When we think about the impacts of rising carbon dioxide emissions, we often think of warming temperatures and changing weather patterns. But the ocean’s role as a carbon sink — absorbing roughly 30 percent of all CO2 emitted by human activity — has a profound and often underappreciated consequence: ocean acidification. As the ocean absorbs more CO2, it becomes chemically more acidic, with far-reaching consequences for marine life and the ecosystems we depend on.
The Chemistry of Ocean Acidification
When carbon dioxide dissolves in seawater, it reacts to form carbonic acid, which then dissociates to release hydrogen ions and bicarbonate. This increase in hydrogen ion concentration lowers the pH of the water — making it more acidic. Since the Industrial Revolution, average ocean pH has fallen from approximately 8.2 to 8.1 — a change that sounds small but represents a 26 percent increase in acidity due to the logarithmic nature of the pH scale.
Impacts on Shell-Forming Organisms
Many marine organisms — including oysters, mussels, sea urchins, corals, and certain plankton — build their shells or skeletons from calcium carbonate minerals such as aragonite and calcite. As ocean acidity increases, these minerals become less stable and more difficult to precipitate. Shell-forming organisms must expend more energy to build and maintain their structures, and in extreme cases their shells can begin to dissolve. This has direct implications for shellfish aquaculture as well as wild populations and food web dynamics.
Broader Ecosystem Consequences
The effects of acidification extend beyond shell-forming organisms. Changes in the abundance and health of calcifying plankton ripple up food webs, affecting fish and marine mammals. Some fish species show altered behaviour, including impaired chemosensory ability and reduced predator avoidance, in more acidic conditions. The combined stress of warming and acidification on coral reefs is particularly alarming, with bleaching and structural degradation occurring at increasing frequency.
Monitoring Australia’s Waters
Australia operates ocean acidification monitoring programs through CSIRO and the Integrated Marine Observing System (IMOS), tracking pH, aragonite saturation, and other chemical parameters across multiple ocean regions. Southern Ocean waters, which absorb disproportionately large amounts of CO2, are showing some of the most rapid acidification signals globally.
What Can Be Done?
Ultimately, addressing ocean acidification requires reducing CO2 emissions at the source. In the shorter term, protecting coastal ecosystems including seagrass meadows and mangroves — which locally buffer acidification by absorbing CO2 — offers some resilience. Research into breeding more acid-tolerant strains of commercially important shellfish species is also progressing, offering practical pathways for industry adaptation.
Further reading: noaa.gov, csiro.au, imos.org.au.
