits — and where most programs stall.
A sequence that actually works
Start with concentration, not coverage. In most supply chains, 20% of suppliers by spend represent 70–80% of Category 1 emissions. Chasing primary data from your smallest 500 suppliers before your largest 20 is a resourcing mistake seen constantly.
Ask for what they already have before asking them to build something new. Many suppliers already track energy use, fuel consumption, or production volumes for cost or compliance reasons — they just haven't converted it to a carbon figure. A request framed as "send us your electricity bills and production output" gets a far higher response rate than "submit your Scope 1/2/3 inventory."
Give suppliers a standard template, not a blank page. Ambiguity kills response rates. A structured data-collection template — ideally aligned with the same emission-factor sources you'll use downstream — turns a research project into a form-filling exercise.
Make it a scored criterion, not a favor. Supplier emissions disclosure moves fastest when it's tied to something the supplier already cares about: RFP scoring, contract renewal, or a standing item in quarterly business reviews. A polite annual email rarely survives contact with a busy procurement contact.
Expect a multi-year ramp, and say so publicly. CDP's supply chain data consistently shows response rates climbing over successive years of the same request, not the first year. Setting an internal target like "supplier-specific data for 60% of Category 1 spend by year three" is more credible — and more achievable — than expecting full coverage immediately.
The payoff
Once even a third of spend is on primary data, the inventory stops being a static compliance number and starts being a genuine management tool — you can see which suppliers are actually decarbonizing, weight sourcing decisions accordingly, and make supplier engagement itself a real reduction lever rather than a reporting exercise.