Ocean Acidification

Ocean acidification is an important topic in Environment and Ecology for the UPSC Civil Services Examination, UPPSC and other State Civil Services Exams. It connects the carbon cycle, climate change, marine biodiversity, coral reefs, fisheries and ocean chemistry. Understanding its chemical mechanism, effects on marine organisms and differences from global warming is essential for solving conceptual questions in competitive examinations.

1. What Is Ocean Acidification?

Ocean acidification refers to the long-term decline in the pH of seawater, primarily caused by the absorption of excess atmospheric carbon dioxide (CO₂) by the oceans.

The oceans act as a major carbon sink and absorb a substantial proportion of human-generated CO₂ emissions. Although this process slows the accumulation of carbon dioxide in the atmosphere, it changes seawater chemistry and creates risks for marine ecosystems. Ocean acidification is often described as the “other CO₂ problem” because rising atmospheric carbon dioxide contributes to both global warming and changes in ocean chemistry. It is important to understand that ocean acidification does not mean that the entire ocean has become acidic. Average surface seawater remains alkaline, with a pH of approximately 8.1. Acidification means that its pH is declining relative to earlier conditions.

2. Chemical Mechanism of Ocean Acidification

Ocean acidification occurs through a series of chemical reactions when atmospheric carbon dioxide dissolves in seawater.

Step 1: Dissolution of Carbon Dioxide

Atmospheric carbon dioxide enters seawater through air–sea gas exchange.

CO₂ (atmosphere) ⇌ CO₂ (dissolved in seawater)

The amount absorbed depends on atmospheric CO₂ concentration, temperature, circulation and the chemical properties of seawater.

Step 2: Formation of Carbonic Acid

Dissolved carbon dioxide reacts with water to form carbonic acid.

CO₂ + H₂O ⇌ H₂CO₃

Carbonic acid is a weak acid that can dissociate into hydrogen and bicarbonate ions.

Step 3: Release of Hydrogen Ions

H₂CO₃ ⇌ H⁺ + HCO₃⁻

The increase in hydrogen-ion concentration lowers seawater pH.

Step 4: Reduction in Carbonate-Ion Availability

Hydrogen ions react with carbonate ions to form bicarbonate ions.

H⁺ + CO₃²⁻ ⇌ HCO₃⁻

Consequently, carbonate-ion availability decreases as more CO₂ enters seawater.

Step 5: Effects on Marine Organisms

Many marine organisms require carbonate ions to produce calcium carbonate shells and skeletons. Reduced carbonate availability can make calcification more difficult and increase the vulnerability of some structures to dissolution.

The overall process can be summarised as:

Increasing atmospheric CO₂ → increasing dissolved CO₂ → increasing hydrogen-ion concentration → declining pH → decreasing carbonate-ion availability → greater difficulty in calcification.

3. Ocean Acidification and the pH Scale

The pH scale measures the acidity or alkalinity of a solution and is logarithmic.

pH = −log₁₀ (hydrogen-ion activity)

A decrease of one pH unit represents a tenfold increase in hydrogen-ion activity. Therefore, even a small numerical decline in pH can represent a substantial chemical change.

Since the beginning of the Industrial Revolution, average surface-ocean pH has declined by approximately 0.1 pH unit. This corresponds to roughly a 26–30% increase in acidity, depending on the reference period and calculation used.

4. Carbonate Chemistry and Calcium Carbonate

The carbonate system is central to understanding ocean acidification. Dissolved inorganic carbon in seawater occurs mainly in three forms:

1. Dissolved carbon dioxide, including carbonic acid.
2. Bicarbonate ions (HCO₃⁻).
3. Carbonate ions (CO₃²⁻).

As more CO₂ dissolves in seawater, dissolved CO₂ and bicarbonate generally increase, while carbonate-ion concentration decreases.

Many marine organisms use calcium and carbonate ions to build calcium carbonate structures.

Ca²⁺ + CO₃²⁻ → CaCO₃

Calcium carbonate occurs in different mineral forms, particularly aragonite and calcite.

