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Ocean acidification caused by the uptake of carbon dioxide (CO2) by the ocean is an important global change problem (Kleypas et al. 1999; Caldeira and Wickett 2003; Doney et al. 2009). Ongoing ocean acidification is closely linked to global warming, as acidification and warming are primarily caused by continued anthropogenic emissions of CO2 from fossil fuel burning (Marland et al. 2008 ), land use, and land-use change (Strassmann et al. 2007). Future ocean acidification will be determined by past and future emissions of CO2 and their redistribution within the earth system and the ocean. Calculation of the potential range of ocean acidification requires consideration of both a plausible range of emissions scenarios and uncertainties in earth system responses, preferably by using results from multiple scenarios and models. The goal of this chapter is to map out the spatiotemporal evolution of ocean acidification for different metrics and for a wide range of multigas climate change emissions scenarios from the integrated assessment models (Nakićenović 2000; Van Vuuren et al. 2008b). By including emissions reduction scenarios that are among the most stringent in the current literature, this chapter explores the potential benefits of climate mitigation actions in terms of how much ocean acidification can be avoided and how much is likely to remain as a result of inertia within the energy and climate systems. The longterm impacts of carbon emissions are addressed using so-called zero-emissions commitment scenarios and pathways leading to stabilization of atmospheric CO 2. Discussion will primarily rely on results from the cost-efficient Bern2.5CC model (Plattner et al. 2008) and the comprehensive carbon cycle– climate model of the National Centre for Atmospheric Research (NCAR), CSM1.4-carbon (Steinacher et al. 2009; Frölicher and Joos 2010). The magnitude of the human perturbation of the climate system is well documented by observations (Solomon e t al. 2007). Carbon emissions from human activities force the atmospheric composition, climate, and the geochemical state of the ocean towards conditions that are unique for at least the last million years (see Chapter 2).

Although the changes in the chemistry of seawater driven by the uptake of CO2 by the oceans have been known for decades, research addressing the effects of elevated CO2 on marine organisms and ecosystems has only started recently (see Chapter 1). The first results of deliberate experiments on organisms were published in the mid 1980s (Agegian 1985) and those on communities in 2000 (Langdon et al. 2000; Leclercq et al. 2000 ). In contrast, studies focusing on the response of terrestrial plant communities began much earlier, with the first results of free-air CO2 enrichment experiments (FACE) being published in the late 1960s (see Allen 1992 ). Not surprisingly, knowledge about the effects of elevated CO2 on the marine realm lags behind that concerning the terrestrial realm. Yet ocean acidification might have significant biological, ecological, biogeochemical, and societal implications and decision-makers need to know the extent and severity of these implications in order to decide whether they should be considered, or not, when designing future policies. The goals of this chapter are to summarize key information provided in the preceding chapters by highlighting what is known and what is unknown, identify and discuss the ecosystems that are most at risk, as well as discuss prospects and recommendation for future research. The chemical, biological, ecological, biogeochemical, and societal implications of ocean acidification have been comprehensively reviewed in the previous chapters with one minor exception. Early work has shown that ocean acidification significantly affects the propagation of sound in seawater and suggested possible consequences for marine organisms sensitive to sound (Hester et al . 2008). However, sub sequent studies have shown that the changes in the upper-ocean sound absorption coefficient at future pH levels will have no or a small impact on ocean acoustic noise (Joseph and Chiu 2010; Udovydchenkov et al . 2010). The goal of this section is to condense the current knowledge about the consequences of ocean acidification in 15 key statements. Each statement is given levels of evidence and, when possible, a level of confidence as recommended by the Intergovernmental Panel on Climate Change (IPCC) for use in its 5th Assessment Report (Mastrandrea et al. 2010).