Response of ocean acidification to atmospheric carbon dioxide removal.

Artificial CO removal from the atmosphere (also referred to as negative CO 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 CO removal scenarios. In our simulations, atmospheric CO 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 CO. However, the change of seasonal cycle in carbonate chemistry lags behind the decline in atmospheric CO. When CO 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 CO change. When CO 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 CO decrease. Our study demonstrates that even if atmospheric CO can be lowered in the future as a result of net negative CO 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.