Oceans are a significant CO2 reservoir, absorbing a significant amount of it (one-third) produced by anthropogenic activities and effectively mitigating climate change. Ocean acidification is a change in ocean chemistry characterized by a decrease in ocean pH (i.e., an increase in hydrogen ion concentration) caused by the ocean's uptake of carbon compounds from the atmosphere. As the ocean absorbs more atmospheric carbon dioxide, the concentration of hydrogen ions increases, the concentration of carbonate ions decreases, the pH of the oceans decreases, and the oceans become less alkaline – this is known as ocean acidification. In this article, we will discuss Ocean Acidification which will be helpful for UPSC exam preparation.

What are pH and Acidity?

• The pH of a liquid solution indicates its acidity or basicity.
• On a scale of 0 to 14, the pH of a solution represents the concentration of hydrogen ions (H+) and hydroxyl ions (OH-).
• Pure water has a pH of 7, is neutral (neither acidic nor basic), and contains equal amounts of H+ and OH.
• A pH less than 7 indicates an acidic solution, while a pH greater than 7 indicates a basic solution.
• Because the pH scale is logarithmic, a decrease of one pH unit results in a tenfold increase in acidity.
• Prior to the Industrial Revolution of the 18th and 19th centuries, the average pH of the ocean was around 8.2. The average pH of the ocean today is 8.1.
• This means that the ocean is about 30% more acidic today than it was before the Industrial Revolution.
• By 2100, the ocean's pH could drop to around 7.8, making it 150 percent more acidic and affecting half of all marine life.

Reactions of Ocean Acidification

• The term "ocean acidification" refers to a group of processes that occur when CO2 reacts with seawater.
• When CO2 dissolves in seawater to form aqueous CO2, carbonic acid is formed (H2CO3).
• Carbonic acid dissociates (splits apart) quickly to produce bicarbonate ions (HCO3-).
• Bicarbonate ions can then dissociate into carbonate ions (CO32-).
• Both of these reactions generate protons (H+), which lower the pH of the solution.

Causes of Ocean Acidification

• Increased Carbon Dioxide Concentration in Ocean: When sea organisms die on the sea floor, their remains accumulate and form corals, which are made up of carbons.
  • These organisms also release calcium into the water. Because they add acidity to the water, these compounds have a far-reaching negative impact on its composition.
• Increased Carbon Dioxide Concentration in Atmosphere: When carbon dioxide is released into the atmosphere as a result of various human activities, it also contaminates the water because the carbon gases dissolve in the seawater, lowering the pH and contributing to acidification.
• Increased Hydrogen Ions Concentration in Water: Some chemical reactions that occur on the sea floor may include an increase in hydrogen ions, which, when combined with other compounds like nitrogen, water, and other gases, cause ocean water acidification.
• Burning of Fossil Fuels: When petroleum, diesel, and coal are burned, they emit a large amount of carbon dioxide. This raises the concentration of carbon dioxide in the atmosphere, which eventually finds its way into the water.
  • Carbon and other atmospheric gases enter the sea via acidic rainfall or direct dissolution into the water.
• Waste Disposal: Many countries that live near sea water masses have been too quick to use the oceans as potential dumping grounds for domestic and industrial waste.
  • Other wastes, in addition to direct sewage waste disposal, increase the acidity of the water.
  • Acidic compounds in industrial and agricultural waste, for example, are extremely dangerous because they lower the PH of ocean water.
• Improper Land Management: Agriculture also plays a role in the problem of ocean acidification.
  • Some of the farmers' methods are ineffective and prone to soil erosion, and the chemicals are washed downstream into the ocean as a result.
  • In short, if the land is not well managed, the impact of acidifying soil mineral content and water pollution can harm water bodies.
• Industrialization: Countries or cities that have embraced industrialization pose serious environmental risks.
  • Their presence can only mean that there is more carbon dioxide in the atmosphere, which, when absorbed by the water, raises the level of acidity.
  • Industries contribute to the release of hazardous gases such as carbon dioxide, sulfur dioxide, nitrogen oxides, and others, which eventually form acid rain or dissolve in the oceans, resulting in acidic conditions.

CO2 Effect on Ocean Acidification

• Carbon dioxide is being absorbed from the atmosphere at a rate that exceeds the ocean's natural buffering capacity.
• Since the beginning of the industrial revolution, the pH of the ocean surface waters has decreased by about 0.1 pH unit (a 26 percent increase in ocean hydrogen ion concentration).
• The ocean currently has a pH of around 8.0 and is thus 'basic,' and it is chemically impossible for all of it to become a pH less than 7.0. But it is still referred to as ocean acidification because acidification is the trend, regardless of the starting point.
• Acidification is the process of lowering the pH of a solution from any starting point to any endpoint on the pH scale.

