Ocean acidification is the lowering of the pH in our oceans (making them more acidic, obviously). However, ocean acidity is largely a result of the CO2 in the atmosphere, which is the same amount of CO2 on the ocean’s surface. As CO2 increases in the atmosphere, more is pushed into the oceans, lowering its pH. Thus, ocean acidification is a sign of accelerating climate change. What scientists have discovered is that the current rate of ocean acidification is alarmingly similar to the Paleocene-Eocene Thermal Maximum (PETM), which one can think of as the most aggressive climatic warming in geological history. PETM was what would inevitably lead to (or contribute to) the Permian-Triassic mass extinction.
There are some things that differ between the PETM and the present day state of our oceans, and that’s the rate at which CO2 is accumulating in the atmosphere. During the Permian-Triassic era there was substantial volcanic activity around the world, but CO2 was, even then, accumulating in the atmosphere much more slowly than it is today. This makes the ocean’s resistance to chemical changes higher at that time than it is today. Some of the other indicators of high ocean acidification is that lowering of certain creatures abilities to form shells, and coral reefs nearly died off. Scientists are seeing these many different pieces of evidence today, though at an accelerated rate than was evident during the PETM.
Previous climate and oceanic research has shown that the ocean can, to a certain extent, function like a carbon sink, absorbing carbon (which drives down the pH of the ocean and has other disastrous effects) which may stave off more accelerated rates of warming. However, when the ocean is carbon saturated, and its acidity has reached that “unknown territory of marin ecosystem change”, the climate will no longer be able to force the excess carbon into the world’s oceans and we will see a dramatic spike in atmospheric warming.