Climate impact of CO2 removals in carbon farming: sequestration durability and implications for compensation

Carbon dioxide removal via soil carbon sequestration is proposed as contribution to climate mitigation, as well as for compensation. The average storage durability of sequestered carbon in soil systems is uncertain, yet shorter than the adjustment time of CO₂ in the atmosphere. In this study, we use the reduced-complexity climate model FaIR v2.1.0 (Finite Amplitude Impulse Response model) to quantify by how much the climate impact of carbon removals changes based on sequestration durability and time horizon (time until which the climate impacts are accounted for). We quantify the climate impact via cumulative radiative forcing, namely the well-established absolute global warming potential (AGWP) metric. We use simplified scenarios with five years of soil organic carbon accrual at a constant rate mimicking the use of cover crops, followed by 20 to 1000 years hold time before re-release of the sequestered carbon. We show that the percentage of climate benefit achieved by temporary carbon removals relative to permanent removals increases near-linearly with longer sequestration durability. However, this percentage and its increase rate also vary with chosen time horizon. The climate benefit of short-term removals diminishes rapidly with time horizon. For example, temporary removals with 20 years of durability before sudden re-release offer only about 15% of the climate benefit of permanent sequestration for a 100-year horizon, dropping below 5% for horizons longer than 400 years. For a sequestration that lasts 100 years, the full climate benefit is maintained for a 100-year horizon but still drops to less than 20% of the climate benefit of permanent sequestration for horizons longer than 400 years. These findings have significant implications with regards to compensation units: the amount of anthropogenic CO₂ emissions compensated (based on AGWP equivalence) by a given temporary removal declines rapidly as we look further in time, i.e., as time horizon increases. For sequestration durations of 100 years or less, this amount drops by over 90% between horizon 2100 and a millennial horizon, whether the carbon is released abruptly or progressively following a 30-year decay pattern. Our results highlight the key role of the storage duration of carbon in soil systems on the climate impact of soil carbon sequestration over time. In the context of compensation and climate mitigation targets, we stress the need to have a physics-based accounting of the large climatic drawbacks of temporary removals relative to permanent ones. The difference in AGWP can be used for such accounting. In addition, potential drawbacks on the temperature pathway should not be overlooked.