Why is Scope 3 so much harder than Scope 1 and 2?

The fifteen categories include purchased goods and services, capital goods, fuel and energy activities not already counted, upstream and downstream transport, waste, business travel, employee commuting, leased assets, processing and use of sold products, and investments. Few businesses have all fifteen.

In practice you have three ways to get a number, in descending order of quality:

• Supplier-specific data — the supplier tells you the emissions of what you bought. Best, and rare.
• Activity data with an average factor — you know you bought 12 tonnes of paper, and apply a published factor for paper.
• Spend-based proxy — you know you spent S$40,000 on paper, and apply a factor per dollar. Weakest, but honest when labelled.

Spend-based proxies deserve a specific warning: they scale with price, not with physical impact. Buying the same goods after a price rise increases your calculated emissions without anything changing in the world. They are a reasonable starting point and a bad basis for a reduction claim.