tion, like climate change, is a growing problem that is linked to the rate and amount of CO2 emissions and is expected to affect ecosystems and society on a global scale. Unlike the uncertainties regarding the extent of CO2-induced climate change, the principal changes in seawater chemistry that result from an increase in CO2 concentration can be measured or calculated precisely. Importantly, these chemical changes are also practically irreversible on a time scale of centuries due to the inherently slow turnover of biogeochemical cycles in the oceans.
The mean pH of the ocean’s surface has decreased by about 0.1 unit (from approximately 8.2 to 8.1) since the beginning of the industrial revolution, representing a rate of change exceeding any known to have occurred for at least hundreds of thousands of years (Figure 1.1) (Raven et al., 2005). Model projections indicate that if emissions continue on their current trajectory (i.e., business-as-usual scenarios), pH may drop by another 0.3 units by the end of the century (e.g., Wolf-Gladrow et al., 1999; Caldeira and Wickett, 2003; Feely et al., 2004). Even under optimistic scenarios (i.e., SRES scenario B13), mean ocean surface pH is expected to drop below 7.9 (e.g., Cooley and Doney, 2009).
Scientific research on the biological effects of acidification is still in its infancy and there is much uncertainty regarding its ultimate effects on marine ecosystems. But marine organisms will be affected by the chemical changes in their environment brought about by ocean acidification; the question is how and how much. A number of biological processes are already known to be sensitive to the foreseeable changes in seawater chemistry. A prime example is the impairment in the ability of some organisms to construct skeletons or protective structures made of calcium carbonate resulting from even a modest degree of acidification, although the underlying mechanisms responsible for this effect are not well understood. Effects on the physiology of individual organisms can be amplified through food web and other interactions, ultimately affecting entire ecosystems. Organisms forming oceanic ecosystems have evolved over millennia to an aqueous environment of remarkably constant composition. There is reason to be concerned about how they will acclimate or adapt to the changes resulting from ocean acidification—changes that are occurring very rapidly on geochemical and evolutionary time scales.
3 The Intergovernmental Panel on Climate Change developed emissions projection scenarios by examining alternative development pathways that considered a wide range of demographic, economic, and technological drivers (IPCC, 2000).