What Is Ocean Acidification: CO2, Coral Reefs, and Marine Life Impacts

The Ocean as a Carbon Sink: A Double-Edged Role

The world's oceans have absorbed approximately 25 to 30 percent of all the CO2 emitted by human activities since industrialization — a service that has substantially slowed the pace of climate change. Without this ocean carbon sink, atmospheric CO2 concentrations and associated warming would be significantly higher than they are today. But this benefit comes with a cost: when CO2 dissolves in seawater, it undergoes chemical reactions that increase the acidity of the ocean, a process termed ocean acidification. Since the beginning of the industrial era, the average pH of the ocean surface has dropped from about 8.2 to approximately 8.1 — a seemingly small change that represents a 26 percent increase in hydrogen ion concentration, because pH is a logarithmic scale.

The rate of acidification is unprecedented in the geological record. While the ocean has been more acidic in Earth's deep past — notably during the end-Permian mass extinction 252 million years ago — those changes occurred over tens of thousands of years, allowing marine organisms time to adapt. The current acidification is occurring orders of magnitude faster, over decades to centuries, presenting a challenge to marine life that may outpace evolutionary response times. Ocean sediment cores and boron isotope proxies show that the ocean has not been as acidic as it will be under business-as-usual emissions scenarios for at least 300 million years.

Ocean acidification is not uniform. Cold polar waters absorb more CO2 than warm tropical waters because gas solubility increases with decreasing temperature, meaning the Arctic and Antarctic oceans are acidifying faster and are more immediately at risk. Coastal and upwelling regions, where deeper, naturally more acidic water wells up to the surface, already experience pH values close to what the broader ocean will experience in coming decades — providing a preview of future conditions. Freshwater inputs can also reduce ocean pH locally around river mouths and estuaries.

The Chemistry: How CO2 Makes Seawater More Acidic

When carbon dioxide (CO2) dissolves in seawater, it reacts with water (H2O) to form carbonic acid (H2CO3). Carbonic acid is unstable and rapidly dissociates into a bicarbonate ion (HCO3-) and a hydrogen ion (H+). It is the increase in hydrogen ions — the definition of increased acidity on the pH scale — that constitutes ocean acidification. Some of these excess hydrogen ions then react with carbonate ions (CO32-) already present in the seawater, forming additional bicarbonate. This reaction reduces the concentration of carbonate ions in the water.

This reduction in carbonate ion concentration is crucially important for marine life. Carbonate ions are the building blocks that many marine organisms use to construct their shells and skeletons, specifically in the mineral forms of calcium carbonate: calcite and aragonite. Aragonite is the mineral form used by corals, pteropods (free-swimming sea snails), and some bivalves; it is more soluble and thus more vulnerable to acidification than calcite, which is used by other organisms including coccolithophores and some foraminifera. As carbonate ion concentrations decrease, the water becomes undersaturated with respect to these minerals — meaning the minerals tend to dissolve rather than precipitate.

A key metric is the aragonite saturation state (Ω_arag), which measures how saturated seawater is with respect to aragonite. Values above 1 indicate supersaturation (favorable for shell formation); values below 1 indicate undersaturation (shells and skeletons tend to dissolve). Pre-industrial surface ocean aragonite saturation was approximately 3 to 4 in tropical regions. By 2025, tropical values have declined to around 2.5 to 3, and polar surface waters already experience seasonal undersaturation (Ω below 1) during winter. Under high-emissions scenarios, tropical coral reef regions could see aragonite undersaturation by the end of this century.

Coral Reefs: The Most Threatened Ecosystem

Coral reefs are among the most biologically diverse and economically valuable ecosystems on Earth, covering approximately 0.1 percent of the ocean floor while supporting an estimated 25 percent of all marine species. They protect coastlines from wave erosion, support fisheries that feed hundreds of millions of people, generate tourism revenue estimated at tens of billions of dollars annually, and are a source of compounds with pharmaceutical applications. They are also among the most vulnerable ecosystems to ocean acidification, in combination with the thermal stress of warming oceans.

Reef-building corals are animals — the coral polyps — that secrete calcium carbonate skeletons to build the reef structure. Each polyp houses symbiotic algae called zooxanthellae that photosynthesize and provide the coral with up to 90 percent of its energy needs through nutrient transfer. When waters are too warm (typically 1°C above the average maximum summertime temperature for a prolonged period), the zooxanthellae are expelled, the coral whitens (bleaches), and without its energy source, it eventually starves and dies. This thermal bleaching, which occurred at catastrophic scale on the Great Barrier Reef in 2016, 2017, 2020, 2022, and 2024, is driven primarily by warming — but acidification compounds the stress.

Ocean acidification reduces the rate at which corals can calcify their skeletons, weakens existing skeleton structure, and makes it more energetically expensive for corals to maintain their calcium carbonate structures against increasing dissolution pressure. Laboratory experiments and field studies show consistent reductions in calcification rates across coral species as CO2 concentrations rise. Perhaps more critically, as aragonite saturation drops, the balance between coral reef growth and dissolution shifts; below certain saturation thresholds, reefs begin dissolving faster than they grow, transitioning from net builders to net dissolvers. The combination of thermal bleaching and acidification creates a compounding stress that many reef scientists describe as an existential threat to most of the world's coral reefs under business-as-usual emissions scenarios.

