Lough., 2007:

"Observed
The oceans absorb carbon dioxide (CO₂) from the atmosphere and are estimated to have absorbed about half of the excess CO₂ released into the atmosphere by human activities in the past 200 years. About half of this anthropogenic CO₂ is in the upper 10 per cent of oceans (less than 1000 metres depth) due to slow ocean mixing processes. This absorbed CO₂ is resulting in chemical changes in the ocean, which it is estimated has already caused a decrease in oceanic pH of 0.1. This is referred to as ocean acidification as the oceans are becoming more acidic, though they are still alkaline.

Projected
With continued emissions of CO₂, oceanic pH is projected to decrease by about 0.4 to 0.5 units by 2100 (a change from 8.2 to 7.8 associated with a surface water decrease in CO₃^- by 47 percent of pre-industrial levels). This is outside the range of natural variability and a level of ocean acidity not experienced for several hundreds of thousands of years. Of particular concern is that the rate of this change in ocean chemistry is about 100 times faster than at any other time over the past several million years. In addition ocean acidification is essentially irreversible during our lifetimes and would take tens of thousands of years to return to pre-industrial levels. The magnitude of projected changes in ocean chemistry can be estimated with a high level of confidence but the impacts on marine organisms and various geochemical processes are much less certain. The scale of changes may also vary regionally with the Southern Ocean most likely seeing the greatest changes in the short term. In addition, changes in ocean chemistry will result in interactions and feedbacks with the global carbon cycle, atmospheric chemistry and global climate – in ways that are currently not understood."

Hoegh-Guldberg et al., 2007:

Figure 4: Projections for ocean acidification include reductions in oceanic pH by as much as 0.4 pH units by the end of this century, with ocean carbonate saturation levels potentially dropping below those required to sustain coral reef accretion by 2050. Changes in ocean acidity will vary from region to region, with some regions, such as the Great Barrier Reef and Coral Sea, and the Caribbean Sea, attaining risky levels of aragonite saturation (the principal crystalline form of calcium carbonate deposited in coral skeletons) more rapidly than others (Source: Hoegh-Guldberg et al., 2007).