This graph shows the correlation between rising levels of carbon dioxide (CO2) in the atmosphere at Mauna Loa with rising CO2 levels in the nearby ocean, at Station Aloha. As more CO2 accumulates in the ocean, the pH of the ocean decreases. (Modified after R.A. Feely, Bulletin of the American Meteorological Society, July 2008.)

By Scott K. Johnson
1 March 2012

Some like to point to cycles when dismissing climate change, brushing off warming as simply being the thing that happens right before cooling. In this view, concern about climate change is akin to the naïve worry that half of schools are performing below average. This is why we need context. We need to know whether an observed change is more like a world premiere or a familiar re-run.

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.

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. […]

Ocean acidification on track to be among the worst of the last 300 million years



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