Ocean acidification refers to a reduction in pH of the ocean over an extended period, typically decades or longer, caused primarily by the uptake of carbon dioxide (CO₂) from the atmosphere. Ocean acidification can also be caused by other chemical additions or subtractions from the oceans that are natural (e.g., increased volcanic activity, methane hydrate releases, long-term changes in net respiration) or human-induced (e.g., release of nitrogen and sulphur compounds into the atmosphere). Anthropogenic ocean acidification refers to the component of pH reduction that is caused by human activity (IPCC, 2011).

Since the beginning of the industrial era, the release of CO₂ from industrial and agricultural activities has resulted in atmospheric CO2 concentrations that have increased from approximately 280 ppm to about 392 ppm in 2012 (Chapter 6). The oceans have absorbed approximately 155 PgC from the atmosphere over the last two and a half centuries (Sabine et al., 2004; Khatiwala et al., 2013). This natural process of absorption has benefited humankind by significantly reducing the greenhouse gas levels in the atmosphere and abating some of the impacts of global warming. However, the ocean’s uptake of carbon dioxide is having a significant impact on the chemistry of seawater. The average pH of ocean surface waters has already fallen by about 0.1 units, from about 8.2 to 8.1 (total scale), since the beginning of the industrial revolution (Orr et al., 2005a; Figure 1; Feely et al., 2009). Estimates of future atmospheric and oceanic carbon dioxide concentrations indicate that, by the end of this century, the average surface ocean pH could be lower than it has been for more than 50 million years (Caldeira and Wickett, 2003).

The major controls on seawater pH are atmospheric CO₂ exchange, the production and respiration of dissolved and particulate organic matter in the water column, and the formation and dissolution of calcium carbonate minerals. Oxidation of organic matter lowers dissolved oxygen concentrations, adds CO₂ to solution, reduces pH, carbonate ion (CO3 2–) and calcium carbonate (CaCO3) saturation states (Box 3.2, Figure 2), and lowers the pH of seawater in subsurface waters (Byrne et al., 2010). As a result of these processes, minimum pH values in the oceanic water column are generally found near the depths of the oxygen minimum layer. When CO₂ reacts with seawater it forms carbonic acid (H2CO3), which is highly reactive and reduces the concentration of carbonate ion (Box 3.2, Figure 2) and can affect shell formation for marine animals such as corals, plankton, and shellfish. This process could affect fundamental biological and chemical processes of the sea in coming decades (Fabry et al., 2008; Doney et al., 2009; WGII Chapters 5, 6, 28 and 30).

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