The uptake of CO₂ by the ocean changes the chemical balance of seawater through the thermodynamic equilibrium of CO₂ with seawater. Dissolved CO₂ forms a weak acid (H2CO3) and, as CO₂ in seawater increases, the pH, carbonate ion (\scriptstyle \mathrm{CO}^{2-}_3), and calcium carbonate (CaCO 3) saturation state of seawater decrease while bicarbonate ion (HCO 3–) increases (FAQ 3.3). Variations in oceanic total dissolved inorganic carbon (CT = CO₂ + \scriptstyle \mathrm{CO}^{2-}_3 + HCO 3–) and pCO₂ reflect changes in both the natural carbon cycle and the uptake of anthropogenic CO₂ from the atmosphere. The mean pH (total scale) of surface waters ranges between 7.8 and 8.4 in the open ocean, so the ocean remains mildly basic (pH > 7) at present (Orr et al., 2005a; Feely et al., 2009). Ocean uptake of CO₂ results in gradual acidification of seawater; this process is termed ocean acidification (Box 3.2) (Broecker and Clark, 2001; Caldeira and Wickett, 2003). The observed decrease in ocean pH of 0.1 since the beginning of the industrial era corresponds to a 26% increase in the hydrogen ion concentration [H+] concentration of seawater (Orr et al., 2005b; Feely et al., 2009). The consequences of changes in pH, \scriptstyle \mathrm{CO}^{2-}_3, and the saturation state of CaCO3 minerals for marine organisms and ecosystems are just beginning to be understood (see WGII Chapters 5, 6, 28 and 30).

A global mean decrease in surface water pH of 0.08 from 1765 to 1994 was calculated based on the inventory of anthropogenic CO₂ (Sabine et al., 2004), with the largest reduction (–0.10) in the northern North Atlantic and the smallest reduction (–0.05) in the subtropical South Pacific. These regional variations in the size of the pH decrease are consistent with the generally lower buffer capacities of the high latitude oceans compared to lower latitudes (Egleston et al., 2010).

Direct measurements on ocean time-series stations in the North Atlantic and North Pacific record decreasing pH with rates ranging between –0.0014 and –0.0024 yr –1 (Table 3.2, Figure 3.18; Bates, 2007, 2012; Santana-Casiano et al., 2007; Dore et al., 2009; Olafsson et al., 2009; González-Dávila et al., 2010). Directly measured pH differences in the surface mixed layer along repeat transects in the central North Pacific Ocean between Hawaii and Alaska showed a –0.0017 yr –1 decline in pH between 1991 and 2006, in agreement with observations at the time-series sites (Byrne et al., 2010). This rate of pH change is also consistent with repeat transects of CO₂ and pH measurements in the western North Pacific (winter: –0.0018 ± 0.0002 yr –1 ; summer: –0.0013 ± 0.0005 yr –1 ) (Midorikawa et al., 2010). The pH changes in southern ocean surface waters are less certain because of the paucity of long-term time-series observations there, but pCO₂ measurements collected by ships-of-opportunity indicate similar rates of pH decrease there (Takahashi et al., 2009).

Uptake of anthropogenic CO₂ is the dominant cause of observed changes in the carbonate chemistry of surface waters (Doney et al., 2009). Changes in carbonate chemistry in subsurface waters can also reflect local physical and biological variability. As an example, while pH changes in the mixed layer of the North Pacific Ocean can be explained solely by equilibration with atmospheric CO₂, declines in pH between 800 m and the mixed layer in the time period 1991–2006 were attributed in approximately equal measure to anthropogenic and natural variations (Byrne et al., 2010). Figure 3.19 shows the portion of pH changes between the surface and 1000 m that were attributed solely to the effects of anthropogenic CO₂. Seawater pH and \scriptstyle \mathrm{CO}^{2-}_3 decreased by 0.0014 to 0.0024 yr –1 and ~0.4 to 0.9 µmol kg –1 yr –1, respectively, between 1988 and 2009 (Table 3.2). Over longer time periods, anthropogenic changes in ocean chemistry are expected to become increasingly prominent relative to changes imparted by physical and biological variability.

The consistency of these observations demonstrates that the pH of surface waters has decreased as a result of ocean uptake of anthropogenic CO₂ from the atmosphere. There is high confidence that the pH decreased by 0.1 since the preindustrial era.

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