Terry M. Bella
Over the past 444 million years there have been 5 major extinction events (8). These major extinction events are characterized by the disappearance of 75% or more of all species. Note that this means the species are gone, it is not so much about the death of individual organisms but more so about the removal of a species from the planet. Historically life has rebounded, even following the Permian extinction event wherein 96% of species went extinct (8). What we, humans, have difficulty comprehending is the timescale for the rebound. Yes, we may currently be living in the sixth major extinction, and the biosphere will likely rebound, but that will take millions of years. Humans have only been extant for a couple hundred thousand years. The entire history of the human race pales in comparison to the amount of time it will take for the planet to reestablish a network of flora and fauna as complex and diverse as we have today. Our actions in just a couple of centuries are impacting the evolutionary product of 65 million years, and longer as some of our current species are well over 200 million years old.
The normal rate of extinctions per year is known as the background rate. This is estimated to be one to five per year. It is believed that currently the extinction rate is 1,000 to 10,000 times the normal background rate (9). We have not identified all of the organisms on the planet and many are going extinct even before they are described. We cannot immediately prove causation between ocean acidification and the extinction of a species, but it is immediately evident that the pH of the ocean has an impact on shell and skeleton formation. Hard corals build calcium carbonate skeletons that become the structures we know as coral reefs. Generally corals are classified as soft or hard. The hard corals are the ones that produce the calcium carbonate deposits. As the ocean becomes more acidic there is an increased likelihood that the aforementioned calcium carbonate structures will dissolve.