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

Capturing atmospheric CO2 through ocean afforestation—basin-scale seaweed farming in open ocean regions—has attracted growing scientific and public interest as a potential climate intervention. Despite advocacy for rapid implementation, the climate-relevant CDR potential and side-effects at scale are poorly constrained. While models can explore scenarios, they miss real-world complexities; in situ experiments are limited in scale. Natural analogues provide a low-risk, cost-effective means to evaluate large-scale responses. The Great Atlantic Sargassum Belt (GASB)—recurrent, trans-basin blooms of floating Sargassum across the (sub)tropical North Atlantic since 2011—offers a unique analogue to assess ocean afforestation under real-world conditions. Historically confined to the Sargasso Sea, Sargassum expanded across the Intra-Americas Sea and tropical Atlantic, potentially initiated by an extreme negative NAO phase (2009–2010) and sustained by increased Amazon nutrient runoff. Satellite-detected blooms are seasonal, peaking in summer; in 2018, coverage reached ~6100 km2 distributed along a ~9000 km belt and produced a net build-up of 0.81 Mt particulate organic carbon (POC). However, this apparent CDR potential is modified by biogeochemical feedbacks. This study investigates how calcification by epibionts and nutrient reallocation from phytoplankton alter the theoretical CDR potential, and evaluates verification challenges and albedo effects relevant to the net climatic impact of ocean afforestation.

The study situates ocean afforestation within broader CDR portfolios, noting prior proposals and discussions on seaweed farming for climate mitigation. It draws on analogues where natural perturbations informed climate interventions (e.g., Mount Pinatubo for solar radiation management; glacial-interglacial iron fertilization for marine C cycling). It synthesizes observations of GASB emergence and dynamics (satellite detection, drivers such as NAO shifts and Amazon runoff). Background biogeochemistry includes: CO2 consumption by photosynthesis vs. CO2 release by calcification through reduced alkalinity; typical elemental ratios (C:N:P) in seaweeds vs. phytoplankton; regional nutrient limitation patterns (N primary, occasional P/Fe co-limitation) in the (sub)tropical North Atlantic; low planktonic PIC:POC ratios regionally. Prior work on air-sea CO2 equilibration timescales and mixed-layer residence time frames the verification challenge. Albedo literature highlights contrasting effects of terrestrial afforestation (albedo decrease) vs. marine vegetation (albedo increase), with seagrass albedo observations used as a proxy for surface or near-surface seaweeds. The review underscores that Earth-system feedbacks (biogeochemical and radiative) can strongly modulate apparent CDR benefits.

