International climate agreements aim to reduce CO₂ emissions, but a gap remains between current trajectories and the reductions needed to limit warming to 2°C. Carbon dioxide removal (CDR) technologies are essential to offset residual emissions, particularly from hard-to-decarbonize sectors. The ocean’s vast capacity to absorb CO₂ offers a key opportunity for marine CDR (mCDR), especially through seaweed cultivation in coastal regions like Exclusive Economic Zones (EEZs). Seaweed farming is promising due to its high productivity and established agricultural practices, but scaling to climate-relevant levels poses challenges related to long-term carbon sequestration and ecosystem impacts. This PhD explores seaweed cultivation as a CDR strategy using the NEMO-PISCES ocean biogeochemical model. Simulations show that seaweed CDR potential is limited by dynamical and geochemical processes, with only partial air-sea carbon equilibration. Biological constraints, including nutrient competition with phytoplankton, further reduce potential.

Additionally, accounting for seaweed’s iron utilization lowers afforestation potential, suppresses phytoplankton production, and reduces CDR estimates. We show that nutrient affinity and demand significantly impact these projections. As CDR will be critical for future climate mitigation, we assess the effects of rising atmospheric CO₂ and climate change on seaweed CDR. Results indicate potential increases at high latitudes due to enhanced stratification and CO₂ concentrations, while other regions may see declines. These findings highlight the importance of accurately representing seaweed physiology and broader biogeochemical processes in future CDR assessments.

Lieu
École normale supérieure – PSL
24 rue Lhomond – aile Erasme
Salle Claude Froidevaux – E314

Visio
https://cnrs.zoom.us/j/92344695910?pwd=1GoT9nJYGbZwtPf86hEH1xrfcIjasT.1

• Sabrina Speich (examiatrice) – Professeure à l’Ecole Normale Supérieure
• Laure Resplandy (rapportrice) – Professeure associée à l’iniversité de Princeton
• Judith Hauck (rapportrice) – Directrice de recherche à l’Alfred Wagner Institute
• Paul Halloran (examinateur) – Professeur à l’université d’Exeter
• Laurent Bopp (directeur de thèse) – Directeur de recherche au CNRS au LMD
• Lester Kwiatkowski (co-encadrant) – Chargé de recherche au CNRS au LOCEAN
• David T. Ho (invité) – Professeur à l’inniversité d’Hawai’i à Mānoa