Ocean Acidification Is Climate Change's 'Equally Evil Twin,' NOAA Chief Says

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12. Acidification is increasing at ten times the rate that preceded the Paleocene-Eocene mass extinction

Wikipedia: Ocean acidification

Current rates of ocean acidification have been compared with the greenhouse event at the Paleocene-Eocene boundary (about 55 million years ago) when surface ocean temperatures rose by 5–6 degrees Celsius. No catastrophe was seen in surface ecosystems, yet bottom-dwelling organisms in the deep ocean experienced a major extinction. The current acidification is on a path to reach levels higher than any seen in the last 65 million years,[23] and the rate of increase is about ten times the rate that preceded the Paleocene-Eocene mass extinction. The current and projected acidification has been described as an almost unprecedented geological event.[24] A National Research Council study released in April 2010 likewise concluded that "the level of acid in the oceans is increasing at an unprecedented rate."[25][26] A 2012 paper in the journal Science examined the geological record in an attempt to find a historical analog for current global conditions as well as those of the future. The researchers determined that the current rate of ocean acidification is faster than at any time in the past 300 million years.

It's not just the coral that is at risk here. This effect has the potential to become a planetary-scale extinction event.