A customer asks for your Scope 3 emissions. You look up the GHG Protocol, find fifteen categories, and the project immediately looks impossible. It is not — but only if you resist the instinct to quantify everything.

Screen before you measure

The Protocol expects you to screen all fifteen categories and quantify those that are material. Screening is a rough order-of-magnitude exercise using spend data, headcount and known volumes. It takes days, not months, and its purpose is to tell you where the emissions actually are.

For most organisations the answer is concentrated. A manufacturer typically finds purchased goods and services dominates everything else combined. A professional services firm usually finds business travel and purchased services. A retailer finds use of sold products or upstream transport. The shape is predictable once you look.

Spend-based is a starting point, not a destination

Spend-based factors get you a defensible first number quickly. Their weakness is that they cannot show improvement — if you buy the same steel from a supplier who has decarbonised, a spend-based method reports the same emissions. That makes them useless as a management tool once you start acting.

The sequence that works is spend-based for screening, then activity-based or supplier-specific data for the categories you intend to manage. You do not need supplier data for everything. You need it for the categories that dominate.

Getting supplier data without stalling

  • Start with your largest suppliers by emissions, not by spend — they are not always the same
  • Ask for what they already have. Many larger suppliers publish a footprint or hold a CDP response
  • Accept partial data. A verified figure for 40% of a category beats an estimate for all of it
  • Record the method and factor for every figure, because you will be asked and you will forget

What the deliverable actually is

The deliverable is not a number. It is a base year with a documented boundary, a stated method per category, and a data trail you can hand to an assurance provider. That is what makes a target credible later — and a target set against an undocumented baseline is not one you can defend.