Effect of Plankton Composition Shifts in the North Atlantic on Atmospheric pCO2.

Marine carbon cycle processes are important for taking up atmospheric CO2 thereby reducing climate change. Net primary and export production are important pathways of carbon from the surface to the deep ocean where it is stored for millennia. Climate change can interact with marine ecosystems via changes in the ocean stratification and ocean circulation. In this study we use results from the Community Earth System Model version 2 (CESM2) to assess the effect of a changing climate on biological production and phytoplankton composition in the high latitude North Atlantic Ocean. We find a shift in phytoplankton type dominance from diatoms to small phytoplankton which reduces net primary and export productivity. Using a conceptual carbon‐cycle model forced with CESM2 results, we give a rough estimate of a positive phytoplankton composition‐atmospheric CO2 feedback of approximately 60 GtCO2/°C warming in the North Atlantic which lowers the 1.5° and 2.0°C warming safe carbon budgets. Plain Language Summary: The marine carbon cycle is important for taking up carbon from the atmosphere and thereby lowering atmospheric CO2 concentrations. One of the ways the marine carbon cycle transports carbon from the surface to the deep ocean is biological production (net primary production and export production). Once in the deep ocean, carbon can be stored for thousands of years. Biological production is dependent on environmental conditions such as nutrient availability and ocean temperature, which can be affected by increasing atmospheric CO2 concentrations. This can lead to a positive feedback loop, where increasing CO2 concentration decrease biological production which in turn decreases uptake of CO2 by the ocean, effectively increasing atmospheric CO2 concentrations. Here, we find in an Earth System Model that under a high emission scenario, biological production decreases significantly in the high latitude North Atlantic Ocean which is primarily the result of a shift in dominant phytoplankton type in this region. Larger diatoms, which are relatively efficient in exporting carbon, are replaced by small phytoplankton which are less efficient. By using a conceptual carbon cycle model, we identify a positive feedback loop where the decrease in biological production increases atmospheric CO2 by approximately 60 GtCO2 per degree warming. Key Points: Biological production decreases significantly in the high latitude North Atlantic in Community Earth System Model version 2 under the SSP5‐8.5 scenarioPhytolankton type dominance shifts from diatoms to small phytoplanktonA positive feedback loop is diagnosed where changes in the physical system decrease biological production, reducing oceanic uptake of CO2 [ABSTRACT FROM AUTHOR]