Ocean acidification

Definition

Ocean acidification describes the decrease in pH of the oceans due to increased concentrations of dissolved carbon dioxide (CO₂). CO₂ is a trace gas found in the atmosphere. The concentration of dissolved oceanic CO₂ is in equilibrium with the gas’s atmospheric concentration, such that an increase in the atmospheric concentration leads to an increase in the dissolved concentration, and vice versa. Throughout geologic time, atmospheric CO₂ reflected a balance between various sources and sinks found on land and in the oceans—such as photosynthesis, respiration, chemical weathering of rocks, and burial of organic and inorganic carbon. Since the onset of the industrial age, human activities, such as fossil fuel combustion and intensification of agriculture, have represented a new source of atmospheric CO₂.

These activities are thought to have caused much of the observed increases in atmospheric CO₂, which in turn led to increased dissolved CO₂concentrations in the oceans. Dissolved CO₂ combines with water to form carbonic acid (H₂CO₃), which subsequently dissociates to produce bicarbonate (HCO₃^-) and carbonate ions (CO₃^2-) and protons (H⁺). The balance among these different reactions is such that increased concentrations of dissolved CO₂ lead to increased proton concentrations (acidity). Since pH is an inverse scale of the concentration of dissolved H⁺, the higher the proton concentrations, the lower the pH and the greater the acidity. Modeling experiments using different scenarios to project future atmospheric CO₂ concentrations indicated that by 2100, the ocean’s pH would drop by up to 0.45 units.

Significance for Climate Change

The reactions by which CO₂ and water form H₂CO₃, HCO_3- and CO₃^2- control the pH of the world’s oceans. Oceanic pH has important ecological consequences because many plankton (including the Coccolithophoridae, which are marine algae, and the Foraminiferida, which are planktic protists) and macrofauna (corals, mollusks, brachiopods, and so on) precipitate the minerals calcite or aragonite (which both have the chemical formula CaCO₃) to make exoskeletons, shells, and tests in a process called calcification. Under normal conditions, calcite and aragonite are stable minerals in surface waters because CO₃^2- concentrations are naturally maintained at levels that prevent their dissolution. However, increasing atmospheric CO₂ concentrations lead to decreased oceanic pH values and a concomitant decrease in carbonate ion concentration.

This effect has been observed in a vertical profile of the oceans, where carbonate ion concentrations decrease with increasing depth. At a depth known as the lysocline, the rate of carbonate mineral dissolution increases rapidly with decreasing carbonate ion concentrations. Thus, in surface and intermediate waters, carbonate minerals are not dissolved. Below the lysocline, carbonate minerals are readily dissolved. Many laboratory experiments have seemed to indicate that ocean acidification will prove detrimental to many marine ecosystems. This finding has been validated by a study of a benthic ecosystem located near a naturally occurring volcanic vent that delivers CO₂ to the surrounding waters. The full-scale consequences of acidification of the global ocean have been considered likely to be numerous and could include extinctions as food webs collapse due to a loss of calcareous planktonic primary producers and consequences for the strength of the biological pump.

The biological pump is the process by which CO₂ is actively removed from the atmosphere by primary producing phytoplankton who convert the CO₂ to organic matter via photosynthesis. When these organisms die, their hard parts act as ballast to help the organic matter sink to the ocean floor, where it is buried in sediments. Ultimately, this sequesters the CO₂ in rocks for geologic time scales. However, if ocean acidification leads to the dissolution of calcareous tests, organic matter will not be effectively buried because it is not dense enough to settle to the ocean floor. Not all scientists have agreed with the negative predicted consequences of ocean acidification. They have argued that carbonate minerals present in sedimentary rocks on the ocean floor should, over time, consume the excess H⁺ produced from increased atmospheric CO₂ concentrations, causing ocean acidification to slow or even stop.

The increase in atmospheric CO₂ concentrations has appeared to be linked to an increase in global temperatures. Temperature is a strong control on the conversion of atmospheric CO₂ to dissolved CO₂ in the ocean. Henry’s Law states that the dissolved concentration of a gas is proportional to the partial pressure of that gas in the atmosphere in contact with that liquid. Gases are characterized by different Henry’s Law constants that give the proportionality of dissolved gas that will be in equilibrium with the overlying atmosphere. For most gases, including CO₂, the value of the Henry’s Law constant decreases with increasing temperature—that is, at higher temperatures, the amount of CO₂ that can be dissolved in the ocean will decrease. This temperature dependence provides negative feedback on the amount of ocean acidification that could occur. However, this also means that a greater proportion of anthropogenic CO₂ would remain in the atmosphere as global temperature rises.

According to the National Oceanic and Atmospheric Administration (NOAA), by the early 2020s, the world's oceans had absorbed more than 150 billion metric tons of CO₂ over two hundred years due to human activities. In 2021, the NOAA's National Centers for Environmental Information (NCEI) provided analyzed highlights from the American Meteorological Society's annual State of the Climate report. Emphasizing the findings' confirmation of 2020 setting a record as one of the top-three warmest years in recorded history, NCEI noted that as well as upper atmospheric temperatures remaining especially high, the world's oceans had absorbed approximately 3 billion metric tons more CO₂ than they had released in 2020. As well as marking the highest since 1982, this amount was nearly 30 percent higher than the average from 1999 to 2019. In its 2022 annual report, the World Meteorological Organization asserted that in 2021, open ocean surface pH had reached its lowest point in at least the previous twenty-six thousand years and that the rate of change was also unprecedented since at least that time. NOAA reported that 2024 saw the biggest increase of CO₂ absorption in a single year, increasing by 3.75 parts per million (ppm) for a total of 422.8 ppm, resulting in pH levels becoming 30 percent more acidic than they were around the time of the Industrial Revolution.

In addition to the increased potential for ecological damage, studies have highlighted some of the negative commercial impacts that oceanic changes related to acidification could create. In 2021, some scientists conducted controlled studies in which they theorized that ocean acidification could be affecting some crucial fish behaviors, specifically that the conditions resultant from acidification were slowing survival escape responses and weakening the cohesion of protective shoal movement. By that time, other scientists had begun studying whether an alkalizing agent could be injected into the ocean in the region of the Great Barrier Reef to aid the particularly highly stressed marine area in offsetting acidification. A 2025 study of shark teeth, which are naturally shed and regrown, found that the teeth were degrading much faster due to the acidity in the oceans. Researchers stated concerns that this may impact sharks' abilities to find and consume food, as well as lead to greater disruptions in marine ecosystems. An earlier study conducted in 2022 had determined shark teeth to be resistant to lower pH levels, so more research needs to be done to determine a definitive answer on the effects ocean acidity may have on the shark population.

Bibliography

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