Ocean Acidification Quiz: Carbon Chemistry and Marine Life Under Threat

When carbon dioxide from the atmosphere dissolves in seawater, it reacts with water to form carbonic acid, which then dissociates to release hydrogen ions and bicarbonate ions. The increase in hydrogen ion concentration lowers the pH of seawater, making it more acidic. Since pre-industrial times, average ocean surface pH has dropped from approximately 8.2 to 8.1, representing a 26 percent increase in acidity. This rate of acidification is faster than any experienced in the ocean's history over the past tens of millions of years.

pH is a logarithmic scale that measures the concentration of hydrogen ions in a solution. Lower pH values indicate higher concentrations of hydrogen ions and therefore greater acidity. Because the pH scale is logarithmic, even small numerical changes represent substantial changes in acidity. The drop in average ocean surface pH from approximately 8.2 to 8.1 since pre-industrial times represents a 26 percent increase in acidity, a change that has significant consequences for marine organisms sensitive to even modest shifts in seawater chemistry.

Ocean acidification and ocean warming are both direct consequences of increased atmospheric carbon dioxide and greenhouse gases, but they arise through different pathways. Warming occurs because greenhouse gases trap infrared radiation, heating the atmosphere and ocean surface. Acidification occurs specifically because the ocean absorbs carbon dioxide, which dissolves to form carbonic acid. Both stressors act simultaneously on marine ecosystems, and their combined effects are often more damaging to marine life than either stressor alone, a phenomenon scientists describe as synergistic stress.

Ocean acidification reduces the availability of carbonate ions in seawater, which shell-building organisms use to construct their calcium carbonate structures. When pH drops sufficiently, seawater can become undersaturated with respect to calcium carbonate minerals, causing shells and skeletons to dissolve rather than form. Organisms such as oysters, mussels, corals, pteropods, and sea urchins are particularly vulnerable. Laboratory experiments and field observations have documented thinner shells, reduced calcification rates, and increased mortality under acidified conditions.

Pteropods, sometimes called sea butterflies, are small free-swimming mollusks found throughout the world's oceans. Their thin, delicate calcium carbonate shells dissolve rapidly under acidified conditions, and scientists have documented shell dissolution in wild pteropod populations in the Southern Ocean and North Pacific. Because pteropods are a major food source for fish, whales, and seabirds, their decline could cascade through marine food webs. Their sensitivity to pH change makes them important sentinel species for monitoring ocean acidification impacts on ecosystems.

Coral reefs, shellfish, and calcifying plankton including pteropods are all highly vulnerable to ocean acidification because their survival depends on forming calcium carbonate structures that become harder to build and easier to dissolve as pH decreases. Large pelagic fish such as tuna do not build calcium carbonate structures, making them directly less vulnerable to acidification, though they can be indirectly affected through disruption of the food webs that support their prey species.

Coral reefs face a dangerous combination of stressors from rising greenhouse gas concentrations. Ocean warming causes coral bleaching by disrupting the relationship between corals and their symbiotic algae, leaving corals energy-starved and vulnerable. Simultaneously, ocean acidification reduces the availability of carbonate ions needed to build and repair the calcium carbonate skeletons that form the reef structure. This dual stress means that bleached reefs recover more slowly and grow less effectively, increasing the likelihood of long-term reef degradation and collapse.

Cold seawater absorbs carbon dioxide more readily than warm water because gas solubility increases at lower temperatures. This means the Southern Ocean and Arctic Ocean are absorbing proportionally more carbon dioxide per unit volume, driving faster acidification in polar and subpolar regions. Some areas of the Southern Ocean are already approaching undersaturation with respect to aragonite, a form of calcium carbonate used by many marine organisms. This makes polar marine ecosystems among the earliest and most severely affected by ocean acidification globally.

Coral bleaching happens when ocean temperatures exceed the normal maximum for a region by even 1 to 2 degrees Celsius for a sustained period. This thermal stress disrupts the relationship between corals and the symbiotic algae called zooxanthellae that live in their tissues and provide up to 90 percent of their energy. Without these algae, corals turn white and face starvation. If temperatures return to normal quickly, corals can recover, but prolonged or repeated bleaching events cause widespread coral mortality and reef degradation.

While fish can regulate their internal pH through physiological mechanisms, research has shown that elevated carbon dioxide levels in seawater can affect their neurological function and behavior. Studies have found that juvenile fish raised in acidified water show impaired responses to predator odors, altered activity patterns, and disrupted sensory processing. These behavioral effects could reduce survival rates and alter population dynamics even in species without calcium carbonate structures, with implications extending across entire marine food webs.

Aragonite is a form of calcium carbonate used by corals, pteropods, and many other marine organisms to build their skeletons and shells. Aragonite saturation state measures whether seawater contains enough carbonate ions to maintain aragonite stability. When this value falls below 1, the water is undersaturated and aragonite structures spontaneously dissolve. As ocean acidification reduces carbonate ion concentrations, aragonite saturation states are declining globally, with polar regions already approaching or below the critical threshold in some areas during winter months.

The biological carbon pump refers to the process by which marine organisms absorb surface carbon dioxide through photosynthesis and, upon death, sink to the deep ocean carrying that carbon with them. Calcifying plankton such as coccolithophores and foraminifera contribute to this pump because their calcium carbonate shells increase their density and sinking rate. Ocean acidification threatens these organisms by reducing calcification rates and shell integrity, potentially reducing the efficiency of carbon export to the deep ocean and weakening a natural feedback that currently helps slow atmospheric carbon dioxide accumulation.

Ocean acidification has already caused documented economic damage to shellfish aquaculture industries in the Pacific Northwest of the United States. Oyster hatcheries in Oregon and Washington have experienced mass failures of oyster larvae linked to the upwelling of corrosive, low-pH seawater along the coast. The Pacific Northwest is particularly vulnerable because cold, carbon dioxide-rich deep water is regularly upwelled to the surface. These hatchery failures prompted the industry to invest in pH monitoring systems and water treatment to protect larvae from acidified water.

Climate models project that if greenhouse gas emissions continue at high rates, ocean surface pH could drop to approximately 7.95 or lower by the end of the 21st century, compared to the pre-industrial value of about 8.2. This trajectory would represent acidification far beyond anything experienced in the ocean for tens of millions of years. The rate of change is particularly concerning because biological evolution cannot keep pace with such rapid environmental shifts, giving many species little time to adapt before conditions become incompatible with their survival.

Ocean acidification is called the other carbon dioxide problem because it is a direct chemical consequence of rising atmospheric carbon dioxide that is entirely separate from the greenhouse warming effect. While warming threatens climate systems globally, acidification specifically threatens marine chemistry and the countless species that depend on stable ocean pH. Both problems share the identical root cause of excess carbon dioxide emissions from human activities, meaning addressing carbon dioxide emissions at their source is the only way to simultaneously reduce both threats to Earth's life-support systems.