Whatever The Warming, Ocean Acidifies From Carbon-dioxide Buildup

A new study indicates that future changes in ocean acidification caused by atmospheric carbon dioxide emissions are largely independent of the amount of climate change caused by those emissions.

The ocean plays a major role in the uptake of carbon dioxide emitted from fossil-fuel burning, helping to moderate future climate change. However, the addition of the gas to the ocean alters marine chemistry by increasing acidity (decreasing pH), posing a threat to shelled organisms and the predators that feed off them.
This increase in acidity occurs regardless of how much of a global-warming-related temperature rise takes place as carbon dioxide builds up to that concentration.

Science Daily