"The best match for current changes was the Palaeocene-Eocene thermal maximum of 55 million years ago, when vast amounts of methane were released into the atmosphere causing rapid global warming, ocean acidification, and mass extinction. But even then, it took at least 3000 years for ocean pH to drop by 0.5. "That is an order of magnitude slower than today," Hönisch says.
A key point (indirectly pointed to in the article) is that the *rate of change* of acidity is what's critical. We've got the accelerator floored and we're close to the cliff.