the surface waters of the open ocean based on the known variations in atmospheric CO2 over the past 150 years (from actual measurements or from ice core data). Independent estimation of past seawater pH have been made using boron isotopes as well (see Box 2.2). Similarly, projections for changes in seawater chemistry can be made for the future on the basis of any future CO2 emission scenario such as those published by the IPCC. Such calculations are shown in Figure 2.5 for the Pacific Ocean; models show that, based on a “business-as-usual” scenario of CO2 emissions, the surface ocean pH will decrease by about 0.3 units within the next 100-150 years (e.g., Wolf-Gladrow et al., 1999; Caldeira and Wickett, 2003; Feely et al., 2004).
Figure 2.6 shows the results of actual measurements of surface seawater chemistry at a station near Hawaii between 1998 and 2008. These data confirm the validity of the calculations and demonstrate the predicted trend of a decrease of about 0.0015 pH units per year. The data also illustrate the seasonal cycle in pH and inorganic carbon species caused
FIGURE 2.5 Projected changes in the pH, and the concentrations of CO2 and CO32– in surface seawater under a business as usual scenario for CO2 emissions over the next two centuries. Calculations were made for a salinity of 35 and temperature of 25°C assuming constant alkalinity using the CO2sys program (Lewis and Wallace, 1998). The projected future values of pCO2 in the atmosphere are based on the estimates of Caldeira and Wickett (2003).