associated factors such as CO 2 uptake and trace gas emissions (e.g. dimethyl sul-
phide in Section 7.3) are essentially unknown.
As mentioned above, decreased seawater pH may seriously affect marine
organisms that secrete a CaCO 3 skeleton (see Section 6.4.4), particularly reef
forming corals (e.g. the Australian Great Barrier Reef), since the process is sen-
sitive to the acidity of the seawater. This is illustrated in Fig. 7.16 where the sat-
uration index (W) (see Section 6.4.4) for aragonite, the main reef-forming CaCO 3
mineral today, is plotted against atmospheric pCO 2. The shaded area shows the
calculated values of Wat water temperatures of 25 and 30°C (the range within
which reef-building corals live) as a function of the atmospheric pCO 2. It is appar-
ent that as pCO 2 increases the degree of aragonite supersaturation decreases.
Since it is supersaturation that enables coral-forming organisms to synthesize
their aragonitic (CaCO 3 ) skeletons, any decrease in Wwill potentially lead to
inhibited growth or death of the organisms, and so to die-back of coral reefs. The
seriousness of this problem is not well established and is currently a topic of active
research.
Given the potentially serious consequences for humans of the climatic changes
and other environmental changes discussed above, it is not surprising that emis-
sions of greenhouse gases are now subject to control and regulation through
international conventions such as the Kyoto Protocol. Such international action
has only come about because the scientific case for action has become compelling.
However, as we have seen in this section there still remain many uncertainties in
Global Change 261pH0 5 10 15 20 25 30
7.77.87.98.08.18.28.38.4Temperature (°C)Atlantic surface water
variability (summer)1750 (pCO 2 = 280 ppm)1992 (pCO 2 = 354 ppm)2100 (pCO 2 = 750 ppm)Fig. 7.15Equilibrium surface seawater pH for various atmospheric pCO 2 concentrations
assuming no change in alkalinity. Also shown are summertime pH values derived from
measurements in the north Atlantic ocean. Courtesy of Doug Wallace, University of Kiel.