The world's oceans may be experiencing a rate of acidification unparalleled in the past 300 million years, according to a paper published on Thursday in the journal Science.
"The geological record suggests that the current acidification is potentially unparalleled in at least the last 300 million years of Earth history," co-author Andy Ridgwell of Bristol University told AFP, "and raises the possibility that we are entering an unknown territory of marine ecosystem change."
The paper's lead author, Bärbel Hönisch of Columbia University's Lamont-Doherty Earth Observatory, noted in a university release that during the past century, ocean pH has fallen by a tenth of a unit to 8.1 – and as all science-minded Reg readers are aware, pH is measured on a logarithmic scale, so a drop of 0.1 units is significant, indeed.
The acidification change-rate noted in the study is at least 10 times faster than that which occurred during the Paleocene-Eocene Thermal Maximum (PETM), which the globe experienced around 56 million years ago, Hönisch said.
During the PETM, mass extinctions occurred on both land and sea. The causes for that acidification surge are not well understood, but one prime suspect is a series of massive methane bursts from ocean sediment, which were triggered by volcanic activity.
The cause of the current acidification is more clear, the study maintains. As Hönisch points out, atmospheric CO2 has risen about 30 percent in the past century to 393 parts per million. The ocean soaks up massive amounts of that gas, which reacts with seawater to form carbonic acid.
That acid will eventually be neutralized, but if the balance of the rates of neutralization and CO2 absorption leans more towards the latter, the report says, the levels of the carbonate ions that various sea creatures require for growing their shells and building coral reefs can be thrown out of whack.
"If industrial carbon emissions continue at the current pace," Hönisch says, "we may lose organisms we care about – coral reefs, oysters, salmon."
Although CO2 increase is cast as the trigger for runaway ocean acidification, it's a pheomenon that's separate from that of the radiative forcing caused by CO2, methane, nitrous oxide, and other factors cited by the Intergovernmental Panel on Climate Change (IPCC) 2007 report and other studies, and blamed for widely accepted increases in global temperatures.
It's estimated that the oceans' acidity rose by a massive 0.45 units during the PETM. The IPCC has estimated that pH may – may – drop another 0.3 units by the end of this century, should atmospheric CO2 concentrations continue to increase.
Since it's impossible to fully understand what caused the PETM and its mass extinctions, this paper's conclusion may be dismissed by some as an example of "confirmation bias", as it's called in scientific circles, by oceanic scientists concerned about rising ocean acidification as it relates to increases in atmospheric CO2.
That said, a 2011 study of reefs in Papua New Guinea showed that when the pH around reefs there dropped to 7.8 due to shallow volcanic CO2 seeps – an acidification level that, the study says, would require atmospheric CO2 to soar to around 750ppm – biodiversity dropped by as much as 40 per cent.
Not that ocean life as we know it is doomed to a watery grave. Some organisms, in fact, can benefit from a more-acidic environment. "We know that life during past ocean acidification events was not wiped out – new species evolved to replace those that died off," Hönisch said.
While that may be encouraging to those worried about a CO2-induced Armageddon, it's cold comfort to those species that may face a PETM II extinction. ®