A new paper in Science examines the geologic record for context relating to ocean acidification, a lowering of the pH driven by the increased concentration of carbon dioxide in the atmosphere. The research group (twenty-one scientists from nearly as many different universities) reviewed the evidence from past known or suspected intervals of ocean acidification. The work provides perspective on the current trend as well as the potential consequences. 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 are several ways acidification events leave their signature in the rock record. The isotopic composition of carbon changes with shifts in the carbon cycle, such as the movement of greenhouse gases like methane and carbon dioxide in the atmosphere. Isotopes of boron present in marine shells track ocean water pH. The ratios of other trace elements in marine shells (such as uranium or zinc) to calcium indicate the availability of carbonate ions. (Ocean acidification is not just about pH, but the reduction of carbonate mineral saturation that makes it more difficult for calcifiers to build their shells.) In addition to all this, the fossil record records the extinctions and morphological changes in marine species that occur around catastrophic events in Earth history.

Reconstructing the past

The paper covers the last 300 million years. That’s not just a round number—it’s about as far back as we can confidently go. Because plate tectonics drives oceanic plates back down into the mantle at subduction zones, there is no oceanic crust or sediment older than 180 million years for us to examine.

To look back farther than that, you’ve got to rely on the limited supply of marine rocks that shifted onto continental plates. That makes it harder to construct a global picture, as some regions become over-represented. Also, as these records extend deeper and deeper into the past, uncertainty in ages and calcifier physiology reduces confidence in the results of these analyses. Beyond 300 million years ago, the unknowns for some of these measures are just too large.

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. The shells of planktic foraminfera decreased by 40-50 percent, while those of coccolithophores went down 25 percent.

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. This is because ocean acidification depends primarily on the rate of atmospheric CO₂ increases, not the absolute concentration.

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.

We don’t have good records of pH over this period, so it’s difficult to tell how much of the extinctions were caused by ocean acidification as opposed to the temperature change or decrease in dissolved oxygen that results from warming ocean water.

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.

Matching the modern to history

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.

Perhaps more significantly, the ocean chemistry was actually less sensitive to change then. The ratio of magnesium to calcium in ocean water changes over time due to differences in volcanic activity along the mid-ocean ridges, among other things. When magnesium is high (as it is today), a form of calcium carbonate called aragonite becomes dominant. Aragonite is more soluble than calcite, so “aragonite seas” are more susceptible to the effects of acidification. Even though the PETM did not feature aragonite seas, it was a tumultuous time for many marine species.

While the authors frequently point out the difficulty in teasing apart the effects of ocean acidification and climate change, they argue that this is really an academic exercise. It’s more useful to consider the witches’ brew with all the ingredients—acidification, temperature change, and changes in dissolved oxygen—since, historically, those have come together. That combination produces unequivocally bad news.

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.”

Citation: “The Geological Record of Ocean Acidification.” By Bärbel Hönisch et al. Science, Vol. 335, No. 6072, Pg. 1058-1063. March 2, 2012. DOI: 10.1126/science.1208277

Read the complete article on Wired Science.