Aragonite

Aragonite is a naturally occurring crystalline form of calcium carbonate. It is used by various marine organisms to form shells and skeletons. Aragonite is generally more soluble than calcite under comparable seawater conditions.

Calcite

Calcite is another crystalline form of calcium carbonate. Several marine organisms use calcite to build shells, skeletal structures or protective coverings.

Calcium Carbonate Saturation State

The saturation state indicates whether seawater favours the formation or dissolution of a calcium carbonate mineral.

• Saturation state greater than 1: Seawater is supersaturated with respect to the mineral.
• Saturation state equal to 1: Seawater is at equilibrium with the mineral.
• Saturation state below 1: Dissolution is thermodynamically favoured.

Ocean acidification reduces carbonate-ion availability and lowers the saturation states of aragonite and calcite.

A saturation state below 1 does not mean that every shell or coral skeleton dissolves immediately. The actual outcome also depends on biological processes, protective structures and local environmental conditions.

UPSC Prelims fact: The primary chemical problem is reduced carbonate-ion availability, not the disappearance of calcium ions from seawater.

5. Causes of Ocean Acidification

A. Anthropogenic Carbon Dioxide Emissions

The principal global driver of ocean acidification is the increase in atmospheric CO₂ due to human activities. Major sources include:

• Burning coal, petroleum and natural gas.
• Cement production.
• Deforestation and land-use change.
• Industrial activities and energy consumption.

As atmospheric CO₂ rises, the oceans absorb additional carbon dioxide, altering seawater chemistry.

B. Fossil-Fuel Combustion

The combustion of fossil fuels releases carbon dioxide into the atmosphere. The subsequent transfer of CO₂ into seawater contributes directly to ocean acidification.

C. Deforestation

Forests absorb atmospheric CO₂ through photosynthesis. Deforestation reduces this carbon sink and can release carbon stored in vegetation and soils, contributing to increased atmospheric CO₂ concentrations.

D. Coastal Nutrient Pollution

Coastal acidification can also be intensified by local processes.

Agricultural runoff, sewage and other nutrient inputs may stimulate excessive algal growth. When this organic matter dies, its decomposition releases CO₂ and consumes oxygen. This can lower local pH and contribute to oxygen depletion.

E. Ocean Upwelling

Upwelling brings deeper water towards the ocean surface. Deep waters often contain higher concentrations of dissolved CO₂ due to the decomposition of sinking organic matter. When these waters reach coastal environments, they may have relatively low pH and low carbonate saturation.

F. Changes in Ocean Circulation

Temperature, salinity, freshwater input, mixing and ocean circulation influence the distribution of dissolved carbon and the severity of acidification in different regions.

Important distinction: Anthropogenic CO₂ uptake is the dominant driver of global ocean acidification. Local coastal acidification can be intensified by nutrient pollution, organic matter decomposition and natural oceanographic processes.

6. Ocean Acidification versus Global Warming

Ocean acidification and global warming are related but distinct consequences of increasing atmospheric CO₂.

Basis	Ocean Acidification	Global Warming
Main mechanism	CO₂ dissolves in seawater and changes carbonate chemistry	Greenhouse gases alter Earth’s energy balance
Primary effect	Declining pH and carbonate availability	Rising global average temperature
Major consequences	Changes in calcification and marine physiology	Heatwaves, ice melt, sea-level rise and thermal stress
Main environmental medium	Seawater chemistry	Climate system
Relationship with CO₂	Direct chemical effect of dissolved CO₂	Radiative forcing from atmospheric greenhouse gases

Both processes can occur simultaneously and intensify ecological stress. Coral reefs, for example, can experience thermal bleaching due to warming and reduced calcification due to acidification. Ocean warming is not the same as ocean acidification. Warming changes the thermal conditions of seawater, whereas acidification changes its chemical balance.

7. Marine Organisms Affected by Ocean Acidification

A. Corals

Reef-building corals secrete calcium carbonate skeletons. Lower carbonate saturation can reduce calcification and make reef structures more vulnerable to erosion. Ocean acidification should not be confused with coral bleaching. Bleaching is principally associated with the loss or reduction of symbiotic algae under stress, particularly unusually high temperatures. However, warming and acidification can jointly weaken coral reef ecosystems.

