Ocean acidification is caused by the ocean absorbing carbon dioxide from the atmosphere. Since the Industrial Revolution, the ocean has soaked up roughly 31% of all human-produced CO2, and that absorption has dropped the average surface pH from 8.21 to 8.10. That may sound small, but because the pH scale is logarithmic, it represents about a 30% increase in acidity.
How CO2 Changes Seawater Chemistry
When carbon dioxide dissolves in seawater, it doesn’t just sit there. It reacts with water to form carbonic acid, a weak acid that quickly splits apart into hydrogen ions and bicarbonate ions. Some of those bicarbonate ions break down further into carbonate ions and yet more hydrogen ions. The buildup of hydrogen ions is what lowers the pH and makes the water more acidic.
Seawater has a natural buffering system that can neutralize some of this added CO2. Dissolved carbonate ions react with the incoming carbon dioxide to form bicarbonate, which effectively soaks up the acid. But this defense has limits. The more CO2 the ocean absorbs, the more carbonate ions get used up in the buffering process. That leaves fewer carbonate ions available in the water overall, which creates a separate, serious problem for marine life (more on that below).
Where the Carbon Dioxide Comes From
The dominant source is fossil fuel combustion. Burning coal, oil, and natural gas for energy releases billions of tons of CO2 into the atmosphere each year. Cement production and land-use changes (such as deforestation) also contribute. Between 1994 and 2007 alone, the ocean absorbed an estimated 34 billion metric tons of this anthropogenic carbon, averaging about 2.6 billion metric tons per year.
Atmospheric CO2 concentrations have climbed from about 280 parts per million before industrialization to well over 420 ppm today. The ocean’s surface is in constant gas exchange with the atmosphere, so as atmospheric CO2 rises, the ocean steadily absorbs more of it. The process is relentless: as long as atmospheric concentrations keep climbing, the ocean keeps acidifying.
Coastal Sources That Make It Worse
In many coastal areas, the global CO2 signal is amplified by local pollution. Nutrient-rich wastewater and agricultural runoff pour nitrogen and phosphorus into nearshore waters, fueling massive blooms of algae and plankton. When that organic matter sinks and decomposes, bacteria consume oxygen and release CO2 directly into the water column, driving pH even lower than global trends alone would predict.
Research in the Southern California Bight illustrates how significant this can be. Wastewater outfalls in that region release roughly 142,000 kilograms of nitrogen per day into coastal waters. That nitrogen feeds biological productivity at the surface, but the decomposition of all that extra organic matter at depth intensifies both acidification and oxygen loss on the continental shelf. In upwelling zones, where deep, naturally CO2-rich water already rises to the surface, these added nutrients make an already vulnerable system even more acidic.
Why Cold Waters Are Hit Hardest
Cold water holds more dissolved gas than warm water. That basic physical property makes polar oceans, especially the Arctic, far more vulnerable to acidification. Some of the fastest acidification rates on the planet have been recorded in Arctic waters, driven by a combination of cold temperatures, naturally elevated baseline CO2 from global ocean circulation, and seasonal processes that concentrate CO2 in certain water layers. Freshwater inputs from melting ice further reduce the buffering capacity of Arctic surface water, compounding the problem.
Tropical and temperate oceans are acidifying too, but at a somewhat slower pace. The concern is that polar ecosystems, which support enormous fisheries and unique food webs, are reaching harmful thresholds sooner than the global average would suggest.
What Acidification Does to Marine Life
The loss of carbonate ions is the most direct biological consequence. Corals, oysters, mussels, sea urchins, and many types of plankton build their shells and skeletons out of calcium carbonate. They pull carbonate ions from the surrounding water to do this. As acidification consumes those ions, building and maintaining hard structures becomes increasingly difficult.
Scientists track this using a measure called the aragonite saturation state. Aragonite is the specific form of calcium carbonate that corals and many shellfish use. When the saturation state is above 3, calcifying organisms generally thrive. Below 3, they become stressed. Below 1, their shells and skeletons begin to physically dissolve. Large stretches of the ocean, particularly in polar regions, are already approaching or dipping below these thresholds seasonally.
The effects ripple through the food web. Tiny shelled plankton called pteropods are a critical food source for fish, whales, and seabirds in polar waters. If their shells thin or dissolve, the animals that depend on them lose a key calorie source. Coral reefs, which support roughly a quarter of all marine species, grow more slowly and become more fragile as the water around them acidifies.
How Fast This Is Happening
The current rate of ocean acidification is essentially unprecedented in the geological record. The closest natural comparison is the Paleocene-Eocene Thermal Maximum, a period of massive carbon release about 56 million years ago. During that event’s main phase, thousands of billions of tons of carbon entered the atmosphere, but over thousands to tens of thousands of years. That slow release gave the ocean time to mix the carbon into deep water, partially cushioning the pH drop at the surface.
Today’s emissions are compressing a similar-scale carbon release into just a few centuries. The ocean’s deep mixing processes operate on timescales of roughly a thousand years, so the CO2 we’re pumping into the atmosphere is accumulating in surface waters far faster than the deep ocean can dilute it. One pre-PETM carbon release event, which involved hundreds of billions of tons of carbon over centuries (closer to the speed and scale of modern emissions), caused rapid surface warming and acidification that took millennia to fully reverse.
From 1750 to 2000, global surface ocean pH fell by about 0.11 units. If emissions continue on a high trajectory, models project pH could drop another 0.3 units or more by 2100, representing a total acidity increase of roughly 150% compared to preindustrial levels. Even under moderate emission reductions, further acidification is locked in for decades because the ocean is still absorbing CO2 that’s already in the atmosphere.