Scenario-based case study: low-carbon steel

This case study uses a hypothetical automotive scenario to explore how market instruments can be used to account for low-carbon steel when lower-carbon and higher-carbon output cannot be physically segregated. Developed by WBCSD in collaboration with Carbon Trust, with input from participating member companies, it reflects the practical sourcing decisions companies face today. All figures are illustrative.

The worked example shows how a buyer’s market-based emissions change once a low-carbon attribute is allocated under mass balance, why that shift occurs, and what remains unresolved under current standards.

Through this case study, readers can:

• Understand how mass balance allocation could be applied across a multi-site supply chain where lower- and higher-carbon steel are mixed;
• See how the same physical production data could support either mass balance allocation or book-and-claim, and why the two cannot be combined for the same volume;
• Use a worked GHG accounting example to inform supplier engagement and chain-of-custody decisions.