B. Molluscs

Oysters, mussels, clams and some snails build calcium carbonate shells. Reduced carbonate availability can affect shell formation, larval development and survival. The degree of sensitivity varies among species and life stages.

C. Pteropods

Pteropods are small planktonic marine gastropods, often called sea butterflies. Many possess calcium carbonate shells and are important components of polar and subpolar food webs. Their shells can be vulnerable to undersaturated seawater, particularly in cold polar regions.

D. Coccolithophores

Coccolithophores are microscopic marine phytoplankton that produce calcium carbonate plates called coccoliths. They contribute to marine food webs and the ocean carbon cycle. Ocean acidification can affect their calcification, although responses differ among species and environmental conditions.

E. Echinoderms

Sea urchins and some other echinoderms produce calcium carbonate structures. Their early developmental stages can be sensitive to changes in pH and carbonate chemistry.

F. Crustaceans

Crabs, lobsters and other crustaceans may be affected through changes in shell formation, growth, reproduction, physiology and sensory functions. Their responses differ across species and life stages.

G. Fish

Fish do not generally build calcium carbonate shells like corals and molluscs, but they can still be affected by altered seawater chemistry. Potential impacts include changes in acid–base regulation, sensory functions, behaviour, reproduction and early development. The magnitude and direction of these effects vary considerably among species.

H. Marine Algae and Seagrasses

Some photosynthetic organisms may benefit from increased dissolved CO₂ under certain conditions because carbon is required for photosynthesis. However, this potential benefit is not universal. Nutrient availability, temperature, light, competition and other environmental pressures can limit growth.

Prelims takeaway: Calcifying organisms are particularly important examples, but ocean acidification can also affect non-calcifying organisms. It is incorrect to assume that all marine species respond identically.

8. Impact on Marine Ecosystems and Food Chains

A. Reduced Calcification

Organisms may expend more energy to build and maintain shells or skeletons. This can reduce the energy available for growth, reproduction and defence.

B. Disruption of Marine Food Webs

If vulnerable planktonic organisms decline or change their distribution, effects may spread to fish and other animals that depend on them. Pteropods, for example, are prey for several marine organisms. Changes in their abundance may affect food-web relationships.

C. Coral Reef Degradation

Coral reefs provide habitat, feeding grounds and nursery areas for many species. Reduced reef growth and greater erosion can weaken reef structures and reduce habitat complexity.

D. Fisheries and Aquaculture

Ocean acidification may affect shellfish aquaculture, fisheries productivity and the distribution of commercially important species. Economic consequences depend on local species, exposure, adaptation and management.

E. Marine Biodiversity

Changes in survival, reproduction, calcification and habitat quality can alter species composition and ecosystem functioning.

However, ocean acidification does not necessarily cause every marine species to decline. Ecosystem responses vary according to species sensitivity, environmental conditions and ecological interactions.

9. Ocean Acidification and the Carbon Cycle

The oceans are an important component of the global carbon cycle. They exchange CO₂ with the atmosphere, transport dissolved carbon through circulation and store carbon in different forms.

Biological Carbon Pump

Phytoplankton absorb dissolved inorganic carbon during photosynthesis. Some of this carbon enters marine food webs, while some organic matter sinks into deeper waters. This transfer of carbon from surface waters to the deep ocean is known as the biological carbon pump. Calcifying organisms also contribute to carbon transport through calcium carbonate production and sinking particles.

However, the relationship between calcification and atmospheric CO₂ is complex. Calcium carbonate formation can release CO₂ locally, while sinking and burial of carbon-containing material can contribute to longer-term carbon storage. Therefore, the effect of ocean acidification on carbon storage cannot be reduced to a simple claim that more or less calcification always results in more or less atmospheric CO₂.

Ocean as a Carbon Sink

The ocean’s role as a carbon sink helps slow atmospheric CO₂ accumulation. However, the resulting chemical changes create ecological costs. Reducing CO₂ emissions helps limit global warming and slow ocean acidification.

