Carbon dioxide (CO₂) is the primary greenhouse gas that dissolves in ocean water. The ocean absorbs roughly 25 to 31% of the CO₂ that human activities release into the atmosphere, pulling in more than 2 billion metric tons of carbon every year. This makes the ocean the planet’s largest active carbon sink, but all that dissolved CO₂ comes with significant chemical consequences for marine life.
How CO₂ Dissolves in Seawater
When atmospheric CO₂ meets the ocean surface, it doesn’t just sit there as a dissolved gas. It kicks off a chain of chemical reactions. First, CO₂ reacts with water to form carbonic acid. That carbonic acid quickly splits apart, releasing a hydrogen ion and forming bicarbonate. Bicarbonate can break down further into another hydrogen ion and a carbonate ion. The end result is that most of the carbon in the ocean isn’t floating around as CO₂ at all. It exists primarily as bicarbonate and carbonate ions, which are forms of what scientists call dissolved inorganic carbon.
Those extra hydrogen ions are the key problem. Each one makes the water slightly more acidic, which is why rising CO₂ levels are driving ocean acidification on a global scale.
What About Other Greenhouse Gases?
CO₂ isn’t the only greenhouse gas that can dissolve in seawater. Nitrous oxide is also soluble and has been measured in ocean water across a wide temperature range. Methane dissolves to a lesser extent and is produced by microbial activity in ocean sediments. However, neither gas dissolves in quantities anywhere close to CO₂, and neither undergoes the same kind of cascading chemical reaction with seawater. CO₂ is unique in how aggressively it reshapes ocean chemistry once it enters the water.
The Solubility Pump
The ocean doesn’t absorb CO₂ evenly. Cold water holds far more dissolved CO₂ than warm water, which is why polar oceans are the most powerful carbon sinks on the planet. In regions near the poles, sea ice formation leaves surrounding waters extremely salty and dense. That heavy, carbon-rich water sinks and feeds into a deep global circulation pattern sometimes called the ocean conveyor belt. This process, known as the solubility pump, transports vast quantities of dissolved carbon into the deep ocean, where it can remain locked away for centuries.
There’s also a biological pump at work. Tiny marine organisms like phytoplankton absorb CO₂ through photosynthesis near the surface. When they die, their carbon-containing remains sink toward the seafloor, pulling carbon downward through a completely different mechanism. Together, these two pumps explain how the ocean stores so much carbon at depth.
Why Warmer Oceans Absorb Less CO₂
As global temperatures rise, the ocean’s ability to keep absorbing CO₂ weakens. The physics are straightforward: warmer water holds less dissolved gas. The effect of temperature on CO₂ solubility is actually the dominant factor in how ocean chemistry responds to warming, outweighing other temperature-driven changes in water chemistry. This creates a troubling feedback loop. A warming climate heats the ocean surface, which reduces how much CO₂ the ocean can pull from the atmosphere, which leaves more CO₂ in the air, which drives further warming.
Ocean Acidification and Its Effects
Since the start of the Industrial Revolution, the ocean’s surface pH has dropped by about 0.1 units, from roughly 8.2 to around 8.1. That might sound tiny, but the pH scale is logarithmic. A 0.1 drop represents approximately a 26% increase in the concentration of hydrogen ions in the water.
The consequences hit hardest for organisms that build shells or skeletons out of calcium carbonate, including corals, oysters, mussels, and small sea snails called pteropods. Calcium carbonate exists in two crystalline forms: calcite and aragonite. Aragonite is the less stable of the two and dissolves first as water becomes more acidic. When acidity rises past a critical threshold (a saturation level below 1.0), the water becomes actively corrosive to unprotected shells and skeletons.
This threshold is no longer theoretical. In parts of the West Antarctic Peninsula, aragonite saturation values near that critical level have been documented during winter months. Globally, the depth at which water becomes corrosive to aragonite has been rising toward the surface, meaning corrosive conditions are creeping into shallower coastal waters where more marine life lives. Coral reefs, which are built from aragonite, are especially vulnerable. Pteropods, a food source for salmon and other commercially important fish, face shell dissolution that can compromise their survival in increasingly acidic polar waters.
How Much Carbon the Ocean Can Hold
The ocean currently absorbs over 2 billion metric tons of carbon annually. That absorption has been a major buffer against the full impact of fossil fuel emissions on climate, effectively slowing the rate of atmospheric warming. But this service comes at a cost to ocean ecosystems, and it has limits. As surface waters warm and become more saturated with dissolved carbon, the rate of absorption could slow. Some research suggests this is already happening in certain ocean regions.
The deep ocean still holds enormous capacity for carbon storage, but getting carbon from the surface to the deep relies on circulation patterns that take hundreds to thousands of years to complete. In the short term, the surface ocean is where absorption happens, and the surface is where the effects of rising CO₂ are felt most acutely by marine life.