across ocean basins (Byrne et al., 2010). Based on ice-core CO2 data and the WOCE/JGOFS Survey, surface ocean pH has already dropped on average by about 0.1 pH units from pre-industrial levels (pH is measured on a logarithmic scale and a 0.1 pH drop is equivalent to a 26% increase in hydrogen ion concentration) (Orr et al., 2005). The patterns of ocean acidification in subsurface waters depend on ocean circulation patterns; thermocline waters in subtropical convergence regions and deep-waters in polar regions where cold surface waters sink into the interior ocean are affected more than other parts of the subsurface.
Future acidification of surface waters can be predicted for a given atmospheric carbon dioxide level (see Figure 4.28). An additional decline of 0.15 pH unit would occur if atmospheric carbon dioxide increases from current levels to 550 ppm, and larger pH changes would occur, approximately