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ABSTRACT
It is now well understood that the global surface ocean, whose pH has been reduced by ~0.1 in response to rising atmospheric CO^sub 2^ since industrialization, will continue to become more acidic as fossil fuel CO^sub 2^ emissions escalate. However, it is unclear how uncertainties in climate sensitivity to future CO^sub 2^ emissions will alter the manifestation of ocean acidification. Using an earth system model of intermediate complexity, this study performs a set of simulations that varies equilibrium climate sensitivity by 1.0°-4.5°C for a given CO^sub 2^ emissions scenario and finds two unexpected and decoupled responses. First, the greater the climate sensitivity, the larger the surface mixed layer acidification signal but the smaller the subsurface acidification. However, taken throughout the ocean, the highest climate sensitivity will paradoxically cause greater global warming while buffering whole-ocean pH by up to 24% on centennial time scales. Second, this study finds a large decoupling between pH and carbonate ion concentration in surface waters whereby these chemical properties show opposite effects under variable climate sensitivity. For every 1°C increase in climate sensitivity, the surface ocean pH reduction grows by 4%, while surface ocean carbonate ion reduction shrinks by 2%. The chemical and spatial decoupling found here highlights the importance of distinguishing the biological impacts of pH and aragonite saturation and understanding the spatial extent of important calcifying biomes so as to truly understand the long-term impacts of ocean acidification.
1. Introduction
Much of the concern with regard to fossil fuel consumption and emissions of CO2 into the atmosphere is focused on climate change. For example,WorkingGroup 2 of Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) has an extensive assessment of adverse effects of enhanced warming and changes in precipitation on human health, native ecosystems, food production, water, and the like (Parry et al. 2007). However, relatively little is discussed about ocean acidification, which occurs as a direct consequence of the oceanic uptake of fossil fuel CO2.
Since human industrialization began in the late eighteenth century, the global ocean has absorbed nearly half of all fossil fuel CO2 emitted to the atmosphere (Sabine et al. 2004). Because CO2 becomes a (weak) acid when hydrated, this net uptake causes the ocean to become more acidic, lowering seawater pH...