Influence of Other Factors

Several factors can locally influence CO2 chemical reactions with seawater, contributing to the effects of ocean acidification.

Acid Rain

• Acid rain has an impact on surface ocean chemistry and has a pH range of 1 to 6.
• It has a significant local and regional impact on ocean acidification, but a negligible global impact.

Eutrophication

• Excess nutrient inputs, mostly nitrogen, from agriculture, fertilizers, and sewage also have an impact on coastal waters.
• The resulting eutrophication causes large plankton blooms, and when these blooms collapse and sink to the sea bed, the subsequent respiration of bacteria decomposing the algae causes a decrease in seawater oxygen and an increase in CO2 (a decline in pH).

Effect of Ocean Acidification

• Sea water absorbs CO2 and produces carbonic acid (H2CO3), bicarbonate (HCO3--), and carbonate ions (CO32-).
• These carbonate ions are required for the calcification process, which allows certain marine organisms to build their calcium carbonate shells and skeletons (e.g. hard tropical corals, cold water corals, molluscs, crustaceans, sea urchins, certain types of plankton, lobsters, etc).
• However, increases in atmospheric CO2 levels cause a decrease in pH, an increase in the concentration of carbonic acid and bicarbonate ions, and a decrease in the concentration of carbonate ions.
• As a result, carbonate ions are less available, making calcification more difficult and possibly impossible.
• The impact of ocean acidification may have potentially disastrous consequences for ocean life and many economically important marine species.

*For detailed notes of this topic, check this link Effect of Ocean Acidification

Mitigation of Ocean Acidification

• The primary cause of ocean acidification is the release of CO2 into the atmosphere as a result of human activity. On a global scale, the only known realistic mitigation option is to limit future atmospheric CO2 levels.
• Appropriate land use and land-use change management can increase the uptake of atmospheric CO2 by vegetation and soils through activities such as wetlands restoration, forest planting, and reforestation.
• Proposals for geoengineering that do not reduce atmospheric CO2 levels, such as methods that focus solely on temperature (such as aerosol backscatter or reduction of greenhouse gases other than CO2), will not prevent ocean acidification.
  • Adding alkaline minerals to the ocean would be effective and cost-effective only on a very small scale in coastal areas, and the unintended environmental consequences are unknown.
• Other stressors on ocean ecosystems, such as higher temperatures and deoxygenation, which are also associated with rising CO2, will be mitigated by limiting CO2 levels.
• The shellfish aquaculture industry faces significant threats, and a risk assessment and analysis of mitigation and adaptation strategies may be beneficial.
  • For example, seawater monitoring near shellfish hatcheries can identify when to limit the intake of lower pH seawater, when to relocate hatcheries, or when to select larval stages or strains that are more resilient to ocean acidification for breeding.

Saturation Horizons

• Deep, cold ocean waters are naturally deficient in carbonate ions, causing most calcifying organisms' shells to dissolve.
• Surface waters are oversaturated with carbonate ions and do not dissolve calcifying organism shells easily.
• The saturation horizon is the point at which calcium carbonate minerals dissolve.
• Those organisms that can survive below the saturation horizon do so because they have special mechanisms in place to keep their calcium carbonate from dissolving.
• As ocean acidification causes this horizon to rise vertically in the water column, more and more calcifying organisms will be exposed to undersaturated water, leaving their shells and skeletons vulnerable to dissolution.
• Calcite's saturation horizon is deeper in the ocean than aragonite's, but both horizons have moved closer to the surface in recent years when compared to the 1800s.

Ocean Acidification and the Short and Long-term Fate of Carbon in the System

• On long timescales (>100,000 years), a natural balance between CO2 uptake and release on Earth is maintained; the CO2 produced by volcanoes, the main natural source of CO2, is taken up by the production of organic matter by plants and rock weathering on land.
• However, because rock weathering takes tens of thousands of years, the current anthropogenic CO2 input to the atmosphere and ocean will not be removed quickly enough.
• On shorter time scales (>1,000 years), the ocean has internal stabilizing feedback known as carbonate compensation that connects the ocean carbon cycle to the underlying carbonate-rich sediment.
• Because the upper layers of the ocean are supersaturated with CaCO3, little dissolution occurs, whereas the deep ocean is undersaturated and carbonate dissolves readily.
• The lysocline, or depth at which dissolution strongly increases in the deep ocean, is the first boundary between these two states.
• CaCO3 in the form of dead shells sinks to the ocean floor.
• The majority is buried in the sediment and trapped for a long time in shallow water, but when the shells sink in deep water, nearly all of the CaCO3 is dissolved, preventing the carbon from being locked away for millions of years.
• The current increased rate of atmospheric CO2 dissolution into the ocean causes an imbalance in the carbonate compensation depth (CCD), which is the depth at which all carbonate is dissolved.
• As the pH of the ocean falls, the lysocline and the CCD become shallower, exposing more shells trapped in the sediments to suboptimal conditions and causing them to dissolve, which will help buffer ocean acidification over a thousand years.

