Testing the climate intervention potential of ocean afforestation using the Great Atlantic Sargassum Belt

<p>Ensuring that global warming remains <2 °C requires rapid CO₂ emissions reduction. Additionally, 100–900 gigatons CO₂ must be removed from the atmosphere by 2100 using a portfolio of CO₂ removal (CDR) methods. Ocean afforestation, CDR through basin-scale seaweed farming in the open ocean, is seen as a key component of the marine portfolio. Here, we analyse the CDR potential of recent re-occurring trans-basin belts of the floating seaweed <i>Sargassum</i> in the (sub)tropical North Atlantic as a natural analogue for ocean afforestation. We show that two biogeochemical feedbacks, nutrient reallocation and calcification by encrusting marine life, reduce the CDR efficacy of <i>Sargassum</i> by 20–100%. Atmospheric CO₂ influx into the surface seawater, after CO₂-fixation by <i>Sargassum</i>, takes 2.5–18 times longer than the CO₂-deficient seawater remains in contact with the atmosphere, potentially hindering CDR verification. Furthermore, we estimate that increased ocean albedo, due to floating <i>Sargassum</i>, could influence climate radiative forcing more than <i>Sargassum</i>-CDR. Our analysis shows that multifaceted Earth-system feedbacks determine the efficacy of ocean afforestation.</p>