Shellfish, Pteropods, and the Broader Marine Food Web

Ocean acidification threatens not only coral reefs but a wide range of shell-forming organisms throughout the marine ecosystem. Pteropods — small, free-swimming sea snails that are abundant in polar and subpolar waters — produce aragonite shells that dissolve rapidly under the moderately acidified conditions already present in some regions. Studies of pteropods collected from the Southern Ocean and North Pacific have documented severe shell dissolution on living animals collected in naturally acidified upwelling waters. Pteropods are a critical food source for salmon, herring, mackerel, whales, and other organisms; their decline could cascade through food webs.

Commercially important shellfish including oysters, clams, mussels, sea urchins, and scallops are also highly sensitive to ocean acidification, particularly during early life stages when shell formation is most critical. Pacific oyster hatcheries on the U.S. West Coast experienced catastrophic larval mortality events from 2007 to 2010 that were traced to corrosive upwelling water — a preview of what may become routine ocean chemistry in coming decades. Hatchery operators have developed monitoring and buffering systems to manage water chemistry, but wild shellfish populations face no such protection. Oyster and shellfish aquaculture industries worth billions of dollars globally face existential uncertainty under projected acidification scenarios.

The impacts extend to fish as well. While fish have sophisticated mechanisms for maintaining internal acid-base balance, elevated CO2 (hypercapnia) impairs sensory abilities in larvae and juveniles of several species, altering their ability to detect predators, respond to chemical cues, and navigate. Studies of clownfish, coral reef fish, and salmon larvae have found impaired behavior under elevated CO2 conditions comparable to projected ocean chemistry. The implications for fish population dynamics and ecosystem function under continued acidification are poorly understood but potentially significant, particularly given the many stressors already pressuring marine fish populations.

Impacts on Marine Chemistry and Biogeochemical Cycles

Ocean acidification affects not only individual organisms but the broader chemical and biological processes that regulate Earth's climate and biogeochemical cycles. The carbonate chemistry changes that harm shell-forming organisms also affect the "biological pump" — the process by which photosynthesis in surface waters fixes atmospheric CO2 into organic carbon that sinks to depth as dead organisms and fecal pellets, sequestering carbon in the deep ocean. Changes in the abundance, physiology, and calcification of key organisms including coccolithophores (which produce calcite shells) and pteropods could alter the efficiency of this pump, potentially changing the ocean's capacity to absorb and sequester future CO2 emissions.

Nitrogen cycling in the ocean is also affected by pH changes. Nitrification — the microbial conversion of ammonium to nitrate — is sensitive to pH: acidification has been shown to slow nitrification rates in some marine environments, potentially altering the availability of nitrogen in forms usable by phytoplankton. Changes in nitrogen cycling could affect primary productivity across vast ocean regions. Similarly, the production of dimethylsulfoniopropionate (DMSP) by some phytoplankton species — a compound that produces dimethyl sulfide (DMS) when it enters the water, and which plays a role in cloud formation — may be altered by acidification, with potential but poorly constrained feedbacks on climate.

Sound propagation in seawater is also altered by pH: more acidic water absorbs less low-frequency sound, meaning that in a more acidic future ocean, sound travels farther. This has potential consequences for marine mammals that rely on acoustic communication and echolocation, and for the behavior of fish that use sound for communication and spawning cues. The interconnected nature of ocean chemistry means that the consequences of acidification ripple through marine systems in ways that are only beginning to be understood.

Future Projections and What Mitigation Can Achieve

The trajectory of ocean acidification is tightly coupled to CO2 emissions. Under high-emissions scenarios (SSP5-8.5), ocean surface pH could decline by a further 0.3 to 0.4 units by 2100 — a total decline of 0.4 to 0.5 units from pre-industrial levels, representing a 150 percent increase in ocean acidity. Under strong mitigation scenarios compatible with the Paris Agreement's 1.5°C target, ocean acidification would be significantly reduced, with pH declining by only an additional 0.07 to 0.1 units by 2100. This difference is substantial: the lower mitigation pathway keeps most tropical coral reefs within the range of aragonite saturation states that have historically supported reef growth, while the high-emissions pathway pushes most reef regions into conditions that support net dissolution by mid-century.

Various ocean alkalinity enhancement (OAE) approaches are being proposed as potential ways to counteract acidification locally or regionally. Adding alkaline minerals like olivine, basalt, or lime to the ocean can raise pH and increase carbonate saturation. This approach simultaneously sequesters additional CO2 and counteracts acidification — a dual benefit that has attracted significant research interest. However, the scale required to meaningfully impact global or even regional ocean chemistry is enormous, the ecological effects of adding large quantities of mineral material are poorly understood, and governance and monitoring frameworks are lacking. Local applications near coral reefs or shellfish beds may be more tractable as targeted protection measures.

Ultimately, the only solution to ocean acidification at a global scale is reducing CO2 emissions. Unlike ocean warming, which is closely tied to cumulative emissions and requires removing CO2 from the atmosphere to reverse, ocean pH will stop declining once emissions stabilize — though it will not return to pre-industrial values without active carbon dioxide removal. Every tonne of CO2 emissions avoided translates directly into less acidification. For coral reefs and the marine ecosystems that depend on them, the urgency of emissions reductions is existential: at current trajectories, many of the world's iconic reef systems will be chemically uninhabitable for reef-building corals within the lifetimes of children alive today.