to more clearly define the contributors to ocean acidification and put its consequences for marine life and human societies into perspective.

Since the beginning of the Industrial Revolution, the average pH of the upper ocean has decreased by about 0.1 pH unit, which corresponds to an approximately 30% rise in acidity (activity of hydrogen ions (H+)). Shallow ocean pH is projected to decrease by an additional 0.2-0.3 pH units by the end of this century, corresponding to a rise in acidity of 100-150% since the mid-18th century (IPCC, 2007 WGI; under the IS92a scenario). The rates of relevant chemical change in deep waters may not necessarily be that much slower than in surface waters because deeper waters naturally have higher concentrations of inorganic carbon, a lower buffer capacity, and are thus more susceptible to CO2 perturbations. This rate of acidification is faster than any rates inferred from the geological record for at least the past 55 million years (Zeebe and Ridgwell, 2011; Hönisch et al., 2012). Due to the mixing of ocean waters across depths, pH is decreasing—and will continue to decrease—in deeper regions of the marine water column as well as at shallow depths.

These changes in pH and carbonate chemistry are expected to have effects on marine organisms at all levels of biological organization, including the physiologies of individual organisms and the composition, productivity, and health of diverse marine ecosystems. Furthermore, the effects of ocean acidification may be compounded by stresses arising from other features of global change, notably rising temperatures and decreases in concentrations of dissolved oxygen. Currently, we are in the early stages of discovering what these diverse and interacting effects are and how they may affect marine ecosystems and the socioeconomic activities that depend on ocean-derived resources. However, even though the field of ocean acidification research is relatively new, it is growing rapidly and beginning to reveal the scope and magnitude of biological, ecological, and societal consequences projected to arise from future acidification. Early studies focused primarily on the many organisms that build shells and skeletons of calcium carbonate, such as reef-building corals and the small calcareous phytoplankton that lie at the base of the marine food web. Recent studies now illustrate that the biological impacts of ocean acidification go far beyond calcification processes and also include photosynthesis, respiration, nutrient acquisition, behavior, growth, reproduction, and survival per se (Gattuso et al., 2011).

Some discoveries of how acidification affects marine species have been unanticipated. For example, it was recently discovered that low pH impairs sensory and neurotransmitter systems of larval marine fish, which leads to maladaptive changes in their behavior and olfactory capabilities (Munday et al., 2009; Nilsson et al., 2012). It seems reasonable to



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