proportionally, for higher CO2 concentrations. Polar surface waters may become under-saturated with respect to aragonite, a key calcium carbon mineral that can affect the ability of organisms to build their shells, for atmospheric CO2 levels of 400-450 ppm for the Arctic and 550-600 ppm for the Antarctic (Orr et al., 2005; Steinacher et al., 2010). For tropical surface waters, large reductions in calcium carbonate saturation state are expected to occur, but waters are expected to remain super-saturated for projected atmospheric CO2 during the 21st century for current scenario projections. Calcium saturation horizons (Ω = 1) have been observed to move upward, that is, shoaled (Feely et al., 2004; Orr et al., 2005), and there is evidence that water undersaturated for aragonite is already upwelling onto the continental shelf off the U.S. west coast due to a combination of strong wind-induced upwelling and the penetration of anthropogenic CO2 into off-shore source waters (Feely et al., 2008). For most of the surface ocean, climate change feedbacks are weak, and warming and altered ocean circulation have a limited effect on changing pH and Ω that are determined primarily by atmospheric CO2. An exception is in the Arctic, where sea-ice retreat and changes in surface freshwater balance amplify atmospheric CO2-driven pH and Ω declines (Steinacher et al., 2010).

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