Current atmospheric CO2 are at greater levels than seen over the last 800,000 years. Similarly, pH levels are already more extreme than those experienced by the oceans over this same period. By the end of the 21st century, the projected decline in seawater pH is expected to be three times larger than any change in pH observed as the Earth’s climate has oscillated between glacial and interglacial periods. The times when seawater pH changed fastest was during glacial terminations when the Earth came out of an ice age. The change in seawater pH over the 21st Century is projected to be around 100 times faster than this rate.
What will be the effect of seawater pH falling to such levels? Let’s look further back at periods when pH fell to the levels projected for the end of the 21st Century. There have been several periods where pulses of CO2 have been injected into the atmosphere, from volcanic activity or melting of methane hydrates. One well known example is the Paleocene-Eocene Thermal Maximum (PETM), which occurred around 55 million years ago. During this event, global temperatures increased by over 5°C over a time frame less than 10,000 years. This coincided with a massive release of carbon dioxide into the atmosphere, which led to ocean acidification. This change caused a series of biological responses, including the mass extinction of benthic foraminifera.
Looking further back, there are other examples of mass-extinctions coinciding with global warming and increases in atmospheric carbon dioxide. Examination of the mass extinction that occured 251 million years ago during the end-Permian find that the patterns of mortality are consistent with the physiological effects of elevated CO2 concentrations (along with the effects of global warming). 205 million years ago at the Triassic–Jurassic boundary, a sudden rise in the levels of atmospheric CO2 coincided with a major suppression of carbonate sedimentation, very likely related to ocean acidification. A similar situation occurred 65 million years ago during the Cretaceous–Tertiary extinction event. Most of the planktonic calcifying species became rare or disappeared.
Future acidification depends on how much CO2 humans emit over the 21st century. By the year 2100, various projections indicate that the oceans will have acidified by a further 0.3 to 0.4 pH units, more than many organisms like corals can stand. This will create conditions not seen on Earth for at least 40 million years.