| Due to anthropogenic emissions of carbon dioxide (CO2), atmospheric CO2 concentrations are increasing continuously, leading to climate change-the global warming. In response to increasing CO2 concentrations and associated climate change, the global ocean is undergoing both physical and chemical changes, among which the changes of ocean carbon cycle, carbonate chemistry and dissolved oxygen have great implications for marine biota.To investigate these changes mentioned above and examine the effects of increasing CO2 concentrations and climate change on ocean physical and chemical environment, we choose a possible future CO2 scenario of intense fossil fuel emissions (RCP8.5) and use an intermediate complexity Earth system model to perform a set of simulations with different climate sensitivities. We analyze the model results from these set of experiments and investigate the individual and combined effects of increasing atmospheric CO2 and associated climate change on the global carbon and oxygen cycle.In our simulations, the changes in atmospheric CO2 concentrations have direct impacts on ocean carbon cycle and carbonate chemistry, while climate change has secondary effects. However, for dissolved oxygen, climate change plays a major role. In the simulations without climate change, by year 2500, oceanic total uptake of anthropogenic CO2 is 1996.04 Pg C (1 Pg C=1015 g C), and ocean-mean pH, carbonate ion concentration and aragonite saturation reduce by 4.52%,50.54% and 56.73% relative to the pre-industrial values, respectively. The inclusion of CO2-induced climate change further modifies the modeled carbon and oxygen cycle. In the simulations with climate sensitivity of 3.0 K, the effect of climate change reduces global ocean carbon uptake by 18.48% by year 2500, relative to the simulation without climate change. The reduced uptake of anthropogenic CO2 is mainly a result of increased sea surface temperature that decreases the solubility of CO2 and weakened ocean circulation that reduces the transport rate of CO2 from the surface to the deep ocean. Climate change also mitigates the CO2-induced reduction of surface ocean carbonate ion concentration and aragonite saturation, but has negligible influence in surface mean pH. Averaged over the entire ocean, relative to the simulation without climate change, the effect of climate change increases ocean-mean pH, carbonate ion concentration and aragonite saturation by 0.90%,23.43% and 22.46%, respectively, due to reduced rate of surface- to-deep ocean transport. Meanwhile, climate change decreases surface and ocean mean dissolved oxygen concentrations mainly as a result of ocean warming that decreases the solubility of oxygen in the ocean. By year 2500, compared to the simulation without climate change, the effect of climate change (with climate sensitivity of 3.0 K) reduces ocean-surface mean and ocean-mean dissolved oxygen concentration by 4.38% and 18.44%, respectively.The set of simulations with different climate sensitivities shows that, with greater climate sensitivity, climate change would have a larger effect on reducing the ocean’s ability to absorb atmospheric CO2 and mitigating ocean acidification, but with greater reduction of dissolved oxygen. By year 2500, for every 1.0 K increase of climate sensitivity, oceanic cumulative anthropogenic CO2 uptake decreases by 109.60 Pg C, and changes in ocean-mean pH, carbonate ion concentration, aragonite saturation, oxygen concentration, and respiration index (anomalies relative to pre-industrial) are 0.015 (or 0.19%),3.47 μmol kg-1 (3.51%),0.03 (or 3.12%),-8.72 μmol kg-1 (or 5.10%), and -0.006 (or-0.27%), respectively.Our study demonstrates that, anthropogenic emissions of CO2 and CO2-induced global warming would lead to significantly changes in global ocean physical and chemical environment, which has great implication on marine biota and ecosystem. |
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