10. Ocean Acidification and Cloud Formation

Marine organisms influence atmospheric chemistry through the production of gases and aerosols. For example, some plankton produce dimethyl sulfide (DMS), which can contribute to the formation of atmospheric sulfate aerosols. These aerosols can influence cloud condensation nuclei and cloud properties.

However, the relationships among ocean chemistry, marine biology, DMS production and cloud formation are complex. Ocean acidification is not generally described as directly preventing cloud formation.

For UPSC Prelims, this distinction is relevant to the 2012 question on ocean acidification, which included cloud seeding and cloud formation among its statements.

11. Why Are Polar Oceans Particularly Vulnerable?

Cold water generally dissolves more CO₂ than warm water. Polar waters can therefore absorb substantial amounts of atmospheric carbon dioxide. Cold-water carbonate chemistry can also make aragonite undersaturation occur more readily under increasing CO₂ conditions. Seasonal sea-ice changes, biological productivity, freshwater inputs and ocean circulation influence local conditions. The Southern Ocean around Antarctica is particularly important because of its role in global ocean circulation and carbon uptake.

UPSC/ UPPSC POV: Ocean acidification is not restricted to tropical oceans or coral reefs. It affects marine environments globally, including polar waters, deep oceans and coastal areas.

12. Ocean Acidification in India

India has a long coastline and extensive coastal ecosystems. Ocean acidification is relevant to the Arabian Sea, Bay of Bengal and Indian Ocean, although local conditions vary.

A. Coral Reefs

India’s major coral reef regions include:

• Gulf of Kachchh, Gujarat.
• Gulf of Mannar, Tamil Nadu.
• Palk Bay, Tamil Nadu.
• Lakshadweep Islands.
• Andaman and Nicobar Islands.

Corals require suitable conditions for calcium carbonate deposition. Acidification can reduce calcification and interact with warming, pollution and other pressures.

B. Fisheries and Coastal Livelihoods

Marine fisheries support food security, employment and coastal livelihoods. Changes in plankton, shell-forming organisms and fish populations could affect marine food webs and fisheries. The degree of impact depends on species sensitivity, local chemistry, habitat conditions and adaptive capacity.

C. Coastal Pollution

Nutrient runoff, sewage and organic pollution can contribute to local coastal acidification and oxygen depletion. These pressures may compound the effects of globally driven ocean acidification.

D. Research and Monitoring

India can strengthen ocean observation, long-term pH and carbonate-chemistry monitoring, marine biodiversity research and the integration of ocean data into coastal management. Protecting mangroves, seagrasses and other coastal ecosystems supports biodiversity and coastal resilience, although it is not a substitute for reducing global CO₂ emissions.

13. Mitigation and Adaptation Measures

A. Reduce Carbon Dioxide Emissions

The most important long-term measure is reducing global CO₂ emissions through cleaner energy, energy efficiency, electrification and protection of natural carbon sinks.

B. Reduce Coastal Pollution

Better wastewater treatment, agricultural nutrient management and control of organic pollution can help reduce local acidification and hypoxia.

C. Protect Marine Ecosystems

Marine protected areas, sustainable fisheries, coral reef conservation and habitat restoration can improve ecosystem resilience. These measures do not eliminate the underlying global chemical driver.

D. Monitor Ocean Chemistry

Long-term monitoring of pH, dissolved inorganic carbon, alkalinity, temperature, salinity and oxygen can improve understanding of changing ocean conditions.

E. Evaluate Local Interventions Carefully

Researchers are investigating approaches such as seaweed cultivation and ocean alkalinity enhancement. These approaches require careful evaluation of effectiveness, ecological risks, carbon accounting, permanence and scalability.

Adding alkaline materials may increase seawater alkalinity under suitable conditions, but poorly designed interventions could cause unintended environmental effects.

F. International Cooperation

Ocean acidification is a transboundary issue because CO₂ emissions and ocean circulation operate at global scales. International climate cooperation, ocean science and marine conservation are essential.

14. International Frameworks Related to Ocean Acidification

United Nations Framework Convention on Climate Change

The UNFCCC provides a framework for international cooperation to address climate change through greenhouse-gas mitigation and adaptation.

