Accounting for short-term durability in carbon offsetting

Durability, the length of time carbon dioxide is stored before potential re-release to the atmosphere, is a critical yet underdeveloped dimension of carbon offset quality. While the voluntary carbon market trades both short-term and long-term carbon storage as fungible “one tonne” credits, the climate implications of these two classes of removal differ fundamentally. In this commentary, we examine the conceptual and practical challenges of accounting for short-term durability in carbon offsetting. We distinguish between two strategies for reconciling temporary storage with robust neutralization claims: horizontal stacking, in which credits are sequentially renewed as they expire, and vertical stacking, in which multiple short-term credits are purchased upfront to approximate the climate benefit of permanent storage. We evaluate the assumptions, trade-offs, and market constraints associated with each approach and argue that transparent, science-grounded frameworks for durability accounting are essential to the integrity of corporate net-zero claims. We conclude by recommending portfolio-based approaches that pair near-term, high-quality temporary removals with investments in durable engineered storage.