Worse than just the direct acidification: if we're already seeing the increase in surface temperatures that we are, with oceans absorbing more carbon than we thought, what happens when ocean sequestration decreases? It will do that nonlinearly, we just don't know exactly when.

(It's expected that when the pH of the Southern Ocean gets past a threshold, phytoplankton (with carbonate shells) will start to be replaced by algae (without). Dead phytoplankton falling to the ocean floor is a major component of carbon sequestration by the oceans.)

It suggests that we could pass a threshold and be in for a world of hurt very soon.