The ocean becomes more acidic by absorbing carbon dioxide from the atmosphere. When CO2 dissolves in seawater, it triggers a chemical reaction that produces hydrogen ions, which are what make any liquid more acidic. Since the start of the Industrial Revolution, the ocean’s surface has become roughly 30% more acidic, corresponding to a pH drop of about 0.11 units. That may sound small, but the pH scale is logarithmic: each full unit represents a tenfold change in hydrogen ion concentration.
The Chemical Reaction Behind Acidification
The ocean absorbs about 31% of the CO2 that humans release into the atmosphere, acting as a massive carbon sink. Once CO2 molecules cross the surface and dissolve in seawater, they react with water molecules to form carbonic acid. Carbonic acid is unstable and quickly breaks apart, releasing hydrogen ions and bicarbonate ions. Those extra hydrogen ions are what lower the water’s pH.
In balanced seawater, four forms of carbon coexist: dissolved CO2, carbonic acid, bicarbonate, and carbonate. When the ocean takes in more CO2, this balance shifts. The concentration of hydrogen ions rises, the concentration of carbonate ions drops, and the water becomes more corrosive to anything made of calcium carbonate. That last part is critical for marine life, because carbonate ions are the building blocks that shellfish, corals, and many tiny organisms need to construct their shells and skeletons.
How Much CO2 the Ocean Absorbs
Human fossil fuel emissions have climbed steeply over the past several decades. In the 1960s, the world burned enough fossil fuel to release an average of about 11 billion tons of CO2 per year. By the 2010s, that figure had more than tripled to roughly 35 billion tons per year. The ocean has kept pace, steadily absorbing close to a third of those emissions. That absorption has slowed the buildup of CO2 in the atmosphere, but the cost is a fundamental shift in ocean chemistry.
From 1750 to 2000, the average pH of global surface waters dropped by about 0.11 units. Because the pH scale is logarithmic, that translates to approximately 30% more hydrogen ions in the water. To put the scale in perspective, a full one-unit pH drop would mean ten times more hydrogen ions. So a 0.11-unit change, while seemingly modest on the scale itself, represents a significant chemical shift for organisms that evolved in relatively stable conditions over millions of years.
Why Cold Water Acidifies Faster
Not all parts of the ocean are acidifying at the same rate. Cold water can hold more dissolved gas than warm water, the same reason a cold soda stays fizzy longer than a warm one. This means polar oceans absorb more CO2 per unit of surface area than tropical oceans do. Warmer waters can actually release CO2 back into the atmosphere rather than absorb it.
The Arctic Ocean is acidifying faster than the global average. According to NOAA, some of the most rapid rates of ocean acidification in the world have been measured there. Cold temperatures are only part of the explanation. The Arctic also has naturally higher baseline CO2 concentrations because of global ocean circulation patterns, seasonal processes that concentrate CO2 in certain water layers, and unique interactions between land runoff and seawater. The Southern Ocean faces a similar trajectory: surface waters around Antarctica are projected to become undersaturated with aragonite (a key mineral for shell-building) during wintertime as early as 2030.
What This Means for Marine Life
The most immediate biological consequence of acidification is a drop in the saturation state of calcium carbonate minerals in seawater. Marine organisms build their shells and skeletons from two crystalline forms of calcium carbonate: aragonite and calcite. Aragonite is the less stable of the two and dissolves first as water becomes more acidic. When the saturation state drops below a critical threshold, seawater actually becomes corrosive to unprotected shells and skeletons.
Pteropods, tiny free-swimming sea snails sometimes called “sea butterflies,” are among the most vulnerable. Their thin aragonite shells are sensitive to even small pH changes, and shell dissolution has already been observed in regions where aragonite saturation is near the tipping point. Lab studies show that under lower pH conditions, pteropods’ ability to build new shell material drops markedly. Their cells ramp up the genes involved in calcification, essentially working harder, but they still can’t keep up. Pteropods sit near the base of many marine food webs, so their decline would ripple upward to fish, seabirds, and marine mammals that depend on them.
Corals face a related problem. They build their reefs from aragonite, and as carbonate ion concentrations fall, reef-building slows. Meanwhile, the boundary in the water column where aragonite begins to dissolve, known as the saturation horizon, is creeping upward toward the surface. Corrosive water that once stayed in the deep ocean can now flow onto continental shelves and into coastal regions, putting shallow-water ecosystems at greater risk.
The Scale of Change Over Time
A 30% increase in acidity over 250 years is fast by geological standards. The ocean’s chemistry has shifted before, during episodes of massive volcanic CO2 release, but those events played out over tens of thousands of years, giving marine life time to adapt. The current rate of change is many times faster. Organisms that build calcium carbonate structures, from microscopic foraminifera to massive coral reefs, are being asked to cope with chemistry their lineages have never experienced on this timescale.
Measurements from Drake Passage, the body of water between South America and Antarctica, show surface pH declining with corresponding reductions in both calcite and aragonite saturation of roughly 0.06 to 0.09 units per decade. That ongoing trend means the pressure on shell-building organisms intensifies with each passing year, not just from the current level of acidity but from the speed at which conditions continue to shift.