Natural-analogue quantification of Sargassum biomass (GASB): Monthly mean cumulative Sargassum wet weight for the Caribbean and Central Atlantic (2001–2018) was obtained from NOAA/NCEI satellite products. Wet weight was converted to total particulate carbon (TPC) using a factor of 0.0543 gC per g wet weight. The 2018 growth cycle (Nov 2017–Dec 2018) provides the focal case. Calcification offset estimation: Sargassum hosts calcifying epibionts (e.g., bryozoans), contributing on average 9.4% (range 4.3–21.4%) CaCO3 by wet weight (from Sargasso Sea collections). Assuming CaCO3, PIC mass was derived from wet weights and converted to moles; TPC was split into POC and PIC to compute PIC:POC (mean ~0.265; range ~0.11–0.9 mol:mol). The CO2 released per mol PIC (ψ) was computed with seacarb (R) under subtropical conditions (TA 2350 µmol kg−1, DIC 2047.5 µmol kg−1, S=35, T=25 °C), yielding ψ=0.63. The calcification offset relative to photosynthetic POC was PIC×ψ/POC (~16.5% at 9.4% CaCO3). Nutrient reallocation discount: Nutrient content associated with 2018 Sargassum growth was computed from wet-weight elemental ratios (N: −0.002 g g−1; P: 0.0002 g g−1; Fe: 1.009×10−5 g g−1 adjusted from literature) and converted to Gmol (N, P, Fe). Potential phytoplankton POC supported by reallocated nutrients was estimated assuming regional nutrient limitation and elemental ratios (N-limited with C:N=8; P-limited with C:P=170) and subtracted from seaweed CDR. The Sargassum organic C:N was corrected for PIC (mean 24.8; range 16.5–28.2 depending on CaCO3 %). The saved CO2 from reduced planktonic calcification (plankton PIC:POC ~0.01) was added. Theoretical CDR was defined as CDR_theoretical = POC_seaweed − PIC_seaweed − POC_plankton + PIC_plankton (in Mt C). DOC production: DOC release rates from incubations (288 ± 24 µg C d−1 per g wet weight) were applied to satellite biomass to estimate cumulative DOC production (Nov 2017–Dec 2018), acknowledging substantial uncertainties and that only ~2% of DOC is typically sequestered ≥20 years. DOC was excluded from CDR_theoretical due to verification challenges. Air–sea CO2 equilibration and residence times: Following Jones et al., τCO2 = (h×R)/(G×B) was computed across the GASB using PyCO2SYS with monthly climatologies (TA, pCO2, nutrients, T, S), Argo-based mixed layer depths (1° grid), and ERA5 winds for gas transfer velocity (Wanninkhof 2014). Seasonal and annual means were mapped. Surface mixed-layer residence times (τres) were taken from Jones et al.’s modelled idealized surface age tracer. Ratios τCO2/τres were used to assess verification feasibility. Process-chain offsets for geological storage (BECCS): Shipping emissions for transporting wet biomass were estimated from bulk carrier CO2 intensities (2.7–33.9 g CO2 t−1 km−1), giving 0.0014–0.017% of stored CO2 per t wet weight per km for a representative carbon content (0.0543 tC per t wet weight). CO2 separation efficiency in BECCS (80–90%) was used to apply a 10–20% discount, acknowledging additional unquantified lifecycle emissions. Albedo effect estimation: Reduced radiative forcing due to increased surface albedo (Δα) from seaweed coverage (6093 km2 seasonal peak) was calculated using RFdaily = Qs↓×Δα×(1−atm), with Qs↓ from MERRA-2 (Giovanni portal) at 10°N, 60°W and atm≈0.2. A range of Δα=0.01–0.1 was used based on seagrass analogues and the expectation that floating Sargassum lies at the upper end. Annual integrated albedo-related RF reductions (PJ y−1) were compared to RF reductions from CDR using ΔRF per tC removed (−99 GJ tC−1 y−1 at 410 ppm). Uncertainty bounds: Best- and worst-case bounds for CDR_theoretical were propagated using extreme assumptions: high calcification (21.4% CaCO3; PIC:POC≈0.9), low Sargassum C:N (16), vs. low calcification (4.3% CaCO3; PIC:POC≈0.11), high Sargassum C:N (108), with plankton C:N=8 and PIC:POC=0.01. Translation from molar example (100 mol TPC) to 2018 GASB totals used TPC≈1.02 Mt C.

• Biogeochemical feedbacks significantly reduce theoretical CDR by Sargassum:
  • Calcification by epibionts: Mean PIC:POC ~0.25–0.265 (mol:mol) implies ~16–17% offset of photosynthetic CO2 uptake; plausible range 7–57% across observed CaCO3 fractions (4.3–21.4% wet weight).
  • Nutrient reallocation: Nutrients consumed by Sargassum in 2018 (N=2.7 Gmol; P=0.12 Gmol; Fe=0.003 Gmol) could have supported ~0.26 Mt POC by phytoplankton (assuming N-limitation, C:N=8), reducing Sargassum CDR by ~32% (31% under P-limitation, C:P=170). Saved CO2 from reduced plankton calcification is minor (plankton PIC:POC ~0.01).
• 2018 GASB CDR_theoretical: POC_seaweed=0.81 Mt C; PIC_seaweed=0.13 Mt C; POC_plankton=0.26 Mt C; PIC_plankton=0.002 Mt C; net CDR_theoretical ≈ 0.42 Mt C (420,000 t C).
• Bounds on CDR_theoretical for 2018 GASB: −0.03 to 0.79 Mt C (i.e., could be a small net source or up to ~0.8 Mt C sink), translating to −0.1 to −2.9 Mt CO2.
• Air–sea CO2 equilibration vs. surface residence: τCO2 is 2.5–15 months (mean ~5) in pronounced Sargassum regions; τres is ~0.2–2 months (mean ~0.9). τCO2 is typically 2–46 times longer (mean ~8) than τres across GASB, implying CO2-deficient waters often subduct before equilibration, complicating verification and crediting.
• Albedo effect magnitude: For ~6100 km2 coverage, annual RF reduction from albedo is ~181–1811 PJ y−1 (Δα=0.01–0.1), exceeding RF reduction from 0.42 Mt C CDR_theoretical (~42 PJ y−1) in early years. CDR would surpass albedo effect only after ~4.4–44 seasonal cycles, under optimistic instantaneous atmospheric compensation assumptions.
• Storage pathways: BECCS incurs shipping emissions of ~0.0014–0.017% per t wet weight per km and capture losses of ~10–20%; deep-sea deposition leads to >90% remineralization with return to the surface on centennial–millennial timescales (700–900+ years in the North Atlantic; >1400 years in the North Pacific).
• Potential contribution to global CDR needs: At GASB 2018 scale, contribution is ~0.0001–0.001% of annual CDR required by 2100 in low-emission scenarios; propagation analysis suggests −0.0001 to 0.0029 Gt CO2 possible depending on assumptions.
• DOC production: Estimated ~1.07 Mt C DOC during 2018, exceeding POC build-up, but with only ~2% likely long-term storage (≥20 years) and major monitoring/accounting challenges.

