Ocean acidification is caused mainly by oceans absorbing additional atmospheric carbon dioxide from human activities, especially fossil-fuel combustion and deforestation. Dissolved carbon dioxide forms carbonic acid, increasing hydrogen ion concentration and lowering seawater pH.
This is SL and HL content in IB ESS subtopic 2.3, Biogeochemical cycles.
This reduces the availability of carbonate ions used by corals, molluscs and some plankton to produce calcium carbonate shells or skeletons. Reduced calcification can weaken organisms and disrupt marine food webs, fisheries and ecosystem services.
The main driver is rising atmospheric carbon dioxide from anthropogenic emissions. Local processes such as respiration and decomposition can also increase dissolved carbon dioxide, but they do not explain the present global trend by themselves.
A common misconception is that ocean acidification means seawater has become acidic. Seawater remains alkaline, with a pH above , but it is becoming less alkaline as its pH decreases. Another misconception is that plastic pollution directly causes global ocean acidification; its principal global cause is carbon dioxide absorption.
For example, acidification threatens calcifying organisms in Australia’s Great Barrier Reef, creating ecological and economic risks for tourism, fisheries and reef-dependent communities.
For an explain question, give the complete causal chain: increased atmospheric carbon dioxide greater ocean uptake carbonic acid formation more hydrogen ions lower pH and fewer carbonate ions. Link the mechanism to a named example and avoid stating only that “pollution makes oceans acidic.”
