Study Finds Carbon Accounting Choices Can More Than Double Reported Supply-Chain Emissions

A study published in Nature Sustainability uses data from 401 European companies and a fixed purchasing basket to show how strongly supply-chain emissions can change with the database and accounting method selected. For the same activities, the study example ranged from 1.7 million to 3.5 million tonnes of CO2 equivalent; the difference came from accounting choices, not operational change.

Corporate net-zero claims often depend heavily on Scope 3 emissions, but supply-chain activity data must be converted through emission factors, industry databases, boundaries and base-year rules. Sources differ in year, geography, product classification and estimation logic, so identical activity data do not necessarily produce directly comparable results.

The study reports that 62.5% of year-to-year variation in supply-chain emissions across 401 European companies could not be explained by company activity, industry change or emission intensity. The comparable unexplained variation for direct emissions was just over 5%. When a company discloses only a final number, users cannot tell whether a reduction came from real action or from changing a database, boundary or calculation rule.

Each inventory should retain activity data, emission-factor sources, database and version, geographic and temporal applicability, organisational and value-chain boundaries, base year and recalculation policy. A change of database or method should trigger a comparable-baseline recalculation and an impact disclosure. Industry bodies and standard setters can also establish minimum common rules and controlled change logs.

The study’s fixed food-and-beverage purchasing basket produced estimates from 1.7 million to 3.5 million tonnes of CO2 equivalent across four commonly used scientific databases, making the highest result more than double the lowest. This draft preserves clickable links to Eco-Business, the underlying Mongabay report and the peer-reviewed Nature Sustainability paper, while separating reported findings from SNN analysis.

For Taiwanese exporters and multinational supply chains, different factor libraries and boundaries can distort supplier performance, procurement comparisons and recognition of reductions. Delivering an emissions total is therefore insufficient; companies also need to deliver the sources, versions, assumptions and change history required to reconstruct the calculation.

SNN editorial analysis: Carbon-inventory credibility cannot rest on a single aggregate. A verifiable evidence chain should bind activity data, emission factors, method versions, boundaries, transformations, recalculation decisions and the final disclosure so reviewers can distinguish operational change from data revision or methodological choice.

Next steps include monitoring whether the proposed industry-consistent rules enter standards, assurance practice and digital reporting, and how updates by institutions such as GHG Protocol address factor selection, method changes, base-year recalculation and Scope 3 comparability.