Response of ocean acidification to atmospheric carbon dioxide removal

Artificial CO₂ removal from the atmosphere (also referred to as negative CO₂ emissions) has been proposed as a potential means to counteract anthropogenic climate change. Here we use an Earth system model to examine the response of ocean acidification to idealized atmospheric CO₂ removal scenarios. In our simulations, atmospheric CO₂ is assumed to increase at a rate of 1% per year to four times its pre-industrial value and then decreases to the pre-industrial level at a rate of 0.5%, 1%, 2% per year, respectively. Our results show that the annual mean state of surface ocean carbonate chemistry fields including hydrogen ion concentration ([H⁺]), pH and aragonite saturation state respond quickly to removal of atmospheric CO₂. However, the change of seasonal cycle in carbonate chemistry lags behind the decline in atmospheric CO₂. When CO₂ returns to the pre-industrial level, over some parts of the ocean, relative to the pre-industrial state, the seasonal amplitude of carbonate chemistry fields is substantially larger. Simulation results also show that changes in deep ocean carbonate chemistry substantially lag behind atmospheric CO₂ change. When CO₂ returns to its pre-industrial value, the whole-ocean acidity measured by [H⁺] is 15%-18% larger than the pre-industrial level, depending on the rate of CO₂ decrease. Our study demonstrates that even if atmospheric CO₂ can be lowered in the future as a result of net negative CO₂ emissions, the recovery of some aspects of ocean acidification would take decades to centuries, which would have important implications for the resilience of marine ecosystems.