After the First SB 253 Deadline, the Harder Work Is Supplier Evidence

In February 2026, the California Air Resources Board announced that it had approved the initial climate-transparency regulation. The first Scope 1 and Scope 2 reporting deadline was 10 August 2026 for covered entities doing business in California with annual revenue above US$1 billion. Scope 3 reporting begins in 2027.

CARB’s corporate greenhouse-gas reporting programme page should be checked for current implementation materials and updates. The fact that a deadline has passed does not by itself establish any company’s compliance outcome. Current official materials remain the operational reference.

Scope 3 data becomes unmanageable when the organisation asks every supplier for every possible metric. Begin with the reporting boundary, relevant categories, organisational structure and material activities. Map which suppliers or partners influence each category and which internal owner will use the result.

The map should distinguish purchased-goods evidence, logistics, waste, business travel and other categories instead of blending them into one generic supplier-data request.

A supplier may provide electricity consumption, fuel use, material mass, product-level emissions or an already calculated corporate figure. These are different evidence types. The receiving organisation should retain the original unit, period, boundary and source before converting data for its own methodology.

Emission factors, allocation rules and currency or unit conversions belong in a controlled calculation layer. This separation allows the company to update a factor without pretending the supplier changed its original activity data.

Not every partner will provide equivalent evidence. A useful register identifies whether a value is measured, estimated, modelled, verified by a third party or unavailable. It also records the date, document, contact and any limitation.

Quality status supports prioritisation. High-impact suppliers with weak evidence can receive targeted engagement, while low-impact gaps can remain visible without consuming disproportionate resources.

The assessment should avoid false precision. A supplier PDF with a polished total may have a different organisational boundary, period or allocation method from the reporting company’s needs. Conversely, a simple meter or shipment record may be strong primary evidence even though it has no assurance badge. Data quality is a documented fit-for-purpose judgement, not a ranking based on presentation.

Scope 3 inventories will change as supplier data improves and CARB implementation materials evolve. The evidence system should preserve prior submissions, calculation versions, approval decisions and the reason for each restatement or correction.

This is particularly important when a supplier revises a corporate boundary or replaces an estimate with measured data. Silent overwrites break the link between what the reporting company knew at the deadline and what it learned later.

Select one high-impact supplier and trace a value from source document through mapping, conversion, factor selection, review and proposed Scope 3 disclosure. Ask a reviewer who did not build the workbook to reproduce the result. Record every manual assumption and inaccessible source.

Repeat the exercise with a supplier that has no primary data. The comparison will show where engagement, contractual clauses, systems integration or a documented estimation method is needed before 2027. Add a timetable for evidence requests, review, escalation and data freeze. A controlled timetable prevents late supplier updates from entering the inventory without the same checks applied to earlier information.