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How does ocean acidification work?

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Quick answer

Ocean acidification works through simple chemistry. Carbon dioxide from the air dissolves in seawater and forms carbonic acid, releasing hydrogen ions that lower pH. Those ions also lock up carbonate, the building block corals and shellfish use for skeletons. Surface ocean pH has dropped from about 8.2 to 8.1 since the Industrial Revolution.

Ocean acidification happens when seawater absorbs carbon dioxide from the air, which reacts with water to form carbonic acid and lowers the ocean's pH. That chemistry also removes carbonate ions that corals, oysters and other shell-builders need, making it harder for them to build and maintain their skeletons.

Step by step

  1. Burning coal, oil and gas raises carbon dioxide levels in the atmosphere.
  2. At the sea surface, some of that carbon dioxide dissolves into the water and mixes downward.
  3. Dissolved carbon dioxide reacts with water to form carbonic acid, which splits into bicarbonate and hydrogen ions.
  4. The extra hydrogen ions lower the pH and combine with carbonate ions, shrinking the carbonate supply.
  5. Shell-building creatures struggle to gather enough carbonate, so their shells and skeletons grow more slowly or weaken.

Key things to understand

  1. The ocean absorbs roughly a quarter of the carbon dioxide released by burning fossil fuels and other human activities.
  2. Dissolved carbon dioxide combines with water to form carbonic acid, which releases hydrogen ions into the seawater.
  3. More hydrogen ions mean lower pH, and average surface ocean pH has fallen from about 8.2 to about 8.1 since pre-industrial times.
  4. Extra hydrogen ions bind with carbonate ions to make bicarbonate, leaving less carbonate available for marine life.
  5. Corals, oysters, sea urchins and tiny plankton then spend more energy building calcium carbonate shells, which may grow thinner or dissolve.

Labelled diagram

  1. Atmospheric CO2

    Carbon dioxide from fossil fuels builds up in the air above the sea.

  2. Dissolving

    Some carbon dioxide crosses the sea surface and dissolves in seawater.

  3. Carbonic acid

    CO2 reacts with water and releases hydrogen ions, lowering the pH.

  4. Carbonate loss

    Hydrogen ions bind carbonate, turning it into bicarbonate that shells cannot use.

  5. Weaker shells

    Corals and shellfish struggle to build calcium carbonate skeletons.

How carbon dioxide lowers seawater pH

Real-world examples

  • Oyster hatcheries in the US Pacific Northwest suffered larval die-offs in the late 2000s linked to corrosive upwelled water.
  • Coral reefs in Lakshadweep and the Gulf of Mannar face acidification on top of heat-driven bleaching.
  • Pteropods in the Southern Ocean show shell damage in waters low in carbonate.
  • Long-term monitoring near Hawaii records seawater pH falling as atmospheric carbon dioxide rises.

Common mistake

✗ Myth: Ocean acidification means the sea is turning into acid that would burn your skin.

✓ Fact: Seawater is still mildly alkaline and harmless to swim in. The concern is a steady chemical shift that reduces carbonate and stresses marine organisms, not water becoming corrosive to people.

People also ask

What causes ocean acidification?
Ocean acidification is caused mainly by the ocean absorbing carbon dioxide released by burning fossil fuels, making cement and clearing forests. The dissolved carbon dioxide forms carbonic acid, which lowers seawater pH. Local factors such as nutrient runoff and upwelling of deep, carbon-rich water can make coastal waters more acidic still.
How does ocean acidification affect coral reefs?
Corals build skeletons from calcium carbonate drawn from seawater. Acidification reduces the carbonate available, so corals grow more slowly and their skeletons can be less dense. Reef structures also erode faster. Combined with warming-driven bleaching, this makes it harder for reefs to recover from damage and keep pace with erosion.
Can ocean acidification be reversed?
Natural processes can restore ocean chemistry, but very slowly, over thousands of years as rocks weather and deep waters mix. On human timescales, the main way to slow acidification is to cut carbon dioxide emissions. Protecting seagrass and mangroves can locally buffer coastal water, though it cannot fix the global problem.
Is the ocean actually becoming acidic?
Not yet. Seawater remains slightly alkaline, around pH 8.1. 'Acidification' describes the direction of change toward lower pH, the same way 'cooling' describes falling temperature without meaning it is cold.
Why does a 0.1 drop in pH matter?
The pH scale is logarithmic, so a fall of 0.1 units means roughly 26 to 30 percent more hydrogen ions. That is a large chemical shift for organisms adapted to stable conditions over very long periods.
Is ocean acidification the same as global warming?
No, but both come mainly from rising carbon dioxide. Warming results from heat trapped in the atmosphere, while acidification is a direct chemical change when carbon dioxide dissolves in seawater.
Which sea creatures are most affected?
Organisms that build calcium carbonate structures are most exposed, including corals, oysters, mussels, sea urchins and pteropods, small swimming snails that many fish eat. Their larvae are often the most sensitive stage.

Part of the guide: Nature & Earth: Living Things and Rocks →