Paris Agreement

The Paris Agreement seeks to limit global temperature rise. Reducing CO₂ emissions under the Agreement also helps address the chemical driver of ocean acidification.

United Nations Convention on the Law of the Sea

UNCLOS includes obligations concerning the protection and preservation of the marine environment and the prevention, reduction and control of marine pollution.

Convention on Biological Diversity

The CBD provides a framework for biodiversity conservation, sustainable use and benefit-sharing.

Sustainable Development Goal 14

SDG 14 focuses on conserving and sustainably using oceans, seas and marine resources.

These frameworks address different aspects of the problem. None should be described as a standalone global treaty devoted exclusively to ocean acidification.

15. Important Terms for UPSC/ UPPSC Prelims

Term	Meaning
Ocean acidification	Decline in seawater pH, mainly due to CO₂ absorption
Carbonic acid	Weak acid formed when dissolved CO₂ reacts with water
Hydrogen ion	Ion whose increasing activity lowers pH
Bicarbonate	Dissolved inorganic carbon species, HCO₃⁻
Carbonate ion	CO₃²⁻, important for calcium carbonate formation
Calcification	Formation of calcium carbonate structures
Aragonite	Crystalline form of calcium carbonate
Calcite	Another crystalline form of calcium carbonate
Saturation state	Measure of seawater chemistry relative to a mineral’s formation or dissolution
Coccolithophore	Calcifying marine phytoplankton
Pteropod	Planktonic marine gastropod; many species have calcium carbonate shells
Biological carbon pump	Transfer of carbon from surface waters to deeper ocean layers through biological processes
Coastal acidification	Local or regional pH decline influenced by global CO₂ uptake and coastal processes
Hypoxia	Low dissolved oxygen in water

16. UPSC Previous-Year Question on Ocean Acidification

UPSC Civil Services Prelims 2012

Question: The acidification of oceans is increasing. Why is this phenomenon a cause of concern?

1. The growth and survival of calcareous phytoplankton will be adversely affected.
2. The growth and survival of coral reefs will be adversely affected.
3. The survival of some animals that have phytoplanktonic larvae will be adversely affected.
4. Cloud seeding and the formation of clouds will be adversely affected.

Select the correct answer:

(a) 1, 2 and 3 only

(b) 2 only

(c) 1 and 3 only

(d) 1, 2, 3 and 4

Correct Answer: (a) 1, 2 and 3 only.

Explanation

Statement 1 — Correct: Calcareous phytoplankton, including coccolithophores, produce calcium carbonate structures. Ocean acidification reduces carbonate-ion availability and can impair calcification.

Statement 2 — Correct: Reef-building corals secrete calcium carbonate skeletons. Reduced carbonate saturation can make calcification more difficult and increase the vulnerability of reef structures to dissolution and erosion.

Statement 3 — Correct: Some marine animals have vulnerable planktonic larval stages. Changes in carbonate chemistry can affect development and survival, while impacts on planktonic food resources can influence marine food webs.

This statement does not mean that every animal with planktonic larvae will necessarily be affected in the same way.

Statement 4 — Incorrect in the context of the accepted answer: Ocean acidification primarily changes seawater chemistry and affects marine organisms and ecosystems. Although marine biological processes can influence atmospheric aerosols and cloud properties, cloud formation is not an established direct consequence of ocean acidification in the context of this question.

PYQ lesson: UPSC tests the ecological consequences of ocean acidification, not merely its definition. The central links are carbonate chemistry, calcifying organisms and marine food webs.

17. Related UPSC PYQ: Phytoplankton and the Carbon Cycle

UPSC Civil Services Prelims 2012 — Related Concept

A related question from the 2012 paper tested the consequences of the complete destruction of ocean phytoplankton.

The key concepts are:

1. The ocean’s role as a carbon sink would be adversely affected.
2. Marine food chains would be adversely affected.

Conceptual explanation: Phytoplankton are primary producers in marine food webs and contribute to the biological carbon pump. Their destruction would disrupt marine food chains and affect biological carbon uptake and transfer.

