Authors: Rick Berg and Radhika Moolgavkar
The durability of sequestered carbon, and credits designed to represent it, has gotten a lot of focus in discussions around the voluntary carbon dioxide removal (CDR) markets and emerging mandatory carbon markets. This attention is well-deserved. Durability characteristics play a key role in the functioning of the markets, including how the carbon removal credits are used, the development of carbon insurance, the management of carbon credit portfolio risks, and the creation of credit ratings.
While the integrity of sequestration for CDR activities must be monitored over time, the processes of monitoring, reporting, and verification (MRV) alone cannot ensure the integrity of a CDR credit or the claims they support. Crediting methodologies must incorporate additional mechanisms to manage durability, creating high-integrity credits that foster market confidence.
But first, what exactly is durability? The concept encompasses both the longevity of carbon storage, often referred to as "permanence," and the potential for the stored carbon to be re-released into the atmosphere, known as "reversal risk"1-3. While those concepts are independently important, a unified definition of durability can provide a useful tool to improve evaluations of carbon removals4.
Rigorously defining durability is not just an academic exercise but a practical necessity for carbon market participants.
- Carbon credit buyers can better understand credits’ abilities to meet the goals of their carbon credit portfolios, such as managing risk or optimizing for impact over various timescales.
- Carbon removal ratings agencies can more transparently compare and evaluate the durability characteristics of credits across removal types and crediting methodologies.
- Carbon insurance companies can more fully assess reversal risk over time, design more robust insurance mechanisms, and better match replacement credits across similarity characteristics.
- Registries and methodology developers can improve the durability assurance mechanisms they employ and better design buffer pool mechanics.
Understanding Durability Through the Durability Curve
To contribute to articulating the concept [of durability] more succinctly4, Nori is introducing the concept of the “durability curve”. The durability curve describes durability in a form that unifies the concepts of permanence and reversal risk, and can be used for evaluation of carbon credits in a way that can better inform buyers, ratings agencies, portfolio developers, consultants, and methodology developers.
The durability curve is the conditional probability5 that a given amount of CDR remains sequestered over time.
At the credit level, it is the conditional probability at each point on a timeline of a ‘yes’ or a ‘no’ answer to the question of whether a given credit represents the claim of removal and continued sequestration of at least one full tonne of CO2. An important distinction to note: the curve does not represent the amount of carbon sequestered over time. Although the curve may approximate a minimum amount of sequestered carbon when constructed based on a large collection of credits from different (non-correlated) projects with similar durability curve profiles, that is not its intended purpose or primary utility.
To illustrate the durability curve concept, we present a conceptualized curve for soil organic carbon sequestration via regenerative farming in Figure 1, with a temporary permanence (typically 10-20 years).
By framing durability in terms of probabilities over time rather than the amount of carbon stored over time, the durability curve demonstrates the relationship between the elements of permanence, additionality, reversal risk, and the concept of a ‘real’6 credit.
A threshold of probability must be set by carbon credit methodology developers to determine when a removal credit is considered ‘real’. Given the non-deterministic nature of the measurements and baselining involved in all CDR activities, a 100% certainty is not achievable. The period of time a CDR credit remains above the set threshold (i.e. remains real) is the permanence of the credit. In this example, a 90% probability threshold is approximately in alignment with approaches that aim for 90% certainty in quantification, at the market level.
Additionality for carbon removal is the quantification of accumulated carbon exceeding a baseline counterfactual, a hypothetical alternate reality where the project doesn’t exist. Dynamic baselining – an alternative scenario that accounts for evolving conditions – can help ensure that the baseline is as realistic as possible over time. Dynamic baselining is particularly important for open-system CDR activities, which have at least a portion of the process subject to uncontrolled natural environments. For the durability curve, this counterfactual scenario defines the zero-probability baseline because, by definition, no additional carbon was sequestered in that scenario.
But what about reversal risk? If we flip the durability curve upside down, as in Figure 2, the curve becomes the cumulative reversal risk over time.
In more technical terms, the accumulated reversal risk is the inverse of the probability of sequestration, so the cumulative reversal risk over time can be derived from the durability curve simply by inverting the y-axis after t=0 (after sequestration). A reversal in this case means that the credit no longer represents the full amount of its stated sequestration, and the curve represents the probability that a reversal has occurred.
This framing as cumulative reversal risk over time also offers an established way to build the curve for a given CDR activity – using the tools within the field of risk assessment.
