Overview
This lecture explains ocean acidification—its causes, chemical processes, impacts on marine life and human society, historical context, and potential solutions.
Definition and Causes
- Ocean acidification is the ongoing decrease in the pH of Earth's oceans.
- The main cause is human-caused carbon dioxide (CO₂) emissions, which the ocean absorbs from the atmosphere.
- Since 1950, ocean surface pH dropped from about 8.15 to 8.05, a 26% increase in hydrogen ions.
- Seawater remains alkaline (pH > 7), but is becoming less so.
Chemical Processes
- Absorbed CO₂ reacts with water to form carbonic acid (H₂CO₃), which dissociates into bicarbonate (HCO₃⁻) and hydrogen ions (H⁺).
- Increased H⁺ lowers ocean pH, making water more acidic.
- Most extra CO₂ becomes bicarbonate and H⁺, reducing carbonate ions (CO₃²⁻), essential for marine calcifiers.
Impacts on Marine Life and Ecosystems
- Marine organisms like corals, mollusks, and some plankton need carbonate to build shells and skeletons.
- Lower carbonate saturation makes calcification harder, leading to weaker shells and coral structures.
- Ocean acidification can disrupt food webs, reduce biodiversity, and cause declines in fisheries.
- Non-calcifying species, harmful algal blooms, and some sea grasses may benefit from higher CO₂.
Economic and Societal Impacts
- Coral reefs support about 25% of marine life; their decline threatens food security, fisheries, and tourism.
- Acidification harms shellfish industries, impacting regional economies (e.g., UK, US).
- Up to a billion people rely on ocean ecosystems for food and livelihoods.
Patterns, History, and Projections
- Acidification rates differ by region due to upwelling, currents, temperature, and river input.
- Current pH declines are unprecedented over at least 26,000 years; future pH may fall to 7.7 by 2100 (high emissions).
- Past mass extinctions were linked to rapid ocean acidification from natural CO₂ spikes.
Solutions and Response Strategies
- Only significant CO₂ emissions reduction addresses root causes of acidification.
- Carbon dioxide removal and ocean alkalinity enhancement are proposed but not widely deployed.
- Sustainable management and international cooperation are critical for mitigation.
Key Terms & Definitions
- pH — A measure of acidity; lower pH means higher acidity.
- Calcification — Process of building shells/skeletons from calcium carbonate.
- Carbonate ions (CO₃²⁻) — Ions needed by marine organisms to calcify.
- Carbon sink — A reservoir (like the ocean) that absorbs and stores carbon.
- Saturation state (Ω) — Indicates whether calcium carbonate will dissolve or form in seawater.
Action Items / Next Steps
- Review recent data on ocean pH and regional acidification trends.
- Study chemical equations for CO₂ dissolution and calcification.
- Read about the economic impact of ocean acidification on fisheries and tourism.