Most California companies preparing for climate disclosure understand their own emissions reasonably well. Energy bills exist, fuel receipts exist, and fleet mileage is recorded somewhere.
Then the requirement extends to everything their suppliers emit, and the data is not there.
That gap is the practical challenge behind SB 253, and it is a data problem long before it becomes a sustainability one.
What the Law Actually Asks For
SB 253 requires large companies doing business in California to disclose greenhouse gas emissions, with Scope 3 reporting due in 2027 and third-party assurance following. The threshold applies to companies with more than $1 billion in revenue.
Scope 1 and 2 cover what you burn and what you buy in energy. Scope 3 covers everything else in your value chain, and for most organizations that is where the emissions sit.
Implementation detail continues to move through California Air Resources Board rulemaking, so confirm current guidance rather than any published timeline, including this one.
Why Scope 3 Is the Difficult Number
The scale catches people out.
Supplier emissions fall under Scope 3, specifically Category 1 for purchased goods and services and Category 4 for upstream transportation and distribution. Analysis published by Sweep puts supplier emissions at, on average, 26 times higher than combined Scope 1 and 2 emissions, with Scope 3 typically accounting for up to 75 percent of a company's total carbon footprint.
So the number you cannot produce is the one that matters most. A company reporting only Scopes 1 and 2 has disclosed roughly a quarter of its footprint and called it a carbon inventory.
The Data Problem Underneath
Your suppliers mostly do not measure this, and the ones who do measure it inconsistently.
Smaller suppliers frequently lack the resources or expertise to report greenhouse gas emissions reliably, and there is no standardized format everyone uses. Manual collection makes it worse, since a spreadsheet emailed to a procurement contact does not scale past a few dozen vendors.
That is the honest starting position for most companies scoping SB 253 readiness, and pretending otherwise wastes the first six months.
Four Ways to Calculate, and Why You Need More Than One
The GHG Protocol defines four methodologies for supplier emissions: spend-based, average-data, hybrid, and supplier-specific. Each trades accuracy against data availability, and companies wanting to learn more about Sweep will find the platform supports all four together.
That combination is the point. Sweep, the sustainability intelligence platform, keeps estimates and verified supplier data in the same system rather than in separate spreadsheets that have to be reconciled at reporting time.
Spend-based estimates apply emissions factors to what you spent, so you can cover your entire supply base immediately without a single supplier responding. Supplier-specific data is far more accurate and takes considerably longer to obtain.
The workable approach uses both. Start with estimates for full coverage, then replace them with primary data supplier by supplier, prioritizing wherever the emissions concentrate.
Getting Suppliers to Respond
Data collection fails on engagement more often than on technology.
A supplier receiving an emissions survey sees unpaid work with no obvious benefit, and response rates reflect that. Escalating through procurement rarely helps, because the person who can supply the data is usually not the one who holds your contract. The approaches that work change the exchange: giving suppliers their own footprint dashboard, providing training on greenhouse gas accounting so they can answer accurately, and automating reminders so chasing does not consume a procurement team's month.
Audit trails matter here too. When a figure is questioned eighteen months later, you need to show where it came from and who approved it.
Capture that at the point of entry rather than retrofitting it. Reconstructing provenance across eighteen months of submissions is considerably harder than recording it as the data arrives.
Assurance Is the Part People Forget
Third-party assurance follows the reporting requirement, and it changes what counts as adequate data.
Numbers that survive an internal review do not necessarily survive an assurance process. Documented methodology, traceable sources and approval workflows stop being administrative tidiness and become the thing an assurance provider actually examines, line by line.
California already has a developed advisory market here. Firms working on the energy transition in the state include specialists auditing carbon claims, and engaging one early tends to reveal data weaknesses while there is still time to fix them.
Conclusion
SB 253 is a reporting obligation with a procurement problem inside it.
Start with the methodology question, because it determines everything downstream. Accept estimates for coverage, target primary data where emissions concentrate, and build the audit trail as you go rather than reconstructing it before assurance.
Companies beginning now have time to do this properly. Those beginning in 2027 will be doing it badly and under scrutiny.
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