Fig. 1: Framework to assess the additional carbon sequestration (C_additional) with ocean afforestation.

The framework considers predominant carbon pools and sequestration pathways of a baseline ecosystem driven by phytoplankton (upper illustration). Carbon pools and sequestration pathways are for example particulate organic carbon (POC), dissolved organic carbon (DOC), carbon export (all three are CO₂ sinks) and particulate inorganic carbon (PIC, a CO₂ source by reducing seawater alkalinity by a factor psi¹⁸). For ocean afforestation (lower illustration), a proportion of the nutrient inventory, which controls the magnitude of all carbon pools and fluxes, are diverted to the corresponding seaweed carbon pools and fluxes. This reduction of a former C-sink has to be considered for the calculation of how much ocean afforestation can add to the marine biotic carbon sink as shown in equations at the bottom. These equations are an extended version of Eq. 1 in Bach et al.² to account for comments by Wang et al.¹. The equations include simplifications, for example that different pools/fluxes sequester carbon for different timescales. Also, it assumes linearity between the baseline and the ocean afforestation scenario although the implementation of ocean afforestation may have synergistic/antagonistic effects on phytoplankton carbon sequestration. Consideration of these and other variables will further complicate the calculation of additionality.