Dissolved carbon dioxide forms carbonic acid, releasing hydrogen ions and reducing carbonate availability. Average surface seawater remains alkaline, but ‘acidification’ accurately describes its shift toward lower pH.
Three things to know
- Main driver
- Added CO₂
- The ocean absorbs a substantial share of human carbon emissions.
- Chemical change
- Lower pH
- The pH scale is logarithmic, so small numeric shifts matter.
- Key ion
- Less carbonate
- This can make calcification harder for some shells and skeletons.
Carbon dioxide changes seawater chemistry
CO₂ crossing the air–sea boundary reacts with water. The resulting sequence increases hydrogen ions, which combine with carbonate ions. Organisms that build calcium-carbonate structures can therefore face both altered pH and reduced saturation state.
Not every animal responds the same way
Some pteropods, corals and shellfish show reduced calcification or survival under particular conditions; others tolerate or compensate. Temperature, food, oxygen and local variability interact with pH, so one result cannot be applied to every species or life stage.
The global change looks different locally
Coastal upwelling, freshwater, respiration and nutrient pollution can produce large natural and human-amplified pH swings. Long-term observations and experiments are both needed to separate the global carbon signal from local conditions.
What people usually ask
Is the ocean becoming acidic?
Average seawater is still above pH 7, so it remains alkaline. It is becoming less alkaline, which scientists call acidification.
Can animals adapt?
Some populations may acclimate or evolve, but capacity and pace differ, and multiple stressors can limit adaptation.
Here’s where the answer came from
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