Constraining ocean carbon sink projections in CMIP6 models

Understanding and limiting the spread of ocean carbon sink projections are crucial to effectively guide the development of climate mitigation policies, determine accurate future carbon budget and subsequently climate change. The North Atlantic and the Southern Ocean are two of the most intense sink regions for anthropogenic CO₂ emissions, while the tropical Pacific is the largest outgassing system. Nevertheless, CMIP6 models simulate growing inter-model spread in future carbon fluxes in these regions. In this study, we apply an emergent constraint approach to reduce the projections uncertainties under the high-CO₂ future scenario. The efficiency of surface-to-deep transport of anthropogenic carbon is commonly identified as the key mechanisms driving the systematic inter-model spread in high latitude regions. For the North Atlantic region, we further use a genetic algorithm to optimize our identified emergent constraint relationship by isolating the region where contemporary model bias strongly correlates with the projection spread. The interior biogeochemical state is key for constraining the future tropical Pacific CO₂ flux. Our study consolidates the importance of improving representations of anthropogenic carbon ventilation mechanisms in models and sustaining carbon and watermass monitoring network in these regions to improve the fidelity of future model projections.