Ocean acidification is the ongoing decrease in ocean pH caused by seawater absorbing carbon dioxide from the atmosphere. Since the Industrial Revolution, the average pH of surface ocean waters has dropped from 8.21 to 8.10. That 0.11-unit shift sounds small, but because pH is measured on a logarithmic scale, it represents roughly a 30% increase in acidity.
How CO2 Changes Seawater Chemistry
The world’s oceans absorb about 31% of the carbon dioxide humans release into the atmosphere. That absorption has long been seen as a helpful buffer against climate change, but it comes with a cost. When CO2 dissolves in seawater, it reacts with water molecules to form carbonic acid, a weak acid that quickly breaks apart into hydrogen ions and bicarbonate ions. Those extra hydrogen ions are what make the water more acidic.
The process also reduces the availability of carbonate ions, which are the building blocks that marine animals use to construct shells and skeletons made of calcium carbonate. So acidification does two things at once: it makes the water more corrosive to existing calcium carbonate structures, and it makes it harder for organisms to build new ones.
Which Ocean Regions Are Hit Hardest
Acidification isn’t uniform across the globe. NOAA research shows that the most vulnerable coastal waters in North America are along the northern stretches of both the east and west coasts. Cold water absorbs more CO2 than warm water, so the northern Atlantic, where the cold Labrador Current flows south from the Arctic, acidifies faster than the warmer waters off the southeastern U.S. or the Gulf of Mexico.
On the Pacific side, the California Current system running from the Canadian border to Baja California faces a double hit. Cold, CO2-rich water naturally wells up from the deep ocean along this coast, and that upwelling combines with the absorption of atmospheric CO2 to accelerate acidification in the region. Shellfish hatcheries in Washington state have already experienced drops in yields linked to this more acidic nearshore water.
Dissolving Shells and Weakening Reefs
Coral reefs are among the most visible casualties. Research on reef-building corals found that by 2100, when ocean pH is projected to reach about 7.8, certain porous coral skeletons could lose roughly 15 kilograms of calcium carbonate per square meter each year. That’s about three times the rate at which modern reefs typically grow, meaning reefs would shrink faster than they can rebuild. In physical terms, that translates to roughly 10.5 millimeters of vertical reef loss per year.
The damage extends well beyond corals. Pteropods, tiny free-swimming sea snails sometimes called “sea butterflies,” are especially sensitive because their shells are made of aragonite, a particularly soluble form of calcium carbonate. Experiments show that as water becomes more acidic, pteropod shells dissolve faster and new shell growth slows. In polar regions, projections suggest an additional 12% of pteropods will show severe shell dissolution by 2050, pushing the total from about 50% to 62% of individuals affected. That matters for the entire ocean food web because pteropods are a critical food source for commercially important fish species like salmon, herring, and cod.
Behavioral Changes in Fish
Acidification doesn’t just dissolve shells. It also rewires how fish perceive the world around them. In laboratory experiments, juvenile clownfish raised in water with elevated CO2 levels lost their ability to detect and avoid predator-rich environments. Under normal conditions, young reef fish avoid daytime reef sounds, which signal the presence of predators. Fish raised in current atmospheric CO2 levels spent 73% of their time swimming away from those sounds. But fish raised in CO2 concentrations expected later this century showed no avoidance at all, spending the majority of their time near the sound source instead.
The same fish also lost their innate ability to recognize predator scents, becoming attracted to odors they would normally flee. These sensory disruptions appear linked to changes in brain chemistry triggered by elevated CO2, and they could significantly reduce survival rates for young fish navigating the dangerous early stages of reef life. Fish hearing may also be affected directly: the dense calcium carbonate ear bones that fish rely on for hearing can grow abnormally under acidified conditions.
Economic Stakes
The financial consequences are already measurable in some regions. Coral reefs alone provide an estimated $9 billion per year in shoreline protection, shielding coastal communities from storm surges and erosion. When reef-supported fisheries are included, that value climbs to $30 billion annually. As reefs weaken and dissolve, coastal communities lose both a natural barrier and a source of income.
Shellfish industries face more immediate pressure. The hatchery failures in Washington state offered an early warning of what acidification can do to economies built around oysters, clams, and mussels. These aren’t distant projections. They’re disruptions that coastal businesses are navigating right now.
What the Future Looks Like
Under high-emission scenarios, acidification will reach far beyond surface waters. Projections for the North Atlantic show that by 2100, roughly 23% of deep-sea canyon floors below 500 meters will experience pH drops exceeding 0.2 units. About 8% of seamounts, some of which have been proposed as marine protected areas, face similar declines. Deep-sea ecosystems that have remained chemically stable for millennia would be pushed into unfamiliar territory within decades.
The trajectory depends heavily on how much CO2 humanity continues to emit. Lower-emission pathways produce significantly less acidification, while continued high emissions lock in changes that will take tens of thousands of years for ocean chemistry to reverse naturally. The ocean’s role as a carbon sink has bought the atmosphere some time, but the price is being paid underwater.