Cost of cooling: The value of reversible carbon storage in a zero-emissions world

Atmospheric carbon dioxide removal (CDR) is required to stabilize global temperature and can be achieved via ecosystem (e.g., soil and forest management) and geological (e.g., direct air capture) carbon storage. Ecosystem strategies are scalable and cost-effective but reversible, making their long-term impact on climate mitigation uncertain. Geological storage is permanent but currently expensive. This paper examines trade-offs between these approaches, focusing on timing, contract structures, and cost. Using agricultural soil management—specifically cover crops—as a case study, we simulated reversible soil carbon accrual for a range of CDR contract structures using a simplified biogeochemical model. We then quantified the resulting impact on atmospheric carbon and global temperature using a climate model emulator. We find that maintaining a patchwork of temporary CDR projects by replacing lapsed projects with new projects can reduce warming and that the magnitude of this cooling effect depends on how successfully the patchwork is maintained. Long term maintenance of temporary CDR projects requires institutional stability that cannot be guaranteed over multiple decades. Consequently, effective CDR ultimately requires replacing temporary projects with permanent projects. To address this problem, we modeled the cost of replacing temporary ecosystem CDR with geologic CDR. We found that using temporary CDR as a bridge to permanent CDR is potentially more cost effective as a global cooling strategy than perpetual maintenance of temporary CDR or an immediate transition to permanent CDR. However, we emphasize that institutional commitments to maintain temporary CDR projects are reversible. Reliance on temporary CDR as a bridge to permanent CDR therefore carries an unknown amount of risk and will only function if efforts to maintain temporary CDR are robust.