Carbon dioxide dissolves in seawater, triggers a chain of chemical reactions that makes the ocean more acidic, and simultaneously serves as the raw material that marine life uses to build tissue and grow. The ocean absorbs roughly 25% of all CO2 released by burning fossil fuels, making it one of the planet’s largest carbon sinks. That absorption slows climate change in the atmosphere but comes at a steep cost to ocean chemistry and the creatures that depend on it.
How CO2 Changes Ocean Chemistry
When carbon dioxide from the atmosphere meets the ocean surface, it dissolves and reacts with water to form carbonic acid. That acid quickly breaks apart into bicarbonate ions, carbonate ions, and hydrogen ions. The extra hydrogen ions are what lower the water’s pH, making it more acidic. This entire process, from gas hitting the surface to acid forming, happens continuously across every square kilometer of open ocean.
Since the pre-industrial era, ocean acidity has increased by about 40%, corresponding to a pH drop from roughly 8.2 to 8.04 as of 2024. A 0.1 to 0.2 unit shift in pH might sound small, but pH is a logarithmic scale, so each tenth of a unit represents a meaningful jump in hydrogen ion concentration. By 2100, ocean pH is projected to fall an additional 0.1 to 0.4 units depending on how aggressively the world cuts emissions.
The Ocean as a Carbon Sink
Of all the CO2 humans emit, about half stays in the atmosphere, about 25% is taken up by land plants and trees, and the remaining 25% is absorbed by the ocean. This absorption happens through two main routes: a physical process driven by gas exchange at the surface, and a biological one powered by marine life.
The biological carbon pump starts with phytoplankton, microscopic organisms that photosynthesize just like land plants. They pull dissolved CO2 out of surface waters and convert it into organic carbon. When phytoplankton die or are eaten, that carbon sinks toward the deep ocean in the form of falling particles, fecal matter, and dead cells. Vertically migrating animals, like tiny zooplankton that feed at the surface at night and retreat to deeper water during the day, also shuttle carbon downward by digesting it at depth. Together, these biological processes transport an estimated 5 to 12 billion metric tons of carbon into the deep ocean each year.
Once carbon reaches the deep ocean, it stays there for centuries. Deep Atlantic water has an average carbon residence time of about 288 years, while deep Pacific water holds onto carbon for roughly 889 years. The Indian Ocean falls in between at about 716 years. This long storage time is what makes the ocean such a powerful buffer against atmospheric CO2, but it also means the chemical changes happening at depth will persist for a very long time.
Why Warmer Water Absorbs Less CO2
Cold water holds more dissolved gas than warm water. As ocean temperatures rise, the surface becomes less efficient at soaking up carbon dioxide. The sensitivity is significant: for every degree of warming, the water’s capacity to hold CO2 drops by roughly 4%. This creates a troubling feedback loop. A warmer ocean absorbs less CO2, which leaves more in the atmosphere, which drives further warming.
This effect varies by latitude. Polar waters, which are cold and can absorb the most CO2, are also where some of the fastest warming is occurring. Tropical waters, already warm, contribute relatively less to ocean carbon uptake.
Damage to Shells and Skeletons
The extra hydrogen ions released when CO2 dissolves don’t just lower pH. They bond with carbonate ions, pulling them out of circulation. Carbonate is a critical building block for any marine organism that constructs a hard shell or skeleton, including corals, oysters, mussels, clams, and sea urchins. With fewer carbonate ions available, these organisms have to spend more energy to build and maintain their structures. If acidity rises enough, existing shells and skeletons begin to physically dissolve.
Corals illustrate the severity. At a projected pH of 7.8, which is expected by 2100 under high-emission scenarios, porous coral skeletons like those of Montipora species would lose about 15 kilograms of calcium carbonate per square meter per year. That translates to roughly 10.5 millimeters of vertical reef loss annually, a dissolution rate about three times faster than the average growth rate of modern reefs. Not all corals are equally vulnerable: denser, less porous species show more resistance to dissolution at the same pH. But the net result for reef ecosystems is that destruction outpaces construction.
Ripple Effects Through the Food Web
The damage extends well beyond reef-building corals. Pteropods, tiny free-swimming sea snails sometimes called “sea butterflies,” are a staple food for salmon, herring, and other commercially important fish. Their shells are made of aragonite, a particularly fragile form of calcium carbonate. Researchers examining live pteropods pulled from the Southern Ocean found severe shell dissolution in regions where the water was already undersaturated with aragonite. Under a scanning electron microscope, the shells showed pitting and erosion that didn’t appear in specimens from healthier waters. Just eight days of exposure to mildly undersaturated conditions in lab experiments produced equivalent damage.
By 2050, the upper layers of the Southern Ocean are expected to become broadly undersaturated for aragonite, expanding the zones where these organisms face active shell breakdown. Because pteropods sit near the base of polar and subpolar food webs, their decline would cascade upward, reducing food availability for fish, seabirds, and marine mammals that depend on them.
A Balancing Act With Limits
The ocean’s role as a carbon sink has genuinely slowed the pace of climate change. Without it, atmospheric CO2 concentrations would be substantially higher than they are today. But this service comes with compounding consequences: acidification, weakened marine ecosystems, and a diminishing ability to keep absorbing at the same rate as waters warm. The chemistry is straightforward, but the biological toll is complex and already measurable in coral reefs, polar waters, and shellfish populations worldwide.