How Crediting Methodologies Ensure Carbon Removal Durability — A Use-Case Example
As we delve into the mechanisms that underpin the durability of carbon removal, it's essential to understand the pivotal role crediting methodologies play in ensuring the long-term integrity of carbon sequestration efforts. These methodologies are not just procedural frameworks, they are the backbone of trust and confidence in the carbon market. In addition to dictating the MRV, legal requirements, and environmental and social safeguards, they incorporate mechanisms designed to address and mitigate the risk of carbon re-release into the atmosphere.
The construction of the durability curve in the sequence below illustrates the effect of crediting methodologies, highlighting how mechanisms within methodologies can help ensure that every tonne of CO2 sequestered today remains out of the atmosphere for the intended duration, robustly contributing to climate mitigation efforts.
- Raw quantification without methodological adjustments (measurement-only) - Given the uncertainties of quantifying soil carbon (or any CDR activity), there is initially a conventionally applied 50% probability that the full amount quantified is accurate7. And without durability assurance mechanisms in place, the probability of sequestration falls rapidly, primarily due to the risks of changes to land management practices.
- Uncertainty deduction - Nori’s methodology reduces the issued credits to account for uncertainty in the quantified carbon removal. This uncertainty adjustment increases the probability that a given Regenerative Tonne represents at least a full tonne of CO2 sequestered. The amount of credits deducted will scale relative to the magnitude of the uncertainties in the quantification
- Contractual period - Nori’s supplier contract requires a legally binding commitment to continuing with regenerative practices for at least 10 years. This increases the probability that the practices will continue during that time, though the risk of breach of contract remains.
- Verifications - Nori currently requires verification of practices and adverse field events at years 0, 3, 6, 9, and 10. This acts to increase the conditional probability of sequestration back to its highest value after each verification is completed. The risks of reversals from natural disasters (floods, wind, storms, etc.) remain.
Another aspect that becomes clear after constructing the durability curve is that there is a non-zero probability of continued sequestration after the permanence period. This period of continued sequestration is a “residual benefit” that is not taken into account for crediting or any subsequent carbon accounting. This means that, given a large number of soil credits issued to a diversity of farms with similar durability profiles, the residual benefit represents carbon that remains sequestered long after the permanence period.
This residual benefit may be written off as a difficult-to-quantify, unguaranteed sequestration for any given project. But it is nevertheless a beneficial aspect of “temporary” carbon removals that should be considered at the market level and policy level. Rigorously constructed durability profiles for temporary carbon removals that include residual benefit can support improved modeling of climate benefits8,9. These improved models can then further inform carbon accounting, policy making, and risk management.
Besides the durability assurance mechanisms illustrated above, additional mechanisms can be envisioned to mitigate reversal risk and increase expected durability of a credited carbon removal. As described by Arcusi & Hagood10, these include governance mechanisms such as transfer of liability, and reversal management mechanisms such as buffer pools. Certain durability assurance mechanisms are more (or less) appropriate for a given CDR activity, and should be considered when designing a crediting methodology to robustly manage durability.
Beyond improving durability of a credit using methodological mechanisms, additional tools are available to ensure the durability of a carbon removal claim, which credits underpin. Carbon insurance, for example, is a mechanism that can be implemented to improve the durability of a carbon removal claim – rather than of a specific credit – as some insurance mechanisms may replace a defunct credit with a new credit, or pay out cash to buy replacement credits. Another mechanism to improve the durability of claims exists through combining credits from various CDR activities via “vertical stacking” or “horizontal stacking”1, such as is done for the Nori Net Zero Tonne – which may be a good topic for a future post.
Durability Insights Across CDR Activities
High-durability CDR activities such as direct air capture with geologic storage, biochar, and enhanced rock weathering will have different shapes to the curve, but the concept is universal and can be used to evaluate any CDR activity. Figure 8 illustrates the conceptual durability profile for DAC (with geologic storage) after crediting has been adjusted for uncertainty and a monitoring and verification process has been established. And Figure 9 illustrates the curve for biochar (applied to agricultural fields and credited through a 100-year methodology) after adjustment for both uncertainty and for the expected natural breakdown of the degradable biochar fraction.
For DAC with geologic storage, the figure illustrates a ~40 year operational and post-operational monitoring period during which active monitoring and verification act to increase the conditional probability of credit integrity. These verifications are particularly important during the greater potential risk periods of initial project implementation and later decommissioning. After the monitoring period and the CO2 has been stabilized, there remains a slight increase in cumulative risk over time due to natural processes or catastrophic events. For the biochar curve, the remaining risk represented in the curve is from the various fractions of biochar that degrade over various timescales.
