The urgent need to mitigate climate change necessitates rapid reductions in CO2 emissions and substantial atmospheric CO2 removal (CDR). Ocean afforestation, involving large-scale seaweed farming, is considered a promising marine CDR method. However, the potential and side effects of upscaled ocean afforestation remain largely unknown. While model simulations offer a preliminary assessment, they lack the complexity of real-world systems. In-situ experiments are crucial, yet logistically challenging at the required scale. This study leverages the Great Atlantic Sargassum Belt (GASB), a naturally occurring, large-scale seaweed bloom, as a valuable natural analogue to evaluate the potential and challenges of ocean afforestation. The GASB, although not reaching the maximum extent envisioned for climate-relevant ocean afforestation, offers a unique opportunity to study large-scale ocean afforestation under real-world conditions before its wider application. The GASB, an unprecedented phenomenon involving large-scale floating seaweed (*Sargassum*) blooms in the subtropical North Atlantic, presents a unique opportunity to study large-scale ocean afforestation. Its emergence, potentially linked to altered wind patterns and increased nutrient runoff, provides a real-world system to analyze the complexities and limitations associated with proposed large-scale seaweed farming for CDR.

Existing literature highlights the potential of seaweed farming for CO2 mitigation and adaptation (Duarte et al., 2017; Ritschard, 1992; Flannery, 2017; N’Yeurt et al., 2012; Smetacek & Zingone, 2013). Policymakers and stakeholders are advocating for rapid implementation (Warren, 2020; Doumeizel et al., 2020), yet knowledge about the upscaled CDR potential and side effects remains limited. Previous studies using model simulations (Orr & Sarmiento, 1992; Chung et al., 2013) have explored these aspects, but their limitations in capturing real-world complexities are acknowledged. The study draws parallels to previous research utilizing natural analogues in climate intervention, such as the Mount Pinatubo eruption (Crutzen, 2006) and glacial-interglacial ocean iron fertilization (Martínez-García et al., 2014), which yielded valuable insights into Earth System responses.

The study uses the 2018 GASB as a case study. Data on Sargassum biomass from satellite observations (Wang et al., 2019; Gower & King, 2019; Wang et al., 2019) were used to estimate particulate organic carbon (POC) and particulate inorganic carbon (PIC). The impact of biogeochemical feedbacks was evaluated by considering calcification by epibionts on Sargassum and nutrient reallocation from phytoplankton. Calcification offsets were calculated using the PIC:POC ratio (Pestana, 1985) and the CO2 released per mol PIC (Frankignoulle et al., 1994). Nutrient reallocation effects were estimated based on nutrient uptake by Sargassum and the potential phytoplankton POC production with the reallocated nutrients (Moore et al., 2013; Martiny et al., 2013a; Martiny et al., 2013b; Devries et al., 2012; Sarmiento & Gruber, 2006; Poulton et al., 2006; Sarmiento et al., 2002). DOC production was estimated from (Powers et al., 2019) with considerations for remineralization (Hansell, 2013). Air-sea CO2 equilibration timescales were calculated following (Jones et al., 2014) using observational data (Takahashi et al., 2014; de Boyer Montégut et al., 2004; Roemmich & Gilson, 2009) and ERA5 wind data. The study accounted for potential CO2 offsets associated with biomass harvesting, transportation, and processing for geological storage (IPCC, 2005; Baker et al., 2018; DeVries & Holzer, 2019). Albedo modification by Sargassum was estimated using equations from (Betts, 2000; Kirschbaum et al., 2011; Fogarty et al., 2018), considering albedo changes and the resulting radiative forcing (Myhre et al., 2013; Trenberth et al., 2009; Payne, 1972). The study also analyzed the data from the International Institute of Applied Systems Analysis (IIASA) to determine the amount of CO2 removal needed to limit global warming (Fuhrman et al., 2019; Bastin et al., 2019).

The study found that biogeochemical feedbacks significantly reduce the CDR potential of ocean afforestation. Calcification by epibionts on Sargassum reduced CO2 removal by 7-57%, averaging approximately 17%. Nutrient reallocation from phytoplankton to Sargassum reduced CDR by 31-32%. Considering both feedbacks, the theoretical CDR potential of the 2018 GASB ranged from -0.03 to 0.8 Mt C. The air-sea CO2 equilibration timescale was significantly longer than the seawater residence time in the surface mixed layer, potentially hindering rapid atmospheric CO2 uptake. This temporal mismatch between CO2 fixation and atmospheric CO2 replenishment poses a substantial challenge to verifying CDR. The study estimated that the increase in ocean albedo due to Sargassum could exceed the radiative forcing reduction from CDR, highlighting the importance of considering albedo effects in assessing the overall climatic impact of ocean afforestation. The analysis demonstrated that even under optimal conditions, ocean afforestation at the GASB scale may only contribute a minuscule fraction (0.0001-0.001%) to the annual CDR needed to meet a 2°C warming target.

The findings highlight the complexities of Earth-system feedbacks in assessing the efficacy of ocean afforestation for CDR. The significant reductions in CDR potential due to biogeochemical processes emphasize the need for a more comprehensive understanding of these interactions. The long equilibration timescales indicate a substantial delay in atmospheric CO2 uptake, posing a challenge for accounting and verification in carbon trading systems. The potential dominance of albedo effects underscores the need to include albedo changes in future assessments of climate intervention strategies. The study's results suggest a need for caution and further investigation before considering ocean afforestation as a significant CDR strategy.

The analysis of the GASB as a natural analogue reveals that the climate impact of ocean afforestation is subject to considerable uncertainty due to complex Earth-system feedbacks. The significant influence of biogeochemical feedbacks and albedo effects necessitates a more nuanced approach to assessing the net climatic impact. Addressing challenges related to CO2 uptake verification and the permanence of carbon storage remains crucial for the successful implementation of ocean afforestation as a climate intervention strategy. Future research should focus on refining the quantification of these feedbacks under various conditions and exploring potential mitigation strategies.

The study's analysis is based on the 2018 GASB event, which may not be fully representative of all potential ocean afforestation scenarios. The assumptions made regarding the PIC:POC ratio, nutrient reallocation, and albedo changes introduce uncertainty into the estimates. The model used for calculating equilibration timescales relies on certain assumptions about wind speed and mixed layer depth, which could affect the results. Future research needs to address these uncertainties using further data and experimental validations.