A high-level overview of CARB’s proposed SB 253 implementation updates
by Lucas Willey & Jeff Baldino
Proposed Reporting Updates
During the July 21st, 2026 public workshop, the California Air Resources Board (CARB) discussed several proposed changes to the implementation of SB 253, providing the clearest picture yet of how California’s corporate greenhouse gas reporting program is expected to operate.
While many of the proposals are built on Greenhouse Gas Protocol guidance, several updates represent changes that organizations should prepare for now.
Under the proposed regulation, key changes include:
- Adjust reporting timelines and Scope 3 requirements:
- Report Scope 1 and Scope 2 emissions by November 10, 2026.
- Phase in Scope 3 reporting beginning in 2027, initially covering the five most commonly reported categories.
- Strengthen inventory documentation and assurance:
- Document methodologies, boundaries, emission factors, GWP values, uncertainty assessments, missing data and estimation procedures, and excluded emissions sources.
- Recalculate prior-year inventories when changes alter base-year emissions by more than five percent.
- Obtain limited assurance for Scope 1 and Scope 2 emissions beginning in 2027.
These updates appear to reflect CARB’s effort to apply elements of its established GHG Mandatory Reporting Regulation (MRR) framework to SB 253 reporting. Similar to GHG MRR, the proposed requirements emphasize documented methodologies, assumptions, supporting data, and controls needed to produce consistent, transparent, and independently verifiable inventories.
Scope 3 Reporting Categories
Perhaps the most significant proposal is CARB’s phased implementation of Scope 3 reporting.
Rather than requiring disclosure across all fifteen Scope 3 categories beginning in 2027, CARB is proposing that companies initially report only:
- Purchased Goods and Services
- Fuel and Energy Related Activities
- Waste Generated During Operations
- Business Travel
- Employee Commuting
The remaining ten Scope 3 categories will remain voluntary during the initial phase. CARB indicated these categories were selected because they are already among the most mature and consistently reported across existing climate disclosure programs.
“While this proposal reduces the immediate reporting burden, it establishes a clearer pathway toward full Scope 3 reporting over time.”
Similar to the Scope 1 and Scope 2 documentation elements described above, companies will also be required to maintain expanded documentation for Scope 3 emissions. These requirements include activity identification, calculation methodologies, data sources, excluded emissions, and the percentage of emissions calculated using primary supplier data. The workshop also signals that CARB expects significantly greater transparency regarding how emissions inventories are developed – also consistent with the CARB Mandatory Reporting Regulation.
Historically, many organizations focused primarily on developing accurate emissions totals. Under the proposed framework, companies will also need to demonstrate how those totals were developed. That includes documenting organizational boundaries, emission factors, estimation methods, assumptions, missing data protocols, methodology changes, and measurement uncertainty.
This is unlikely to change how most companies calculate emissions, but it will require more supporting documentation throughout the reporting process.
Recognized Assurance Standards
Beginning with reports submitted in 2027, reporting entities would obtain limited assurance over Scope 1 and Scope 2 emissions using recognized assurance standards.
Although assurance is being phased in, organizations that wait until it becomes mandatory often find that the biggest challenges are not emissions calculations, but the supporting documentation, repeatability, and transparency.
Additional Economic Analysis
CARB also updated its economic analysis in response to stakeholder feedback. Staff revised their assumptions using intentionally conservative cost estimates, including upper-bound reporting costs and the assumption that all compliance costs are attributable solely to SB 253. CARB acknowledged that actual costs would vary substantially depending on an organization’s existing reporting systems, prior disclosure experience, internal resources, and Scope 3 maturity.
Taken together, these proposals provide additional certainty regarding the direction of California’s climate disclosure program.
While the regulations are not yet finalized, organizations should begin evaluating whether their current reporting programs can support the additional documentation and governance expectations now being proposed.
How to Prepare
At EcoCira, we’ve been helping organizations prepare for this transition by focusing on the underlying systems that support long-term compliance. A few practical steps companies should consider include:
- Document greenhouse gas calculation methodologies before annual reporting begins.
- Establish formal written procedures for handling missing data and estimation methodologies.
- Develop consistent documentation supporting organizational boundaries, emission factors, and methodological decisions.
- Begin engaging suppliers for the five proposed Scope 3 categories and identifying where primary data can reasonably be obtained.
- Evaluate whether existing data systems provide sufficient traceability to support future assurance engagements.
- Perform internal readiness reviews before reporting deadlines to identify documentation gaps and strengthen internal controls.
The proposed regulations make one thing increasingly clear: SB 253 is evolving into a robust greenhouse gas reporting program. Organizations that invest now in their program will be better positioned to manage compliance costs, streamline assurance, and adapt as the regulations continue to evolve.