Efficacy of seaweed-based carbon dioxide removal reduced by iron limitation and nutrient competition with phytoplankton

Carbon dioxide removal (CDR) is a crucial component of climate change mitigation strategies, and ocean afforestation via seaweed cultivation has been touted as a promising marine CDR (mCDR) approach due to high productivity and favorable carbon-to-nutrient ratios. However, global mCDR models generally overlook iron limitation, a potential bottleneck for sustainable seaweed cultivation. While competition with phytoplankton for nutrients could even reduce ocean carbon uptake. Here we assess the potential for this unintended consequence using an ocean biogeochemical model. We find that iron limitation reduces afforestation potential three-fold after already accounting for N and P limitation. Variations in nutrient dynamics contribute to substantial uncertainty in projections of CDR efficiency, with global CDR efficiency ranging from -43% to +78%. This study underscores the need for iron dynamics to be included in projections of ocean afforestation. Failing to account for such nutrient dynamics risks overestimating the efficacy of seaweed-based CDR as a mitigation strategy.