Responses of carbon uptake and oceanic pCO2 to climate change in the North Atlantic: A model study with the Bergen Earth System Model

Several model studies diagnose the carbon uptake of the North Atlantic as most sensitive to climate change when considered per unit area. Yet the main drivers of the modeled sensitivity and the share of biological production and physical transport are under debate. In order to contribute to this ongoing discussion, two simulations with the Bergen Earth System Model were carried out for period 1850–2099. One of the simulations (COU) includes the radiative effect of rising CO2 (i.e., climate change), while the second simulation (BGC) excludes this effect. The modeled carbon fluxes show substantially different responses to climate change for different parts of the North Atlantic. Based on these differences, we divide the North Atlantic into two regions, namely, the subpolar gyre (SPG) and the rest of the North Atlantic (rNAT*, covering mainly the subtropical gyre). The highest climate sensitivity is found in the SPG region (accounting for an uptake reduction of 8.06 Pg C over the period 1850–2099), while the response of the rNAT* region is moderate (reduction of 4.00 Pg C). We show that the changing CO2 fluxes in both SPG and rNAT* regions are driven by increasing oceanic pCO2. The pCO2 changes in the rNAT* region are caused by both changing physical and biogeochemical processes, while changes in dissolved inorganic carbon (DIC) and alkalinity are the primary contributor to the high climate sensitivity of the SPG region. We identify a reduced biological production to be responsible for the modeled response of DIC and alkalinity, yet the differences between biological contribution and contributions of ocean circulation and CO2 uptake are small, highlighting our need for a better understanding of the marine biological cycle.