The primary cause of modern ocean acidification is the increased concentration of atmospheric carbon dioxide produced largely by human activities. When carbon dioxide accumulates in the atmosphere, the ocean absorbs part of that excess gas through air sea exchange. The absorbed carbon dioxide then participates in chemical reactions that increase hydrogen ion concentration and lower seawater pH. Human sources include combustion of coal, oil and natural gas, cement production and land use changes such as deforestation.
The ocean would naturally exchange carbon dioxide with the atmosphere even in the absence of human influence, so the critical distinction is that modern atmospheric carbon dioxide concentrations and the rate at which they are increasing have been strongly altered by human activities. Greater atmospheric carbon dioxide therefore drives greater oceanic carbon dioxide uptake and associated acidification.
The chemistry of the cause can be described through the carbonate system. Carbon dioxide entering seawater forms dissolved carbon dioxide and reacts with water to form carbonic acid. Carbonic acid can dissociate into bicarbonate and hydrogen ions. The increase in hydrogen ions lowers pH, while some hydrogen ions combine with carbonate ions to produce bicarbonate. As a result, carbonate ion concentration decreases even though total dissolved inorganic carbon increases.
This distinction is important because ocean acidification is not simply the addition of an external strong acid to seawater. It is a redistribution among different forms of dissolved inorganic carbon. Seawater also contains buffering compounds, especially bicarbonate and carbonate, which resist rapid changes in pH. That buffering capacity slows the pH response but does not eliminate it when large quantities of carbon dioxide are continually added.
Fossil fuel combustion is therefore an indirect chemical cause and atmospheric carbon dioxide is the immediate global driver. Coal, petroleum and natural gas contain carbon that was stored underground for millions of years. Burning these fuels transfers that geological carbon into the atmosphere primarily as carbon dioxide.
Cement manufacture also releases carbon dioxide because limestone is chemically transformed during production, while land use change can release carbon from vegetation and soils and reduce the capacity of ecosystems to remove carbon dioxide. Part of this additional atmospheric carbon eventually enters the ocean. NOAA estimates that the ocean has absorbed roughly one third of human carbon dioxide emissions since the industrial era, meaning that the ocean has acted as a major buffer against atmospheric accumulation while simultaneously experiencing substantial chemical alteration.
Natural processes can also cause local or temporary acidification without being the primary explanation for modern global ocean acidification. Coastal upwelling can bring deep water containing naturally elevated dissolved carbon dioxide to the surface. Respiration and decomposition consume oxygen and release carbon dioxide, particularly in biologically productive coastal waters. Nutrient pollution can stimulate algal blooms, and when the resulting organic matter decomposes, microbial respiration can increase carbon dioxide and reduce pH.
River inputs, freshwater dilution, changes in circulation and changes in temperature can further modify coastal carbonate chemistry. These processes can make particular bays, estuaries and coastal ecosystems more acidic than surrounding offshore waters. They are especially important for understanding local conditions because a global atmospheric driver interacts with regional physical and biological processes.
Ocean warming is not the fundamental chemical cause of ocean acidification, but it can influence the conditions under which acidification develops. Temperature affects the solubility of gases, the equilibrium among carbonate species and the physical movement and mixing of seawater. Warming can also strengthen stratification in some regions, altering the exchange between surface and deeper waters. Biological productivity and respiration respond to temperature and can therefore modify local carbon dioxide concentrations.
Ocean circulation transports dissolved carbon over large distances and carries surface waters into the interior ocean, where carbon can remain isolated from the atmosphere for long periods. Consequently, observed acidification patterns result from the interaction of atmospheric carbon dioxide uptake with physical circulation, temperature, biology, alkalinity and other chemical processes rather than from a single uniform mechanism operating identically everywhere.
The availability of carbonate ions is particularly important because acidification changes the balance between bicarbonate and carbonate. Seawater contains a large reservoir of dissolved inorganic carbon, predominantly in the forms carbon dioxide, bicarbonate and carbonate. When additional carbon dioxide enters the system, equilibrium shifts toward relatively more bicarbonate and less carbonate. This reduces the saturation state of calcium carbonate minerals such as aragonite and calcite.
Organisms that precipitate these minerals may then need to expend more energy to maintain calcification, while exposed calcium carbonate structures can become more vulnerable to dissolution under sufficiently low saturation conditions. Thus, the causal chain can be expressed as human carbon dioxide emissions, atmospheric carbon dioxide increase, ocean carbon dioxide uptake, carbonate system reorganization, lower pH and lower carbonate availability, followed by biological and ecological effects.
The causes of ocean acidification differ somewhat between open ocean acidification and coastal acidification. Open ocean acidification is dominated by the uptake of atmospheric carbon dioxide and therefore closely tracks the global carbon cycle. Coastal acidification can be strongly influenced by additional local processes, including eutrophication, wastewater, agricultural runoff, organic carbon inputs, freshwater discharge, upwelling and restricted circulation. These factors can interact, making coastal systems highly variable over hours, days, seasons and years.
A coastal ecosystem may therefore experience short periods of very low pH even when the surrounding open ocean changes more gradually. This distinction is important for monitoring and management because reducing local nutrient pollution may improve local water chemistry, but it cannot remove the global atmospheric carbon dioxide driver responsible for widespread long term ocean acidification.
The ultimate driver of modern widespread ocean acidification is the disruption of the global carbon cycle by the addition of carbon dioxide faster than natural processes can remove it from the atmosphere and redistribute it through long term reservoirs. Natural carbon exchange between air, ocean, vegetation, soils, rocks and sediments continues, but anthropogenic emissions have shifted the balance. The ocean consequently takes up substantial additional carbon, which helps moderate atmospheric warming while causing chemical consequences within seawater.
This illustrates why ocean acidification cannot be attributed simply to pollution entering the ocean directly. Most global acidification originates from carbon dioxide entering the atmosphere and subsequently entering seawater. Local pollutants and natural processes can intensify acidification in particular environments, but they are additional drivers rather than replacements for the dominant global mechanism.
