Historical simulation and twenty-first century prediction of oceanic CO₂ sink and pH change

A global ocean carbon cycle model based on the ocean general circulation model POP and the improved biogeochemical model OCMIP-2 is employed to simulate carbon cycle processes under the historically observed atmospheric CO₂ concentration and different future scenarios (called Representative Concentration Pathways, or RCPs). The RCPs in this paper follow the design of Intergovernmental Panel on Climate Change (IPCC) for the Fifth Assessment Report (AR5). The model results show that the ocean absorbs CO₂ from atmosphere and the absorbability will continue in the 21st century under the four RCPs. The net air-sea CO₂ flux increased during the historical time and reached 1.87 Pg/a (calculated by carbon) in 2005; however, it would reach peak and then decrease in the 21st century. The ocean absorbs CO₂ mainly in the mid latitude, and releases CO₂ in the equator area. However, in the Antarctic Circumpolar Current (ACC) area the ocean would change from source to sink under the rising CO₂ concentration, including RCP4.5, RCP6.0, and RCP8.5. In 2100, the anthropogenic carbon would be transported to the 40°S in the Atlantic Ocean by the North Atlantic Deep Water (NADW), and also be transported to the north by the Antarctic Bottom Water (AABW) along the Antarctic continent in the Atlantic and Pacific oceans. The ocean pH value is also simulated by the model. The pH decreased by 0.1 after the industrial revolution, and would continue to decrease in the 21st century. For the highest concentration scenario of RCP8.5, the global averaged pH would decrease by 0.43 to reach 7.73 due to the absorption of CO₂ from atmosphere.