Definitions and mechanisms for managing durability and reversals in standards and procurers of carbon dioxide removal

What should be the definition and implementation of durable carbon storage? As carbon markets are expanding to include various forms of Carbon Dioxide Removal (CDR), the time is right to revisit this topic in relation to standard development. Particularly, we look for mechanisms that could be implemented across CDR methods so that certification guarantees sequestration on climate-relevant timescales while preserving the collective sequestration effort. Collecting data from 29 international standard developing organizations and one large carbon removal purchaser, we review the different definitions and mechanisms proposed to implement durability and manage reversals. We find the most often used stipulated contractual definition of durable storage by standard developing organizations is 100 years but ranges from 10 to beyond 100 years. Six different durability mechanisms are employed for various goals, which may not be intended for or aligned with climate mitigation purposes. Even with four different types of reversal management mechanisms, we see that most organizations are choosing mechanisms that are supportive of preserving the collective sequestration effort. However, our analysis also shows no single durability mechanism will work across the full range of CDR methods nor for the goal of keeping carbon sequestered on climate-relevant timescales. Our results highlight that complications arise when including short-term sequestration into a climate-relevant certification system and when attempting to safely end the responsibility of monitoring. Definitions of durable storage are being pushed beyond what can be managed contractually. These complications make innovation necessary and could be addressed using a combination of existing and improved mechanisms tailored to specific implementations.