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Ocean Acidification: Causes, Impacts and Measures | UPSC Environment Notes

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Ocean Acidification

Ocean acidification refers to a reduction in the pH of the ocean over an extended period of time, caused primarily by uptake of carbon dioxide (CO2) from the atmosphere.

When CO2 is absorbed by seawater, a series of chemical reactions occur resulting in the increased concentration of hydrogen ions. This increase causes the seawater to become more acidic and causes carbonate ions to be relatively less abundant.

Primary chemical processes involved in ocean acidification

  • Ocean acidification occurs when the ocean absorbs excess CO2 from the atmosphere. 
    • Oceans absorb about 25–30% of all human-emitted CO₂ — acting as a crucial buffer but at the cost of their own chemistry. 
  • The CO2 reacts with seawater to form carbonic acid, which then dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-). 
  • The increase in H+ ions in the ocean results in a decrease of the seawater pH, thus leading to ocean acidification

Causes

  • Fossil fuel combustion — Burning coal, oil, and gas is the largest source of CO₂ — the more released, the more oceans absorb and acidify.
  • Deforestation —  Clearing forests releases stored carbon, adding more CO₂ to the atmosphere which then dissolves into the ocean.
  • Agricultural runoff — nutrient pollution
    • Excess nitrogen and phosphorus from fertilisers cause algal blooms; decomposing algae consume oxygen and release CO₂, causing localised acidification.
  • Industrial emissions — Cement, steel, chemicals, fertilisers and thermal power plants contribute to atmospheric carbon dioxide. 
  • Transport emissions — Vehicles, ships and aircraft release carbon dioxide, adding to the ocean acidification process.

Impacts

  • Coral bleaching and reef dissolution
    • Reduced carbonate ions make it harder for corals to build skeletons; existing reefs begin dissolving 
  • Shell-forming organisms threatened
    • Oysters, mussels, clams, sea urchins, and pteropods (sea butterflies) cannot form shells in acidic water — their populations collapse, disrupting entire food chains.
      • As ocean acidification increases, available carbonate ions (CO32-) bond with excess hydrogen, resulting in fewer carbonate ions available for calcifying organisms to build and maintain their shells, skeletons, and other calcium carbonate structures. 
  • Threat to plankton — Some plankton species build calcium carbonate shells. Their decline affects the marine food chain. 
  • Fish behaviour and physiology
    • Higher CO₂ impairs fish sensory systems — clownfish, for example, lose the ability to detect predator
  • Impact on Marine Food Webs
    • Disruption of food chains — If plankton and shellfish decline, fish, seabirds, marine mammals and humans depending on them are affected.
    • Reduced biodiversity — Sensitive marine organisms may disappear from affected regions.
    • Change in species composition — Acid-tolerant species may increase, while sensitive species decline. 
  • Impact on Fisheries and Livelihoods
    • Decline in fishery productivity — Damage to coral reefs, shellfish and plankton affects fish breeding and feeding grounds.
    • Loss of income — Fishers, aquaculture workers, coastal communities and seafood industries may suffer income loss.
    • Threat to food security — Coastal and island communities dependent on marine protein may face food insecurity.
    • Impact on aquaculture — Shellfish farming and coastal aquaculture become more vulnerable due to poor larval survival and weak shell formation.
  • Impact on Coastal Protection
    • Weakening of coral reefs — Coral reefs act as natural barriers against waves and storms. Their degradation increases coastal vulnerability.
    • Higher erosion risk — Loss of reef-building capacity weakens natural coastal defence.
    • Threat to islands — Coral islands such as those in Lakshadweep depend on healthy reefs for sediment supply and protection.
  • Economic Impacts
    • Loss to tourism — Coral reef tourism, scuba diving, snorkelling and marine eco-tourism decline when reefs degrade.
    • Impact on the blue economy — Fisheries, tourism, aquaculture, coastal livelihoods and marine biodiversity-based industries are affected. 
  • Scientific and Ecological Impacts
    • Loss of marine genetic resources — Acidification threatens species that may have medicinal, ecological or research value.
  • Decreased Absorption Capacity: As seawater becomes more acidic and its natural carbonate buffers are depleted, the ocean’s “buffer capacity” drops. This makes the water progressively less efficient at absorbing new atmospheric carbon dioxide

Measures to Address Ocean Acidification

  • Aggressive CO₂ emission reduction
    • The only lasting solution — since ocean acidification is directly driven by atmospheric CO₂, cutting emissions is the primary fix.
    • Reduce carbon emissions — Since excess atmospheric carbon dioxide is the root cause, rapid decarbonisation of energy, transport, industry and agriculture is essential. 
    • Promote renewable energy — Solar, wind, green hydrogen and clean energy technologies should be expanded to reduce fossil fuel dependence. 
    • Improve industrial efficiency — Cement, steel, chemicals and thermal power sectors should adopt cleaner technologies, energy efficiency and carbon capture where feasible.
    • Promote nature-based carbon sinks — Forests, mangroves, seagrasses, salt marshes and wetlands should be conserved as natural carbon sinks.
  • Control agricultural and sewage runoff
    • Reducing nitrogen and phosphorus pollution prevents localised acidification and dead zones — watershed management and better fertiliser practices are key.
  • Promote seaweed and seagrass farming
    • Seaweeds absorb CO₂ and raise local pH, creating refugia for marine life — large-scale ocean farming can buffer acidification in coastal waters.
  • Ocean alkalinity enhancement (OAE)
    • Adding minerals like olivine or lime to seawater neutralises acidity and increases CO₂ absorption — an emerging geoengineering approach under research.
  • Scientific and Institutional Measures 
    • Create ocean acidification monitoring network — Strengthen long-term monitoring of pH, temperature, salinity, dissolved oxygen and carbonate chemistry in coastal and island waters.

Conclusion

Ocean acidification is a silent but serious threat to marine ecosystems. It weakens corals, shellfish, plankton, fisheries, tourism, coastal protection and the blue economy. The solution lies in reducing carbon emissions globally while controlling local stressors such as pollution, overfishing and coastal degradation. Ocean acidification must be treated not only as a climate issue, but also as a marine biodiversity, livelihood and coastal security challenge.

Sample Mains Question

Q1. What is ocean acidification? Explain the chemical process responsible for it.
(150 words, 10 marks)

Q2. Ocean acidification is a silent threat to marine biodiversity and coastal livelihoods. Discuss.
(150 words, 10 marks)

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