Ocean dynamics and biological feedbacks limit the potential of macroalgae carbon dioxide removal

• Published in: Environmental Research Letters
• Main Authors: Berger, Manon, Kwiatkowski, Lester, Ho, David T., Bopp, Laurent
• Other Authors: Laboratoire de Météorologie Dynamique (UMR 8539) (LMD), Institut national des sciences de l'Univers (INSU - CNRS)-École polytechnique (X)-École des Ponts ParisTech (ENPC)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Département des Géosciences - ENS Paris, École normale supérieure - Paris (ENS-PSL), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-École normale supérieure - Paris (ENS-PSL), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL), Nucleus for European Modeling of the Ocean (NEMO R&D ), Laboratoire d'Océanographie et du Climat : Expérimentations et Approches Numériques (LOCEAN), Muséum national d'Histoire naturelle (MNHN)-Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Institut Pierre-Simon-Laplace (IPSL (FR_636)), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Institut national des sciences de l'Univers (INSU - CNRS)-École polytechnique (X)-Centre National d'Études Spatiales Toulouse (CNES)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Université Paris Cité (UPCité)-École normale supérieure - Paris (ENS-PSL), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-École polytechnique (X)-Centre National d'Études Spatiales Toulouse (CNES)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Université Paris Cité (UPCité)-Muséum national d'Histoire naturelle (MNHN)-Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Institut Pierre-Simon-Laplace (IPSL (FR_636)), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-École polytechnique (X)-Centre National d'Études Spatiales Toulouse (CNES)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Université Paris Cité (UPCité), Department of Oceanography Halifax (DO), Dalhousie University Halifax, European Project: 821003,CCiCC, European Project: 820989,H2020-EU.3.5.1.,COMFORT(2019)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2023
• Subjects: carbon dioxide removal; CDR; macroalgae cultivation; air-sea equilibrium; seaweed; kelp; MRV; [SDU]Sciences of the Universe [physics]; Pacific; Southern Ocean
• Online Access: https://insu.hal.science/insu-03993948; https://insu.hal.science/insu-03993948/document; https://insu.hal.science/insu-03993948/file/Berger_2023_Environ._Res._Lett._18_024039.pdf; https://doi.org/10.1088/1748-9326/acb06e

International audience In combination with drastic emission reduction cuts, limiting global warming below 1.5 °C or 2 °C requires atmospheric carbon dioxide removal (CDR) of up to 16 GtCO 2 yr -1 by 2050. Among CDR solutions, ocean afforestation through macroalgae cultivation is considered promising due to high rates of productivity and environmental co-benefits. We modify a high-resolution ocean biogeochemical model to simulate the consumption of dissolved inorganic carbon and macronutrients by idealised macroalgal cultivation in Exclusive Economic Zones. Under imposed macroalgal production of 0.5 PgC yr -1 with no nutrient feedbacks, physicochemical processes are found to limit the enhancement in the ocean carbon sink to 0.39 PgC yr -1 (1.43 GtCO 2 yr -1 ), corresponding to CDR efficiency of 79%. Only 0.22 PgC yr -1 (56%) of this air-sea carbon flux occurs in the regions of macroalgae cultivation, posing potential issues for measurement, reporting, and verification. When additional macronutrient limitations and feedbacks are simulated, the realised macroalgal production rate drops to 0.37 PgC yr -1 and the enhancement in the air-sea carbon flux to 0.21 PgC yr -1 (0.79 GtCO yr -1 ), or 58% of the macroalgal net production. This decrease in CDR efficiency is a consequence of a deepening in the optimum depth of macroalgal production and a reduction in phytoplankton production due to reduced nitrate and phosphate availability. At regional scales, the decrease of phytoplankton productivity can even cause a net reduction in the oceanic carbon sink. Although additional modelling efforts are required, Eastern boundary upwelling systems and regions of the Northeast Pacific and the Southern Ocean are revealed as potentially promising locations for efficient macroalgae-based CDR. Despite the CDR potential of ocean afforestation, our simulations indicate potential negative impacts on marine food webs with reductions in phytoplankton primary production of up to -40 gC m -2 yr -1 in the eastern tropical Pacific.