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 Sargassum 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 Sargassum by 20–100%. Atmospheric CO₂ influx into the surface seawater, after CO₂-fixation by Sargassum, 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 Sargassum, could influence climate radiative forcing more than Sargassum-CDR. Our analysis shows that multifaceted Earth-system feedbacks determine the efficacy of ocean afforestation.

确保全球变暖保持在<2 °C 需要快速减少CO ₂排放。此外，到 2100 年，必须使用一系列 CO ₂去除 (CDR) 方法从大气中去除 100–900 吉吨 CO ₂ 。海洋造林，即通过在公海进行流域规模海藻养殖的 CDR，被视为海洋投资组合的关键组成部分。在这里，我们分析了北大西洋（亚）热带地区最近重新出现的漂浮海藻马尾藻跨流域带的 CDR 潜力，作为海洋造林的自然模拟。我们发现，两种生物地球化学反馈，即养分重新分配和海洋生物结壳的钙化，使马尾藻的 CDR 功效降低了 20-100%。在马尾藻固定 CO ₂后，大气中的 CO ₂流入表层海水的时间比缺乏 CO ₂的海水与大气接触的时间长 2.5-18 倍，这可能会阻碍 CDR 验证。此外，我们估计由于漂浮的马尾藻而增加的海洋反照率可能比马尾藻-CDR 对气候辐射强迫的影响更大。我们的分析表明，多方面的地球系统反馈决定了海洋造林的有效性。