Heterogeneous future Arctic Ocean primary productivity changes projected in CMIP6

Abstract. The Arctic Ocean is experiencing profound environmental changes due to climate change, with Net Primary Production (NPP) broadly projected to increase this century. This study analyzes NPP trends and their drivers across pan-Arctic and sub-regional scales throughout the 21st century, comparing Coupled Model Intercomparison Project Phase 6 (CMIP6) and Phase 5 (CMIP5) projections to assess how model generations differ. Using a multi-model approach, we assess projections for different Phytoplankton Functional Types (PFTs), diatoms and nanophytoplankton, and examine the role of physical and biogeochemical constraints including light, nutrient, and temperature limitations. Our results reveal that Arctic Ocean NPP increases are primarily driven by reduced sea ice cover, leading to longer ice-free seasons in the expanding seasonal ice zone. However, NPP changes exhibit pronounced spatial heterogeneity, with strong increases in Arctic inflow shelf regions, tempered by decreases in Baffin Bay and Nordic Seas. These differences are due to the varying balance between physical and biogeochemical NPP constraints across the Arctic Ocean. The multi-model mean Arctic Ocean NPP increase is four times larger in CMIP6 than in CMIP5, under comparable radiative forcing, with a three times higher uncertainty at the end of the century. This difference is attributed to higher baseline nutrient levels in CMIP6, combined with more pronounced sea ice loss and greater warming than in CMIP5. Key aspects to better simulate future Arctic Ocean NPP remain the representation of present-day nutrient levels, light transmission through sea ice and reduced model uncertainty in climate sensitivity.