Recent Developments in Ocean Acidification

Recent research indicates that continued decreases in the pH of ocean water – a process called ocean acidification – can turn into a marine life catastrophe.

The oceans and saline seas cover 71 percent of the planet’s surface and contain 97 percent of global waters, and they all form a single body of water. OA has been studied mostly over the past two decades. Let’s look at some recent developments around this topic.

The Chemistry Behind Ocean Acidification

Oceans absorb about 30 percent of atmospheric carbon dioxide. As carbon dioxide builds up in the atmosphere, oceanic carbon dioxide also increases. Dissolved carbon dioxide in ocean water produces carbonic acid, which then splits into bicarbonate (HCO₃–) and hydrogen (H+) ions. The added hydrogen ions lower the pH value of ocean water and dissolve carbonate minerals – the building blocks of shells and skeletons for calcifying marine creatures, from coral reefs and oysters to pteropods.

London’s Natural History Museum houses specimens collected during the 1872–76 expedition of the HMS Challenger. A group of British micropaleontologists have compared these planktonic foraminifer samples with those of the same species collected in 2009–16 from the same region in the eastern equatorial Pacific Ocean. Their study published in Nature Scientific Reports shows that the 21st-century specimens had much thinner shells.

Accelerated Ocean Acidification

In a study published in Science Advances, Swiss scientists reconstructed the progression of OA from 1800 to 2014. They estimate that the average pH values for ocean waters at 0 to 100-meters deep decreased from 8.17 to 8.05. For water depths of 100 to 500 meters, it decreased from 8.0 to 7.9 units. The aragonite saturation state decreased from 3.31 to 2.7 for water depths from 0 to 100 meters and from 1.87 to 1.53 for water depths from 100 to 500 meters. The researchers pointed out that nearly 50 percent of these decreases occurred from 1994 to 2014.

In another study in Journal of Sea Research, a group of Chinese scientists analyzed the ocean chemistry database from the Copernicus Marine Environment Monitoring Service and found that for the period 1985–2022, the surface ocean pH level decreased 0.024 units per decade; the aragonite saturation state decreased by -0.012 per year; and the calcite saturation state decreased by -0.020 per year for the tropical and subtropical regions where coral reefs live. The researchers also noted that these declines accelerated by 1.4 times in the 21st century.

Environmental Impacts of Low pH Oceans

Ocean water is usually slightly alkaline because it contains salts essential to marine life. The ocean pH scale ranges from 7.8 to 8.3 regionally, depending on latitude, proximity to coasts, water depth, temperature, and biological activity. The long-term global average of surface water is about 8.1 which has decreased by 0.1 units since the 19th century. This value still indicates alkaline water; nevertheless, the pH decrease trends toward acidification. A pH drop of 0.1 might seem to be insignificant, but pH scale is logarithmic, so a 0.1-unit decrease is a 26-percent increase in the concentration of hydrogen ions (100.1 is equal to 1.26).

Small but steady changes in the pH level can have drastic impacts on marine creatures. Experimental work verifies this. For instance, an experimental study of gastropod shells by Chinese scientists reported in Anthropocene shows that after 85 days of exposure to waters with pH values from 8.1 to 7.1, the shell growth was significantly hindered at pH levels lower than 7.5, and shell dissolution became prominent at pH lower than 7.1.

Drops in ocean pH levels during the mass extinction events of the geological past offer additional insights. According to a study by Mingsong Li and colleagues in Nature Geoscience, during the Paleocene-Eocene Thermal Maximum 56 million years ago, ocean pH decreased by 0.46 units from 7.91 to 7.45. Another study by Molly Trudgill and colleagues in Nature Communications indicates that during the Triassic-Jurassic extinction at 201 million years ago the ocean pH level dropped by 0.3 units. While such studies depend on proxy data, it appears that major and long-term decreases in pH levels of ocean water can have far-reaching impacts on marine life.

Currently, all de-acidification methods (see sidebar) are small-scale operations. They require additional financing and policy shifts. Plus, their side effects on ocean ecosystems still need to be investigated. Some scientists believe that ocean de-acidification is probably a much slower process than OA and encourages carbon dioxide emissions into the atmosphere; Nevertheless, these technologies might have their own niches.