Radiative Forcing of Carbon Farming practices: Insights into climate impacts of non-permanence, greenhouse gas trade-offs, and biogeophysical effects

The widely used CO2-equivalent metric does not systematically provide an accurate representation of the climate impacts of carbon farming (CF) practices and projects over time, for instance, in the case of non-permanent carbon sequestration or trade-offs between several greenhouse gases (GHG). This report presents a refined climate impact estimate of carbon farming practices, by looking at their mitigation effect over time, accounting for trade-offs in emissions from different GHGs, longevity of projects, and effects beyond changes in GHG emissions, such as changes in albedo. This work is relevant in the context of Monitoring, Reporting and Verification (MRV) methodologies as the applied methodology could further be used to refine the mitigation potential estimate of a project, derive more accurate permanency discounts, or quantitatively inform about the key role of the choice of time horizon in promoting short or long-term mitigation. Key results from this report include: o Non-permanence considerably reduces the climate benefit of soil carbon sequestration. A 15-year project with a 10-year activity period that is not renewed and whose sequestered carbon starts to be re-released generates 4 to 43% less climate benefit than a permanent sequestration by 2050, and 44 to 89% less on a 100-year horizon. On a 100-year horizon and beyond, the percentage loss in climate benefit is larger than “permanency discounts” currently applied in some carbon farming schemes. Temporary sequestration can lower the magnitude of global mean peak warming, but only if its longevity extends beyond the time of global peak warming or if enough carbon is still sequestered at that time. o Even if climate-friendly practices lead to increased N2O emissions alongside carbon sequestration, as is the case with residue management, the net effect tends to remain beneficial for the climate. However, this beneficial effect decreases over time and the practices can generate a warming effect after several centuries. Implementing these practices in combination with N2O management activities, such as a 20% reduction in mineral N fertilizers, can partially balance this loss and provide longer-lasting climate benefits from these practices. o Peatland rewetting provides both short- and long-term mitigation (mostly from emissions reduction) as compared to drained peatlands. Yet, rewetted peatlands have a net warming balance for centuries to millennia, until they eventually become net cooling ecosystems. o Albedo effect from carbon farming practices is overlooked in carbon farming schemes while this effect can be substantial relative to its GHG counterpart. This deliverable presents the core outcomes of Task 4.2: Refined reporting and sink valuation by implementing radiative forcing calculation. Using simple climate models[1], the radiative forcing and global surface mean temperature change generated by carbon farming practices embedded in projects of different longevity have been computed. Targeted audience encompasses decision-makers involved in land-use planning or policy development, as well as MRV methodologies developers. Certification developers that use discount factors to account for non-permanence could use this report to adapt their discount based on a physical accounting. Compensation of agricultural CH4 or N2O emissions by temporary soil C sequestration could be envisioned since this compensation is fairer in terms of climate impacts whereas full compensation of fossil CO2 emissions requires guaranteed project lifetimes far beyond current practice. Further research on the relevance and implications of such compensation would be needed. This report showed that effects on climate change mitigation and adaptation coming from non-GHG effects such as changes in surface albedo are still understudied for carbon farming practices. This is an avenue for future research, which could leverage the co-benefits of some projects (e.g., reduction in local temperature) and acknowledge existing trade-offs in some others. The choice of time horizon for equating climate impacts is a policy-decision. This report objectively quantifies how large of a difference this choice can make on the valuation of temporary sequestration and on the long-term net climate impacts of associated compensation. Temporary certified units for soil C sequestration circumvent the time horizon dilemma for equivalent climate impacts quantification. However, CF projects need to have a certain longevity in order to have a mitigation effect on global peak warming. If not, then renewal of projects or replacement by new projects is key. [1] A broad class of lower dimensional models of the energy balance, radiative transfer, carbon cycle, or a combination of such physical components. (IPCC, 2021)