This is a related question rather than a direct ocean-acidification question. Ocean acidification may affect some phytoplankton, particularly calcifying groups, but it does not mean that all phytoplankton will be destroyed.

18. UPSC/ UPPSC Practice Questions

The following questions are practice questions based on the topic and are not official UPSC previous-year questions.

Question 1: Ocean Chemistry

Consider the following statements:

1. Ocean acidification increases the hydrogen-ion concentration of seawater.
2. Ocean acidification generally increases carbonate-ion availability.
3. Average surface seawater can remain alkaline while undergoing acidification.

Which of the statements given above are correct?

(a) 1 only

(b) 1 and 2 only

(c) 1 and 3 only

(d) 1, 2 and 3

Answer: (c) 1 and 3 only.

Explanation: Increased CO₂ raises hydrogen-ion concentration and lowers pH. Carbonate-ion availability generally decreases, but seawater may remain alkaline.

Question 2: Marine Organisms

Which of the following are particularly vulnerable to changes in carbonate chemistry?

1. Reef-building corals
2. Coccolithophores
3. Pteropods with calcium carbonate shells
4. All marine fish to the same extent

Select the correct answer:

(a) 1 and 2 only

(b) 1, 2 and 3 only

(c) 2, 3 and 4 only

(d) 1, 2, 3 and 4

Answer: (b) 1, 2 and 3 only.

Explanation: Calcifying organisms are particularly important examples. Fish can also be affected, but not necessarily to the same extent or in the same way.

Question 3: Carbonate Chemistry

Consider the following statements:

1. Aragonite is a form of calcium carbonate.
2. Ocean acidification can reduce aragonite saturation state.
3. A saturation state below 1 favours dissolution of the relevant calcium carbonate mineral.

Which of the statements given above are correct?

(a) 1 only

(b) 1 and 2 only

(c) 2 and 3 only

(d) 1, 2 and 3

Answer: (d) 1, 2 and 3.

Explanation: All three statements are correct. A saturation state below 1 means dissolution is thermodynamically favoured, although the actual rate depends on local and biological conditions.

Question 4: Ocean Acidification and Warming

Consider the following statements:

1. Ocean acidification and ocean warming are identical processes.
2. Ocean warming is primarily associated with increased retention of heat in the climate system.
3. Ocean acidification is primarily associated with the uptake of atmospheric CO₂ and changes in seawater chemistry.

Which of the statements given above are correct?

(a) 1 only

(b) 2 only

(c) 2 and 3 only

(d) 1, 2 and 3

Answer: (c) 2 and 3 only.

Explanation: Warming and acidification are distinct but related consequences of rising atmospheric CO₂ and other human influences.

Question 5: Coastal Acidification

Which of the following can contribute to coastal acidification?

1. Excess nutrient runoff
2. Decomposition of organic matter
3. Upwelling of CO₂-rich deep water
4. Absorption of atmospheric CO₂

Select the correct answer:

(a) 1 and 2 only

(b) 1, 2 and 3 only

(c) 2, 3 and 4 only

(d) 1, 2, 3 and 4

Answer: (d) 1, 2, 3 and 4.

Explanation: Global CO₂ uptake and local biological and hydrographic processes can all influence coastal pH.

Question 6: Ocean Acidification and Coral Reefs

Consider the following statements:

1. Ocean acidification can reduce coral calcification.
2. Coral bleaching and ocean acidification are exactly the same phenomenon.
3. Ocean warming and acidification can jointly stress coral reef ecosystems.

Which of the statements given above are correct?

(a) 1 only

(b) 1 and 2 only

(c) 1 and 3 only

(d) 1, 2 and 3

Answer: (c) 1 and 3 only.

Explanation: Bleaching is principally associated with the loss or reduction of symbiotic algae under stress, particularly heat stress. Acidification affects carbonate chemistry and calcification.

Question 7: Carbon Cycle

Consider the following statements:

1. The ocean absorbs a substantial proportion of anthropogenic CO₂ emissions.
2. Ocean acidification is caused mainly by the ocean absorbing atmospheric oxygen.
3. The biological carbon pump transfers some carbon from surface waters to deeper waters.

