Toward more robust NPP projections in the North Atlantic Ocean

Abstract. Phytoplankton plays a crucial role in both climate regulation and marine biodiversity, yet it faces escalating threats due to climate change. Projecting the future changes in phytoplankton biomass and productivity under climate change requires the utilization of Earth System Models capable of resolving marine biogeochemistry, and exploits the averaged responses across an ensemble of models (within the Coupled Model Intercomparison Project Phase 6, CMIP6) as the most probable projection. However, in the North Atlantic, this straightforward method falls short in providing robust projections of phytoplankton net primary production (NPP) over the 21st century. This is because the processes controlling NPP strongly differ from one model to another, thus causing model divergence. A new inter-comparison approach was therefore developed and applied to 8 CMIP6 models exhibiting substantial divergence in their NPP projections in the North Atlantic. This approach is based on the identification of the mechanisms causing model divergence and the assessment of their reliability, in order to conduct an informed selection of the most reliable models. The basin was first divided into 3 bioregions tailored to the characteristics of each model using a novel method based on a clustering procedure. Two key mechanisms causing model divergence were then identified in the subtropical and subpolar regions (namely, diazotrophy and the presence of an ammonium pool, respectively). This allowed for an informed selection of models in each region, resulting in reduced uncertainty and a more pronounced decrease in total NPP in the subtropical North Atlantic and a stronger increase of small phytoplanton NPP in the subpolar North Atlantic. Our model selection strengthened carbon export and phytoplankton biomass decreases under climate change, but had no impact on zooplankton biomass. By leveraging the diversity of CMIP6 models, this innovative approach identifies the key mechanisms influencing NPP projections and provides valuable insights into the future trajectory of the Earth’s climate system.