Physical Climate Change in the 21st Century - Climate Stabilization Targets - page 157

FIGURE 4.28 Projected evolution of the annual-mean, zonally averaged aragonite saturation, W , plotted
as a function of the annual-mean atmospheric CO 2 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 CO 2 , respectively.
Undersaturated conditions occur for individual months at even lower atmospheric CO 2 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 CO 2 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 logarith-
mic 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 at-
mospheric carbon dioxide level (see Figure 4.28). An additional decline of
0.15 pH unit would occur if atmospheric carbon dioxide increases from cur-
rent levels to 550 ppm, and larger pH changes would occur, approximately