Science has published a troubling but not entirely surprising article on the fact that the oceans are acidifying at the fastest rate in 300 million years. Actually, it could be the fastest rate over an even longer time period, but we can only go back with any degree of accuracy for 300 million years.

We first wrote about this issue in early 2007, and this section, which quoted Stormy from Angry Bear, will help bring readers up to speed:

….there are side effects to our love affair with CO2 that are not often mentioned. In fact, whether the earth cools or warms is absolutely irrelevant to these effects. I repeat: Absolutely irrelevant. One of the most startling effects is the acidification of the oceans. Since 1750, the oceans have become increasingly acidic. In the oceans, CO2 forms carbonic acid, a serious threat to the base of the food chain, especially on shellfish of all sizes. Carbonic acid dissolves calcium carbonate, an essential component of any life form with an exoskeleton. In short, all life forms with an exoskeleton are threatened: shell fish, an important part of the food chain for many fish; coral reefs, the habitat of many species of fish…. The formation of carbonic acid does not depend upon temperature. Whether the oceans warm or cool is irrelevant. Of concern only is the amount of CO2 that enters the oceans.

Fast forward to today. Consider the scope of the paper in Science, per a very good discussion in ars technica:

A new paper in Science examines the geologic record for context relating to ocean acidification…The research group (twenty-one scientists from nearly as many different universities) reviewed the evidence from past known or suspected intervals of ocean acidification…They find that the current rate of ocean acidification puts us on a track that, if continued, would likely be unprecedented in last 300 million years.

There is an important driver of this process that this overview mentions only in passing further on, and it’s useful to have it in mind when you review the discussion of the historical record: ocean acidification depends primarily on the rate of atmospheric CO2 increases, not the absolute concentration. Look at how attenuated the rate of past CO2 changes was in the past versus the speed now:

The first period the researchers looked at was the end of the last ice age, starting around 18,000 years ago. Over a period of about 6,000 years, atmospheric CO2 levels increased by 30 percent, a change of roughly 75 ppm. (For reference, atmospheric CO2 has gone up by about the same amount over the past 50 years.) Over that 6,000 year time period, surface ocean pH dropped by approximately 0.15 units. That comes out to about 0.002 units per century. Our current rate is over 0.1 units per century—two orders of magnitude greater, which lines up well with a model estimate we covered recently. The last deglaciation did not trigger a mass extinction, but it did cause changes in some species… During the Pliocene warm period, about 3 million years ago, atmospheric CO2 was about the same as today, but pH was only 0.06 to 0.11 units lower than preindustrial conditions. This is because the event played out over 320,000 years or so. We see species migration in the fossil record in response to the warming planet, but not ill effects on calcifiers… Next, the researchers turned their focus to the Paleocene-Eocene Thermal Maximum (or PETM), which occurred 56 million years ago. Global temperature increased about 6°C over 20,000 years due to an abrupt release of carbon to the atmosphere (though this was not as abrupt as current emissions). The PETM saw the largest extinction of deep-sea foraminifera of the last 75 million years, and was one of the four biggest coral reef disasters of the last 300 million years… The group also examined the several mass extinctions that defined the Mesozoic—the age of dinosaurs. The boundary between the Triassic and Jurassic included a large increase in atmospheric CO2 (adding as much as 1,300 to 2,400 ppm) over a relatively short period of time, perhaps just 20,000 years. The authors write, “A calcification crisis amongst hypercalcifying taxa is inferred for this period, with reefs and scleractinian corals experiencing a near-total collapse.” Again, though, it’s unclear how much of the catastrophe can be blamed on acidification rather than warming. Finally, we come the big one—The Great Dying. The Permian-Triassic mass extinction (about 252 million years ago) wiped out around 96 percent of marine species. Still, the rate of CO2 released to the atmosphere that drove the dangerous climate change was 10-100 times slower than current emissions… In the end, the researchers conclude that the PETM, Triassic-Jurassic boundary, and Permian-Triassic boundary are the closest analogs to the modern day, at least as far as acidification is concerned. Due to the poor ocean chemistry data for the latter two, the PETM is the best event for us to compare current conditions. It’s still not perfect—the rate of CO2 increase was slower than today… The authors conclude, “[T]he current rate of (mainly fossil fuel) CO2 release stands out as capable of driving a combination and magnitude of ocean geochemical changes potentially unparalleled in at least the last ~300 [million years] of Earth history, raising the possibility that we are entering an unknown territory of marine ecosystem change.”

Translation: “We’re probably fucked, but the data is so far outside of historical parameters, we can’t say anything with a high degree of certainty.”