Geologists have shown that a massive release of carbon dioxide, or CO₂, 56 million years ago caused global temperatures to increase by 9 to 14°F (5 to 8°C). Carbon dioxide gas in the atmosphere also dissolves in ocean water, forming carbonic acid. Carbonic acid causes the oceans to become more acidic, leading researchers to think that these changes could have triggered mass migrations and worldwide extinctions. 

Scientists call this event the Paleocene-Eocene Thermal Maximum or PETM. Researchers have a limited understanding of ocean acidification during the PETM because they have no direct way to measure the acidity of ancient oceans. This lack of data leaves a blind spot in their knowledge of how the atmosphere and oceans interacted during the PETM. 

To understand more about the PETM, a research team led by Mingsong Li modeled ocean acidity using newly available data. One type of data they used was the ratio of boron atoms with different numbers of neutrons, known as isotopes. Boron isotopes are contained in a mineral called calcium carbonate that gets deposited on the seafloor. Scientists have demonstrated that geologic periods with higher concentrations of the heavy isotope of boron, known as boron-11, also had more dissolved CO₂ in the oceans. 

Another type of data they used to study ocean acidity during the PETM was the concentration of carbonate ions, or CO₃^2-. Researchers in the past found that carbonate ions decreased during this time. They claimed this reduction in carbonate triggered a 30% to 50% loss of plankton in the oceans, since plankton need carbonate to make their shells. 

The team used this data to generate a computer model of the PETM called the carbon-centric grid-enabled Earth system data model, or cGENIE. They used this model to map the pressure exerted by CO₂ gas, called its partial pressure, in the oceans and atmosphere. The scientists set the partial pressure of CO₂ in their model to match ocean values they inferred for the PETM from boron-11 data. When they did so, they saw an increase in atmospheric CO₂. 

The team estimated that atmospheric CO₂ levels were 890 parts per million before the PETM, and 1980 parts per million during the PETM, which agreed with other PETM models. They proposed that this increase in atmospheric CO₂ led to a domino effect, whereby higher CO₂ in the atmosphere meant the oceans dissolved more CO₂. More dissolved CO₂ in the oceans led to higher concentrations of carbonic acid, which dissociated into hydrogen and carbonate ions, acidifying the ocean.

Their model suggested that the global average ocean acidity increased nearly 5-fold during the PETM. However, they found that the changes in acidity varied around the globe. For example, polar regions experienced a 6-fold increase in acidity compared to a 4-fold increase near the equator. The team also found that their model showed a greater increase in ocean acidity than past PETM models. They proposed that this increase in ocean acidity would have decreased calcium carbonate in the ocean by 26% on average. 

Li’s team concluded that more carbon dioxide in the air and oceans during the PETM disturbed the circulation of carbon atoms through the Earth’s atmosphere, land, and oceans, due to the increased partial pressure of CO₂ and ocean acidity. They found that the increases in ocean acidity and dissolution of carbonate they calculated for the PETM parallel what we are seeing today, due to the similar increase in atmospheric CO₂.

The researchers aim to include ratios of carbon isotopes in their PETM model in the future, allowing them to determine the atmospheric concentration of CO₂ more accurately. They suggested that these data could also help determine how much carbon dioxide dissolved in the ocean and how acidic it became, aiding scientists in understanding this period of climate change.