Ocean acidification

Topic: Oceans: Hydrological Cycle, Relief of the Ocean Floor, Temperature and Salinity · NCERT: Beyond NCERT

Meaning

Ocean acidification is the slow fall in the pH of seawater as the ocean absorbs carbon dioxide (CO₂) from the air. Since industrialisation, surface pH has dropped from about 8.2 to about 8.1, which means the water is about 30% more acidic.

It matters because animals that build shells or skeletons from calcium carbonate find it harder to do so. These include corals, pteropods and oysters. The safe limit for ocean acidification (its planetary boundary) has already been crossed [2].

Explanation

How it happens (cause → process → result)

• Cause: more CO₂ in the air.
• Burning fossil fuels puts extra CO₂ into the atmosphere.

• The ocean absorbs part of this CO₂ at its surface.

• Process: carbonic acid forms.

• CO₂ reacts with water to form carbonic acid.

• This acid makes seawater more acidic.

• Result: pH falls.

• pH is a 0–14 scale. A lower number means more acidic.
• Surface pH has fallen from about 8.2 to about 8.1 since industrialisation.

Why a small drop is a big change

• The pH scale is logarithmic. Each full step means ten times more acidity.
• So a fall of only about 0.1 means about 30% more acidity.
• The ocean is still above pH 7, so it is still slightly alkaline (basic). "Acidification" means it is moving towards acid. It has not become an acid.

Who is harmed and why

• Many sea animals build shells or skeletons from calcium carbonate.
• More acidic water makes this building harder. The main victims are:
• corals, which build reefs
• pteropods ("sea butterflies": tiny swimming snails), which are an important food for fish

• oysters and other shellfish

• Damage to these animals spreads up the food chain and hurts fisheries.

The planetary boundary has been crossed

• Planetary boundary: a safe limit for an Earth system. Going past it risks harmful changes that may not reverse.
• The revised boundary: a 10% fall, compared with pre-industrial times, in the seawater chemistry that shell-builders need. Ocean conditions passed this limit around 2000 [2].
• How far it has spread: by 2020, more than 40% of the surface ocean and up to 60% of the subsurface ocean (down to 200 m) had crossed the boundary. This comes from a study published in 2025 [2].
• Habitat lost by shell-builders:
• tropical and subtropical coral reefs: down 43% [2]
• polar pteropods: down by up to 61% [2]
• coastal bivalves (two-shelled animals such as oysters and mussels): down 13% [2]

Where it occurs

• The whole world ocean: wherever the sea surface meets the air and absorbs CO₂. It is not limited to one region.
• Below the surface too: up to 60% of the subsurface ocean, down to 200 m, had crossed the boundary by 2020 [2].
• Polar seas: polar pteropods have lost up to 61% of their suitable habitat [2]. Cold water dissolves more gas, so cold polar water takes in CO₂ easily.
• Tropical and subtropical coral reef belts: suitable reef habitat is down 43% [2].
• Coastal waters: oysters, mussels and other bivalves have lost 13% of their suitable habitat [2].
• India: the coral reefs of the Gulf of Mannar, Lakshadweep and the Andaman and Nicobar Islands are at risk. Here acidification adds to the damage already done by heat stress and bleaching.

Don't confuse with

• Ocean deoxygenation: the ocean losing dissolved oxygen because of warming and fertiliser runoff (about 2% lost since 1960). Acidification is about the ocean gaining CO₂ and its pH falling.
• Coral bleaching: caused by heat (marine heatwaves). Stressed corals push out the colourful algae that live in them and feed them, and turn white. Acidification weakens the corals' skeleton-building. It is a chemical problem, not a heat problem.
• Acid rain: acidic rain formed when sulphur and nitrogen oxides from the air mix with rainwater. It falls on land and lakes. Ocean acidification is caused by CO₂ dissolving in seawater.
• "The ocean has become acidic": wrong. Surface pH is about 8.1, which is still above 7 (alkaline). The pH is only falling towards acidic.

Prelims Hooks

• pH change: surface seawater pH fell from about 8.2 to about 8.1 after industrialisation. That is about 30% more acidity, because the pH scale is logarithmic.
• The chemistry: CO₂ + water → carbonic acid. The main victims are animals with calcium carbonate shells or skeletons: corals, pteropods, oysters.
• Planetary boundary: the ocean acidification boundary was crossed around 2000. By 2020, more than 40% of the surface ocean and up to 60% of the subsurface ocean (down to 200 m) had crossed it [2].
• Habitat loss match: coral reefs down 43%, polar pteropods down by up to 61%, coastal bivalves down 13% [2].
• Trap: "Ocean acidification has made seawater acidic (pH below 7)." Wrong. The ocean is still alkaline at about pH 8.1.
• Trap: "Ocean acidification is caused mainly by warming water." Wrong. Its cause is CO₂ absorption. Warming drives deoxygenation and marine heatwaves.

Mains Points

• The "deadly trio" (GS-III, Environment):
• Acidification, marine heatwaves and deoxygenation all act on the ocean at the same time.
• Each one makes the others worse. Together they push coral reefs and fisheries towards collapse.

• More than 40% of the surface ocean had already crossed the acidification boundary by 2020 [2].

• The Indian stakes:

• The reefs of the Gulf of Mannar and Lakshadweep are already under stress from bleaching.

• Weaker shellfish and shrinking reefs threaten coastal fishing livelihoods and the Blue Economy.

• Policy responses:

• Root fix: cut CO₂ emissions from fossil fuels, because no local measure can reverse the change in ocean chemistry.
• Global frameworks: SDG 14 "Life Below Water" and the UN Decade of Ocean Science (2021–2030).
• Building resilience: the BBNJ Agreement lets countries create marine protected areas on the high seas. This helps the 30x30 target (protecting 30% of land and sea by 2030) and reduces other pressures on stressed ecosystems [1].

Related concepts

• Ocean deoxygenation
• Marine heatwave
• Microplastics
• Great Pacific Garbage Patch
• Atlantic Meridional Overturning Circulation
• BBNJ Agreement

Read more

• Oceans and life, oceans under stress, and the High Seas Treaty