Physical Climate Change in the 21st Century - Climate Stabilization Targets - page 158

proportionally, for higher CO 2 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 at-
mospheric CO 2 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 CO 2 during the 21st century for current scenario projections.
Calcium saturation horizons ( W = 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 con-
tinental shelf off the U.S. west coast due to a combination of strong wind-
induced upwelling and the penetration of anthropogenic CO 2 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 W that are determined primarily
by atmospheric CO 2 . An exception is in the Arctic, where sea-ice retreat
and changes in surface freshwater balance amplify atmospheric CO 2 -driven
pH and W declines (Steinacher et al., 2010).