FIGURE 4.28 Projected evolution of the annual-mean, zonally averaged aragonite saturation, Ω, plotted as a function of the annual-mean atmospheric CO2 mixing ratio at the ocean surface. The corresponding years for the SRES A2 and B1 scenarios are given at the top. The largest decreases in aragonite saturation values occur in the tropics. Arctic and Southern Ocean surface waters transition from supersaturation to undersaturation in the annual-mean beginning at approximately 460 ppm and 550 ppm CO2, respectively. Undersaturated conditions occur for individual months at even lower atmospheric CO2 levels, beginning at approximately 410 ppm for the Arctic and 490 ppm for Southern Ocean. Source: Adapted from Steinacher et al. (2009).

FIGURE 4.28 Projected evolution of the annual-mean, zonally averaged aragonite saturation, Ω, plotted as a function of the annual-mean atmospheric CO2 mixing ratio at the ocean surface. The corresponding years for the SRES A2 and B1 scenarios are given at the top. The largest decreases in aragonite saturation values occur in the tropics. Arctic and Southern Ocean surface waters transition from supersaturation to undersaturation in the annual-mean beginning at approximately 460 ppm and 550 ppm CO2, respectively. Undersaturated conditions occur for individual months at even lower atmospheric CO2 levels, beginning at approximately 410 ppm for the Arctic and 490 ppm for Southern Ocean. Source: Adapted from Steinacher et al. (2009).

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



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