"studies have shown that a more acidic environment has a dramatic effect on some calcifying species, including oysters, clams, sea urchins, shallow water corals, deep sea corals, and calcareous plankton. When shelled organisms are at risk, the entire food web may also be at risk. Today, more than a billion people worldwide rely on food from the ocean as their primary source of protein. Many jobs and economies in the U.S. and around the world depend on the fish and shellfish in our oceans."
My understanding is that this is a near-term effect, just like the current climate effects are. Mixing into the deep ocean happens on hundreds-of-years timescales and will eventually absorb everything we're emitting today without significant net Ph change.
So, it's really bad, but not irreversible or unmitigatable (again, from the "human extinction" angle).
And as far as the "billion people" thing goes: yeah. Disruption is unavoidable. Nonetheless there is sufficent arable land to feed the world, and alternative ocean-sourced proteins exist (jellyfish, say) that are unexploited for historical reasons.
Which, too, is really bad. But not apocalypse-level bad, just mass extension bad.
[0]http://www.pmel.noaa.gov/co2/story/What+is+Ocean+Acidificati...
"studies have shown that a more acidic environment has a dramatic effect on some calcifying species, including oysters, clams, sea urchins, shallow water corals, deep sea corals, and calcareous plankton. When shelled organisms are at risk, the entire food web may also be at risk. Today, more than a billion people worldwide rely on food from the ocean as their primary source of protein. Many jobs and economies in the U.S. and around the world depend on the fish and shellfish in our oceans."