Which of the statements given above are correct?

(a) 1 only

(b) 1 and 2 only

(c) 1 and 3 only

(d) 1, 2 and 3

Answer: (c) 1 and 3 only.

Explanation: The main chemical driver of ocean acidification is CO₂ uptake, not oxygen absorption.

Question 8: India and Ocean Acidification

Which of the following Indian marine ecosystems may be affected by ocean acidification?

1. Coral reefs of Lakshadweep
2. Coral reefs of the Gulf of Mannar
3. Coral reefs of the Andaman and Nicobar Islands
4. Coral reefs of the Gulf of Kachchh

Select the correct answer:

(a) 1 and 2 only

(b) 1, 2 and 3 only

(c) 2, 3 and 4 only

(d) 1, 2, 3 and 4

Answer: (d) 1, 2, 3 and 4.

Explanation: All four are recognised Indian coral reef regions. Their local exposure and sensitivity may differ.

Question 9: pH

A decrease in seawater pH from 8.1 to 8.0 indicates:

(a) A decrease in hydrogen-ion activity

(b) An increase in hydrogen-ion activity

(c) That seawater has become acidic because its pH is below 7

(d) No chemical change because the pH difference is small

Answer: (b) An increase in hydrogen-ion activity.

Explanation: The pH scale is logarithmic. A decline of 0.1 pH unit corresponds to an increase of approximately 26% in hydrogen-ion activity.

Question 10: Mitigation

Which of the following measures would most directly address the main global driver of ocean acidification?

(a) Increasing artificial cloud seeding

(b) Reducing anthropogenic CO₂ emissions

(c) Increasing deep-sea fishing

(d) Removing all marine algae

Answer: (b) Reducing anthropogenic CO₂ emissions.

Explanation: Reducing CO₂ emissions addresses the main global chemical driver. Pollution control and ecosystem protection remain important complementary measures.

19. Quick Revision: Facts to Remember

• Primary cause: Absorption of excess atmospheric CO₂ by seawater.
• Chemical sequence: Dissolved CO₂ increases, hydrogen ions increase, pH decreases and carbonate-ion availability declines.
• Average surface ocean: Still alkaline, with a pH of approximately 8.1.
• Minerals: Aragonite and calcite are forms of calcium carbonate.
• Particularly vulnerable organisms: Reef-building corals, coccolithophores, pteropods and several shell-forming organisms.
• Other affected organisms: Fish and non-calcifying organisms can also be affected.
• Ocean acidification versus coral bleaching: Different processes.
• Coastal acidification: Can be intensified by nutrient runoff, organic matter decomposition and upwelling.
• India’s coral reef regions: Gulf of Kachchh, Gulf of Mannar, Palk Bay, Lakshadweep and Andaman and Nicobar Islands.
• Direct UPSC PYQ: 2012 question on the ecological consequences of ocean acidification.
• Most important exam principle: Reduced carbonate availability makes calcification more difficult for many marine organisms.

Important Facts for Exams

• Ocean acidification is mainly caused by the absorption of atmospheric CO₂ by seawater.
• It lowers pH and reduces carbonate-ion availability.
• Ocean acidification does not mean that seawater has necessarily become acidic; average surface seawater remains alkaline.
• Aragonite and calcite are crystalline forms of calcium carbonate.
• Corals, coccolithophores and pteropods are important examples of calcifying organisms vulnerable to changes in carbonate chemistry.
• Ocean warming and ocean acidification are distinct processes that can jointly affect marine ecosystems.
• The Southern Ocean and other cold-water regions are particularly important in discussions of CO₂ uptake and carbonate chemistry.
• India’s coral reef regions include the Gulf of Kachchh, Gulf of Mannar, Palk Bay, Lakshadweep and Andaman and Nicobar Islands.
• The direct UPSC Prelims PYQ identified on ocean acidification appeared in 2012.
• The most effective long-term response is to reduce anthropogenic CO₂ emissions, alongside marine conservation and coastal pollution control.