A7 Acidification and Change in Physical-Chemical Properties

Topic

Ocean acidification is the direct result of the ocean absorbing a portion of the carbon dioxide emitted into the atmosphere by human activities: this dissolved carbon dioxide reacts with water to form carbonic acid, which then dissociates to release hydrogen ions; it is precisely this increase in free hydrogen ions that causes ocean water to become more acidic. Since the pre-industrial era, the average acidity level of the ocean has shifted from a value near 8.18 to one near 8.08 in 2024—a change that, while seemingly small on that scale, equates to a 26 percent increase in the hydrogen ion concentration of seawater.

In turn, this ocean acidification reduces the water's calcium carbonate saturation—in both its aragonite and calcite forms—which is the mineral that calcifying organisms (such as corals, mollusks, and echinoderms) require to build their skeletons and shells. The lower the calcium carbonate saturation, the harder it becomes for these calcifying organisms to extract the necessary mineral from the water; this growing difficulty means that ocean acidification poses a direct threat to their survival.

Ocean acidification alters not only the ocean's chemical composition but also its physical properties. While the speed of sound in seawater increases by approximately 4.6 meters per second for every additional degree of temperature, it also changes due to acidification itself, as the process modifies the seawater equation of state. Furthermore, the increase in dissolved carbon dioxide reduces water density—because dissolved carbon dioxide occupies less volume, molecule for molecule, than when it exists as part of a carbonate ion—and this reduction in density is what causes acidification to ultimately affect ocean stratification as well.

The relationship between this ocean acidification and the carbon cycle also forms a self-reinforcing feedback loop. The ocean absorbs carbon dioxide from the atmosphere, and this very absorption causes acidification; however, a more acidic ocean has a reduced capacity to absorb additional carbon dioxide—a capacity measured by the Revelle factor. Consequently, the already acidified ocean allows more carbon dioxide to accumulate in the atmosphere, thereby accelerating global warming. This warming, by heating the ocean itself, further diminishes its ability to absorb carbon dioxide—since warmer water dissolves fewer gases than cold water—and this reduced capacity leaves even more carbon dioxide in the atmosphere, closing a loop that continues to accelerate both global warming and the ocean acidification that initiated the process.

Under high-emission scenarios, projections indicate that the ocean's average pH level could drop to around 7.8 by the end of the twenty-first century, making the ocean more acidic than it has been in millions of years. In such a scenario, the zone where water remains saturated with aragonite is expected to disappear first from polar regions and subsequently from increasingly shallower depths across the rest of the ocean, to the point where certain calcifying organisms will simply no longer be able to form their skeletons. Thus, from the carbon dioxide absorbed by the ocean to the ultimate inability of a coral or mollusk to build its own skeleton, ocean acidification follows a single chain that begins with the most basic chemistry of the water and ends by determining which forms of marine life can continue to exist.

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