Multi‐Century Changes in the Ocean Carbon Cycle Controlled by the Tropical Oceans and the Southern Ocean

The oceanic absorption of anthropogenic carbon dioxide (CO2) is expected to continue in the following centuries, but the processes driving these changes remain uncertain. We studied these processes in a simulation of future changes in global climate and the carbon cycle following the RCP8.5 high emission scenario. The simulation shows increasing oceanic uptake of anthropogenic CO2 peaking towards the year 2080 and then slowing down but remaining significant in the period up to the year 2300. These multi‐century changes in uptake are dominated by changes in sea‐air CO2 fluxes in the tropical and southern oceans. In the tropics, reductions in upwelling and vertical gradients of dissolved carbon will reduce the vertical advection of carbon‐rich thermocline waters, suppressing natural outgassing of CO2. In the Southern Ocean, the upwelling of waters with relatively low dissolved carbon keeps the surface carbon relatively low, enhancing the uptake of CO2 in the next centuries. The slowdown in CO2 uptake in the subsequent centuries is caused by the decrease in CO2 solubility and storage capacity in the ocean due to ocean warming and changes in carbon chemistry. A collapse of the Atlantic Meridional Overturning Circulation (AMOC) predicted for the next century causes a substantial reduction in the uptake of anthropogenic CO2. In sum, predicting multi‐century changes in the global carbon cycle depends on future changes in carbon chemistry along with changes in oceanic and atmospheric circulations in the Southern and tropical oceans, together with a potential collapse of the AMOC.

人为排放的二氧化碳（CO₂）在海洋中的吸收预计在接下来的几个世纪中会持续，但驱动这些变化的过程仍不确定。我们在一个遵循RCP8.5高排放情景的全球气候和碳循环未来变化模拟中研究了这些过程。模拟显示，海洋对人为CO₂的吸收不断增加，在2080年左右达到峰值，然后减缓，但在直至2300年的期间内仍保持显著。这些长达数世纪的吸收变化主要由热带和南大洋的海 - 气CO₂通量变化所主导。在热带地区，上升流的减少以及溶解碳的垂直梯度降低，将减少富含碳的温跃层水的垂直平流，抑制CO₂的自然释放。在南大洋，溶解碳相对较低的海水上升流使表层碳含量相对较低，从而在接下来的几个世纪中增强了对CO₂的吸收。随后几个世纪中CO₂吸收的减缓是由于海洋变暖以及碳化学变化导致的CO₂在海洋中的溶解度和储存能力下降。预计下个世纪大西洋经向翻转环流（AMOC）的崩溃会导致人为CO₂吸收大幅减少。总之，预测全球碳循环长达数世纪的变化取决于碳化学的未来变化以及南大洋和热带大洋中海洋和大气环流的变化，还有AMOC可能的崩溃。