About 55 million years ago, in the Eocene, volcanic activity spewed enormous amounts of carbon dioxide into the air. The earth warmed by 4-5 degrees Centigrade. All surface ice melted, and every place on earth became tropical, even Antarctica. Sea levels rose a great deal and a significant amount of land was lost to the sea. It is estimated that sea levels rise some 10 to 20 meters (yards) for every 1 degree C increase in the average surface temperature, over the long term.
But along with all that dramatic change came something else. The seas absorbed a lot of the new carbon dioxide, creating carbonic acid. About 50% of some sorts of sea creatures did not survive the change.
The earth is repeating the experiment today, with human beings spewing out enormous amounts of carbon dioxide. And faster. Much faster indeed than in the Eocene.
Because of acidification and over-fishing, the world could lose a large number of ocean species just in the next 40 years or so.