Emergence of the ocean CO2 uptake hole under global warming

Abstract

The air-sea CO2 flux, the amount of CO2 absorbed into the ocean by an exchange of CO2 between the ocean and the atmosphere, is a critical process in global carbon cycle and plays an important role in determining future carbon systems. According to future climate change scenarios, the ability of the ocean to absorb CO2 will diminish as it loses its buffer capacity, ultimately leading to maximum efficiency of air-sea CO2 flux over time. The Subpolar North Atlantic (SPNA) plays a crucial role in uptaking both natural and anthropogenic CO2 in the present. Therefore, this study shows the significant change of air-sea CO2 flux in the SPNA based on experiments with different CO2 emission rates using a state-of-the-art Earth System Model. The remarkable increase of the ocean surface partial pressure of CO2 in the SPNA, which is not observed in the rest of the ocean, ultimately leads to a significant weakening of the CO2 uptake. This feature, the so- called “the CO2 uptake hole”, is triggered by an explosive rise of the dissolved inorganic carbon (DIC) concentration near the surface as a consequence of a dramatic change in the physical processes, in particular the cessation of local deep convection. While the DIC concentration rises throughout the Atlantic basin in response to increasing CO2 emissions, the shutdown of the local deep convection in the SPNA amplifies the accumulation of surface DIC, following reduction in the regional CO2 uptake capacity. These findings offer a more precise understanding of the ocean carbon cycle in the SPNA under global warming and provide insights into regional strategies for achieving carbon neutrality and pathways for carbon dioxide removal in the future. Key words: Ocean carbon cycle, Dissolve Inorganic Carbon (DIC), Atlantic Meridional Overturning Circulation (AMOC), Deep convection shutdown