California Air Resources Board

Thank you for the opportunity to provide comments on the Concepts for Potential Regulations developed pursuant to SB 905. My comments focus on carbon accounting, permanence, monitoring, and technology inclusiveness. I appreciate CARB's efforts to establish a durable framework for Carbon Capture, Removal, Utilization, and Storage (CCUS) and Carbon Dioxide Removal (CDR) activities in California.

§ 95701. Definitions

Consider adding a definition for a carbon reservoir.

The current concepts include a definition of storage that is specific to geologic reservoirs. However, it may be beneficial to define a carbon reservoir more broadly to accommodate future expansion of the program and to support consistent carbon accounting across storage pathways.

From an accounting perspective, disputes regarding carbon losses ultimately depend on determining where carbon is and is not located. Reservoir boundaries therefore become fundamental. Reservoir size and extent may also become subjects of disagreement, making clear definitions important for regulatory certainty.

One possible definition is:

Carbon Reservoir: A physical location where carbon is stored, or a conceptual means of outlining the boundaries of a reservoir that contains stored carbon. To be well defined, it must be possible to determine whether physical carbon is or is not contained within a particular reservoir.

Defining reservoirs based on agreed-upon boundaries would improve accounting transparency and facilitate resolution of future disputes concerning storage performance and carbon losses.

§ 95703. General Project Reporting of CCUS and CDR Technologies

Consider requiring disclosure of reservoir boundaries.

For transparency, independent review, and safety, it is important to know where stored carbon is expected to be located. Knowing where carbon should be located necessarily requires knowing the boundaries of the reservoir.

If reservoir boundaries cannot be identified, this may indicate that the reservoir is insufficiently characterized. In addition, non-disclosure of reservoir boundaries invites speculation and complicates independent evaluation of storage performance.

CARB should consider requiring project developers to disclose reservoir boundaries. A reservoir can be defined substantially larger than the storage plume itself, allowing flexibility while still providing transparency regarding the expected location of stored carbon.

§ 95704.2. Subsurface and Ambient CO₂ Monitoring Plan and Report

Consider evolving monitoring requirements toward direct accounting of stored carbon.

Current approaches to geologic storage generally assume that careful site selection minimizes leakage risk. Under this framework, monitoring focuses primarily on detecting leaks through ambient CO₂ monitoring and other surveillance approaches. This assumption is often reasonable, but the effectiveness of monitoring depends on the nature of the leakage pathway.

Known plausible leakage pathways, including wells and faults, can be monitored directly. However, very small leaks from unexpected pathways may be difficult to detect because of natural background fluctuations in atmospheric CO₂ concentrations and sensor detection limits (Lackner & Brennan, 2009). While individually small, such leaks could accumulate over long periods and eventually represent a significant fraction of stored carbon. At the same time, geophysical monitoring of the storage plume may not always provide sufficient sensitivity to quantify small losses.

For accounting purposes, CARB should consider establishing a long-term objective that monitoring systems evolve toward directly determining the amount of carbon in storage, rather than relying primarily on leak detection. Such an approach would improve transparency and accounting confidence while encouraging research and development to reduce uncertainty over time.

Consider requiring transparency for leak quantification methods.

The Archer Daniels Midland project provides a useful example. Although the reported leak was relatively small, with recent estimates ranging from approximately 2,670 to 3,940 metric tonnes, the methods used to estimate the leak size are not readily transparent. Operators were instructed to model the leak and constrain those models using monitoring, geochemical, pressure, and operational data. At the same time, one verification well sensor has reportedly been nonfunctional since 2020, making it difficult for independent observers to understand how leak estimates are being derived.

For accounting purposes, transparency is essential. CARB may consider requiring that methodologies used to estimate leakage quantities be publicly available and sufficiently documented to allow independent technical review.

Reference:

Lackner, K. S., & Brennan, S. (2009). Envisioning carbon capture and storage: Expanded possibilities due to air capture, leakage insurance, and C-14 monitoring. Climatic Change, 96(3), 357–378. https://doi.org/10.1007/s10584-009-9632-0

§ 95706. Carbon Capture, Removal, Utilization and Storage Protocols

Durability differences among pathways should be explicitly addressed.

A central challenge associated with expanding CCUS and CDR pathways is that storage durability varies substantially across technologies. It would therefore be inconsistent to classify all pathways as permanent unless a mechanism is established to create temporal equivalence between storage outcomes.

I recommend the following approach:

- Define permanence on climate-relevant timescales, which are at minimum 10,000 years (Arcusa & Lackner, 2025).
- Continue using permanence certification for pathways that can satisfy this definition.
- For pathways that cannot satisfy this definition, utilize contractual management. A contract period of approximately 25–50 years could be established, with an upfront requirement to finance a future replacement contract (Lackner & Arcusa, 2024). The future contract could involve the same storage pathway, provided it is prepaid, or transition to a pathway that qualifies for permanence certification. This approach, sometimes referred to as horizontal stacking, functions similarly to a retirement savings account for carbon management.
- Avoid discounting temporary storage relative to permanent storage. Discounting approaches are inconsistent with climate science because they equate temporary carbon storage with permanent emissions reductions (Brander & Broekhoff, 2023). They also create intergenerational equity concerns by shifting climate damages into the future (Arcusa & Lackner, 2025).

References:

Arcusa, S. H., & Lackner, K. S. (2025). Carbon sequestration ought to be permanent on climate-relevant timescales. Environmental Science & Policy, 173, 104223. https://doi.org/10.1016/j.envsci.2025.104223

Brander, M., & Broekhoff, D. (2023). Methods that equate temporary carbon storage with permanent CO₂ emission reductions lead to false claims on temperature alignment. Carbon Management, 14(1), 2284714. https://doi.org/10.1080/17583004.2023.2284714

Lackner, K. S., & Arcusa, S. H. (2024). Permanence is cheap – is it really too difficult? Closing the Carbon Loop. Available at: https://open.substack.com/pub/carbonmanagement/p/permanence-is-cheap-is…