Quantifying negative radiative forcing of non-permanent and permanent soil carbon sinks

Reversibility of soil carbon sinks is a major obstacle in assigning soil carbon sequestration as negative emission technology and it is still unclear how a non-permanent CO2 removal shall be accounted for. In this study, we combine various scenarios of reversible and non-reversible soil carbon sinks with atmospheric CO2 impulse response functions and calculations of the resulting radiative forcing. A time horizon of up to 500 years was considered. Results show that any soil carbon sink generates negative radiative forcing (i.e., cooling) when aggregated over longer time scales. Whereas also non-permanent CO2 removals from the atmosphere provide negative average radiative forcing, their effect is substantially smaller than that of permanent removals of the same magnitude. We show that the average annual soil organic carbon balance over the integrated time window largely determines the average radiative forcing independently of rates of carbon gain or loss and longevity of the sink. This basic principle allows an unbiased assessment, comparison, and rating of mitigation projects that take advantage of soil carbon. The suggested approach is based on quantitative and relatively simple metrics and may therefore support guidance to climate policies and soil carbon markets.

Automated summary

This study examines the impact of soil carbon sequestration on radiative forcing by combining various scenarios of reversible and non-reversible soil carbon sinks with atmospheric CO2 impulse response functions. The results show that any soil carbon sink generates negative radiative forcing, but non-permanent CO2 removal has a smaller effect compared to permanent removal of the same magnitude.