Ocean acidification is the ongoing decrease in the pH of seawater, driven by the ocean absorbing carbon dioxide from the atmosphere. Before the 1700s, average ocean pH sat around 8.2. Today it’s about 8.1, which translates to a 25% increase in acidity. That shift, small-sounding on a 14-point pH scale, is already reshaping marine ecosystems and threatening industries that depend on them.
How CO2 Changes Seawater Chemistry
The ocean acts as a massive carbon sink, absorbing roughly a quarter of the carbon dioxide humans release. When CO2 dissolves in seawater, it reacts with water molecules to form carbonic acid, a weak acid. Carbonic acid then breaks apart into hydrogen ions and bicarbonate ions. Those extra hydrogen ions are what lower the pH and make the water more acidic.
The problem compounds from there. The freed hydrogen ions bond with carbonate ions already present in seawater. Carbonate ions are a critical building block that marine organisms use to construct shells and skeletons out of calcium carbonate. As more hydrogen ions soak up the available carbonate, less of it remains for the creatures that need it. Atmospheric CO2 has risen 40% since pre-industrial times, ten times faster than anything Earth has experienced for millions of years, and the ocean’s chemistry has shifted in lockstep.
Why Shells and Skeletons Are at Risk
Many marine species build their protective structures from two forms of calcium carbonate: aragonite and calcite. Corals, oysters, clams, mussels, and tiny swimming snails called pteropods all depend on water that’s saturated enough with carbonate ions to make shell-building possible. As CO2 absorption drives down carbonate availability, the water becomes undersaturated, meaning it can actually start dissolving existing shells rather than supporting new growth.
Research in the Southern Ocean has documented this in real time. Scientists comparing pteropod shells from undersaturated waters with shells from healthier regions found severe dissolution in the acidified zone alone. Laboratory tests confirmed that just eight days of exposure to low-saturation conditions produced equivalent damage. Pteropods are a foundational food source for fish, seabirds, and whales in polar regions, so their decline has consequences that ripple through the entire food web.
Coral reefs face a similar threat. Reefs depend on the ability to deposit calcium carbonate faster than waves and biological erosion wear it away. When the surrounding water becomes more acidic, corals calcify more slowly, and the balance tips toward net erosion. Reefs support roughly a quarter of all marine species, so even modest declines in reef-building capacity carry outsized ecological weight.
Economic Costs Already Underway
The effects aren’t hypothetical. Washington State’s shellfish industry, valued at $270 million a year and supporting thousands of jobs, became one of the first to experience direct losses. Between 2005 and 2009, oyster seed production in the Pacific Northwest plummeted by as much as 80%. Hatcheries couldn’t figure out why larvae were dying until researchers traced the problem to increasingly corrosive seawater being pumped into their facilities.
Once growers understood the cause, they began monitoring water chemistry and timing their operations to avoid the most acidic conditions. That adaptation helped recover nearly 75% of their losses. But the broader outlook remains serious. Projections suggest acidification could reduce U.S. shellfish harvests by as much as 25% over the next 50 years. Nationally, the shellfish industry is worth $740 million, putting tens to hundreds of millions of dollars at stake from acidification alone.
How Much Worse It Could Get
Future ocean pH depends almost entirely on how much more CO2 humanity emits. The IPCC’s most recent assessment lays out a range of scenarios for the end of this century. Under the most aggressive emissions-cutting pathway, surface ocean pH drops another 0.08 units from recent levels. Under a moderate scenario, it falls 0.17 units. In a high-emissions future with little policy action, the decline reaches 0.37 units, which would bring ocean pH down to roughly 7.7, a level the oceans haven’t seen in tens of millions of years.
Each of those numbers represents an exponential change in hydrogen ion concentration because the pH scale is logarithmic. A 0.37-unit drop doesn’t mean 37% more acidity. It means the ocean would be roughly 130% more acidic than it was in pre-industrial times. At that level, large swaths of the ocean would become undersaturated for aragonite, making shell-building impossible for many species that currently thrive in those waters.
Local Buffers: What Seagrass Can Do
While the core solution to ocean acidification is reducing CO2 emissions, some natural systems offer localized relief. Seagrass meadows absorb carbon dioxide during photosynthesis, which can temporarily raise the pH of the surrounding water. Researchers at Carnegie Science tested whether this buffering effect was meaningful and found that during specific windows, when low tides coincide with daytime photosynthesis, seagrass meadows provided substantial pH buffering to their immediate surroundings.
The effect is limited in scale and timing, but it could still matter for nearby shellfish communities. Some organisms, like blue mussels, appear able to shift the time of day when they do most of their shell-building. If other species can do the same, aligning their calcification with the hours when seagrass is actively buffering, even brief windows of relief could provide real protection. For aquaculture operations near seagrass beds, this natural buffering adds another tool alongside water monitoring and intake timing.
The Bigger Picture
Ocean acidification operates alongside warming temperatures, deoxygenation, and pollution, creating compounding stress on marine life. An organism that might tolerate slightly more acidic water in isolation faces a harder fight when that water is also warmer and holds less oxygen. This combination is what makes acidification particularly dangerous: it doesn’t act alone, and its effects are essentially irreversible on any human timescale. CO2 absorbed by the ocean will take thousands of years to cycle back out naturally.
The chemistry is straightforward, but the consequences are sprawling. Every ton of CO2 that enters the atmosphere changes the ocean a little more, and the ocean holds that change for a very long time.