Once CO2 gas dissolves in seawater, it reacts with water to create carbonic acid and a series of chemical compounds that behave differently. Just as more fish can hide on a coral reef if they all look and behave differently, more CO2 "disguised" as other compounds can dissolve in water than if it wasn't in disguise.
When CO2 dissolves in water (H2O), some of this gas becomes carbonic acid (H2CO3), which dissociates into bicarbonate [HCO3(-)] and hydrogen ions [H(+)]. Then, some of the HCO3(-) dissociates into carbonate [CO3(2-)] and more H(+):
H2O + CO2 = H2CO3,
H2CO3 = HCO3(-) + H(+), and
HCO3(-) = CO3(2-) + H(+).
When more H(+) are free in the water, the acidity of the liquid is greater. Compared to other common gases, CO2's knack for disguise allows it to reach much higher concentrations in liquids. This is why CO2 is used to make soda's bubbles --- so much gas can be forced into the liquid that the drink will stay bubbly long after the bottle has been opened, as carbonate and bicarbonate recombine with H(+) in the soda and make CO2 gas bubbles. Freshly opened cola contains so much dissolved CO2, and thus so many H(+) ions, that it has an acidic pH of about 2.
CO2 in the ocean
Ocean pH is slightly basic because seawater contains many negatively charged minerals produced by both CO2 dissolution and rock weathering. During recent preindustrial times, ocean pH averaged about 8.2 (Key et al., 2004). As the ocean has taken up CO2 from the atmosphere, seawater pH has decreased to around 8.1 (See blue arrow in Figure 1 at right, and Key et al., 2004), increasing H(+) concentrations by 30%. Models predict additional decreases to as low as 7.7 pH as ocean CO2 levels rise (WBGU report), increasing H(+) concentrations up to 70% above preindustrial levels.
The relationships among the many CO2-related compounds in the ocean are complex; as H(+) concentrations increase and pH drops, CO3(2-) concentrations also decrease (black line in Figure 1). Once CO3(2-) drops below a certain threshold, minerals containing the ion will dissolve. This naturally happens in the deep ocean below the "saturation horizon," whose depth varies from ocean to ocean. However, as ocean pH has decreased, oceanic saturation horizons of calcite and aragonite, the most common carbonate minerals in marine organisms' shells and exoskeletons, have become shallower and are expected to continue (Figure 2). Consequently, available habitats for marine organisms that form calcified shells are shrinking.
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