Understanding and enhancing the durability of CDR credits of all types is crucial for the integrity and efficacy of both voluntary and mandatory carbon markets. The concept of the "durability curve" offers one potential way to assess and manage the permanence and reversal risk of sequestered carbon and associated claims, addressing a critical need in the industry for robust, science-based approaches to carbon crediting.
We encourage all stakeholders to contribute to the ongoing dialogue around improving carbon removal methodologies. By collaboratively refining our approach to durability and reversal risk mitigation, we can help ensure that our efforts to mitigate climate change are effective and enduring.
Acknowledgments
The authors would like to acknowledge the following individuals for their contributions in clarifying and refining the ideas presented in this article:
- Tom Merriman, Kita
- Matt Trudeau, Nori
- Ross Kenyon, Nori
- Shuting Zhai, Nori
References and Footnotes
- Carbon Direct notes that “Durability can refer to either the planned duration of carbon storage or the risk of reversal before that time is up.” Bodie Cabiyo, Alex Dolginow: Accounting for Short-Term Durability in Carbon Offsetting. Carbon Direct, Feb 28, 2022. https://www.carbon-direct.com/insights/accounting-for-short-term-durability-in-carbon-offsetting
- Rocky Mountain Institute has stated, “Durability (often used interchangeably with permanence) describes how long a project is expected to reduce or remove carbon, or the likelihood that the emissions reduction or removal will be reversed.” Aijing Li, Raymond Song, Caroline Ott: Can We Count on Forest Carbon Credits? Rocky Mountain Institute, October 2022. https://rmi.org/can-we-count-on-forest-carbon-credits/
- BeZero has gone into more detail, describing the nuances of the term as distinct from permanence and reversal risk: “Durability refers to the robustness of carbon storage. Durability is frequently used interchangeably with permanence to define this storage component. But there is an important difference between the two. Permanence is a finite characteristic - carbon dioxide is either stored or it is not. Durability is a characteristic subject to change. Durability recognises that the storage component is influenced by external factors, whether human or natural.” Victoria Harvey, Ted Christie-Miller: Durability Assessment for Carbon Removal. BeZero Carbon, April 5, 2023. https://bezerocarbon.com/insights/durability-assessment-for-carbon-removal-introduction
- An analysis from the Global Carbon Removal Partnership came to the conclusion that “As for durability, the community must articulate the concept more succinctly. This may mean moving away from conceptualizing “permanence” as “how long of a storage duration should the certification instrument represent” to “what are the systems to address the risk of reversal.” Understanding, defining, and managing durability is a scientific endeavor and a business practice.” Stéphanie Arcusa, Starry Sprenkle-Hyppolite, Aditya Agrawal: Addressing Open Questions in the Development of Standards for the Certification of Carbon Removal Critical Insights from an International Consultation Process. Global Carbon Removal Partnership, Thunderbird School of Global Management, Global Futures Laboratory, & Conservation International, November 9, 2022. https://keep.lib.asu.edu/items/170838
- Conditional probability is the probability of an event given that another event has occurred, incorporating new evidence into the calculation. In this case it is the probability that a stated amount of CO2 remains sequestered, given all the latest information known by the market. For example, the probability of continued soil carbon sequestration can decrease over time due to known risks of catastrophic weather events, but then that probability would increase if it is verified that no catastrophic weather events have occurred.
- ICROA Carbon Crediting Programme Endorsement Criteria, Version 3.0, ICROA, November 2023. https://icroa.org/wp-content/uploads/2024/03/Programme-Endorsement-Review-Criteria.pdf
- The beginning probability for any quantification technique is ~50% since there is an uncertainty associated with any measurement, LCA calculation, or baseline estimate. Given a symmetrical distribution of uncertainty, where the mean = median, the quantification lands in the middle of the uncertainty probability distribution, meaning there is a 50% chance that the actual amount is that number or greater. This percentage may vary depending on the deviation of the uncertainty distribution from a symmetrical distribution.
- H. Damon Matthews, Kirsten Zickfeld, Alexander Koch, Amy Luers: Accounting for the climate benefit of temporary carbon storage in nature. Nature Communications, September 2023. https://www.nature.com/articles/s41467-023-41242-5
- Jens Leifeld, Sonja G. Keel: Quantifying negative radiative forcing of non-permanent and permanent soil carbon sinks. Geoderma, October 2022. https://www.sciencedirect.com/science/article/pii/S0016706122002786
- Stéphanie Arcusa, Emily Hagood: Definitions and mechanisms for managing durability and reversals in standards and procurers of carbon dioxide removal. 2023. https://osf.io/preprints/osf/6bth5