The analysis demonstrates that the apparent CO2 uptake by large-scale Sargassum blooms is substantially reduced by Earth-system feedbacks. Calcification by epibionts partially negates photosynthetic uptake through alkalinity reduction and CO2 release, while nutrient reallocation from phytoplankton diminishes the natural biological pump that would otherwise sequester carbon efficiently in oligotrophic (sub)tropical Atlantic regions. The net theoretical CDR in 2018 (~0.42 Mt C) is highly sensitive to these feedbacks, with bounds spanning small net source to modest sink. Crucially, even when CO2 is fixed into biomass, the disequilibrium-driven atmospheric CO2 invasion into the ocean occurs on longer timescales than the surface residence time of CO2-depleted waters, leading to spatial and temporal decoupling between fixation and air-sea uptake. This complicates accountability and independent verification required for participation in carbon markets. Storage permanence further constrains net benefits: BECCS introduces process-chain emissions and capture inefficiencies, while deep-ocean deposition offers long isolation times but significant remineralization and difficult leakage monitoring. Beyond carbon, afforestation-induced albedo increases can have a larger short-term cooling effect than CDR at the examined scale, implying that ocean afforestation should also be evaluated within a solar radiation management context, though current carbon markets do not account for albedo benefits. Overall, results indicate that the efficacy and sign of ocean afforestation’s net climatic effect are controlled by multifaceted, location- and practice-dependent feedbacks, necessitating rigorous, scalable monitoring frameworks and governance on verification and permanence to assess viability.

Using the Great Atlantic Sargassum Belt as a natural analogue, this study provides quantitative constraints on the theoretical CDR potential of ocean afforestation and the magnitude of key feedbacks. Biogeochemical feedbacks—calcification and nutrient reallocation—can reduce CDR by 20–100%, yielding a 2018 net CDR_theoretical of ~0.42 Mt C with bounds from −0.03 to 0.79 Mt C. Air–sea equilibration lags and storage-permanence issues pose major verification challenges, and the direct albedo effect may outweigh initial CDR-induced radiative forcing reductions at GASB scale. These findings imply that, at least at the scales examined, ocean afforestation could contribute only a minute fraction of the global CDR required this century and may even act as a net source under certain conditions. Future research should: (1) better quantify epibiont calcification across species, growth rates, and environments; (2) constrain nutrient sourcing and reallocation impacts, including potential use of external nutrient supplies; (3) develop and demonstrate scalable methods to verify air–sea CO2 uptake and track storage permanence; (4) refine albedo impact estimates, including indirect aerosol-mediated effects; (5) perform full lifecycle assessments of harvesting, transport, and BECCS pathways; and (6) establish governance frameworks for verification and permanence. Given complexity and uncertainties, prioritizing tractable, verifiable, and durable CDR approaches, potentially with more engineered, abiotic solutions, may be warranted while continuing to investigate marine biological options.

• Epibiont calcification fractions (4.3–21.4% CaCO3 wet weight) are derived from Sargasso Sea samples and may not represent GASB conditions or other seaweed taxa; MgCO3 contributions were neglected.
• DOC production estimates rely on limited temporal sampling (September conditions) and assume uniform rates across seasons and environments; long-term DOC sequestration fraction (~2%) is generalized.
• Air–sea equilibration calculations depend on climatologies, parameterizations (e.g., Wanninkhof 2014 gas transfer), wind products (ERA5), and mixed layer depth estimates, introducing uncertainties; regional data gaps (e.g., Intra-Americas Sea) exist.
• Nutrient content and limitation assumptions (N vs. P) may vary spatiotemporally; Fe estimates use ratios from a different Sargassum species/region.
• Albedo estimates use seagrass as proxy for seaweed Δα and do not include indirect aerosol/cloud feedbacks; depth and farm design strongly influence Δα.
• Storage offsets consider only selected steps (shipping, capture efficiency); broader lifecycle emissions (harvesting, drying, handling, CO2 transport/storage) were not fully quantified.
• Extrapolation from a single large bloom year (2018) to broader scales is uncertain; satellite detection may miss small/submerged rafts, affecting biomass estimates.