Conclusion

Ocean acidification makes it difficult for marine animals like planktons and corals to form shells and skeletons, which has a significant impact on the marine food web. Some organisms will survive or even thrive in more acidic environments, whereas others will struggle to adapt and may become extinct. Aside from biodiversity loss, acidification will have an impact on fisheries and aquaculture, jeopardizing food security for millions of people, as well as tourism and other sea-related industries.

FAQs

MCQs

1. What is the primary cause of ocean acidification?

A) Overfishing
B) Increased atmospheric CO2
C) Oil spills
D) Plastic pollution

2. How does ocean acidification impact coral reefs?

A) Increases coral growth
B) Weakens coral structures
C) Improves biodiversity
D) Reduces water temperature

3. Which marine organisms are most affected by ocean acidification?

A) Sea turtles
B) Shellfish and plankton
C) Dolphins
D) Seaweeds

4. What socio-economic impact can ocean acidification have?

A) Increase in seafood prices
B) Decrease in fishery yields
C) Improved coastal tourism
D) Enhanced biodiversity

5. What is one way to mitigate ocean acidification?

A) Increase plastic use
B) Reduce CO2 emissions
C) Promote overfishing
D) Destroy coastal ecosystems

GS Mains Questions and Model Answers

Q1: Analyze the impact of ocean acidification on marine ecosystems.

Answer: Ocean acidification has significant impacts on marine ecosystems, primarily due to the increased acidity of seawater affecting the physiological processes of marine organisms. Coral reefs, often referred to as the “rainforests of the sea,” are particularly vulnerable, as the dissolution of calcium carbonate makes it difficult for corals to build their structures. This not only threatens biodiversity but also disrupts the habitats of numerous marine species. Furthermore, the impairment of shell formation in organisms such as mollusks and certain plankton species can lead to cascading effects throughout the food web. As these foundational species decline, the entire marine ecosystem faces destabilization, highlighting the urgent need for addressing the root causes of ocean acidification.

Q2: Discuss the socio-economic implications of ocean acidification for coastal communities.

Answer: The socio-economic implications of ocean acidification for coastal communities are profound, particularly for those relying on fishing and marine resources for their livelihoods. As key species decline in response to changing ocean chemistry, fishery yields may decrease, directly impacting food security and local economies. Communities dependent on tourism, particularly those with coral reef ecosystems, may also suffer economic losses due to degraded marine environments, leading to reduced visitor numbers and income. Additionally, the rising costs of adapting to these changes can strain local economies, disproportionately affecting vulnerable populations. Therefore, addressing ocean acidification is crucial not only for ecological health but also for the socio-economic well-being of coastal communities.

Q3: Evaluate strategies for mitigating ocean acidification in the context of global climate change.

Answer: Mitigating ocean acidification requires a multi-faceted approach in the context of global climate change. First and foremost, reducing greenhouse gas emissions is vital to curtailing the primary driver of acidification. Transitioning to renewable energy sources, enhancing energy efficiency, and implementing sustainable land use practices are essential steps. Furthermore, protecting and restoring coastal ecosystems, such as mangroves and seagrasses, can improve carbon sequestration and bolster resilience against acidification effects. International cooperation and policy frameworks that address both climate change and ocean health are necessary to ensure comprehensive strategies that protect marine environments. Additionally, ongoing research and monitoring will be critical to understanding the dynamics of ocean acidification and informing adaptive management practices.

Previous Year Questions on Ocean Acidification

1. UPSC CSE Prelims 2021:

Question: Ocean acidification primarily results from which of the following?

A) Ozone depletion
B) Increased CO2 levels
C) Marine pollution
D) Overfishing

Answer: (B)

Explanation: Ocean acidification primarily results from increased CO2 levels in the atmosphere, which is absorbed by the oceans.

2. UPSC CSE Mains 2019 (GS Paper 1):

Question: "Examine the relationship between ocean acidification and climate change." Discuss its implications for marine biodiversity.

Answer: Ocean acidification and climate change are interconnected phenomena resulting from increased greenhouse gas emissions. As atmospheric CO2 levels rise, a significant portion is absorbed by the oceans, leading to acidification. This process disrupts marine ecosystems, particularly affecting calcifying organisms such as corals, mollusks, and some plankton species. The implications for marine biodiversity are severe, as these foundational species are critical for maintaining healthy marine food webs. The decline of such species can result in a loss of biodiversity, altered species interactions, and compromised ecosystem services, underscoring the urgency of addressing both ocean acidification and climate change simultaneously.