Ocean Acidification: Science and Policy Responses

Introduction

Ocean acidification — often called the 'evil twin' of climate change — refers to the ongoing decrease in the pH of Earth's oceans caused by the absorption of excess carbon dioxide (CO₂) from the atmosphere. Since the Industrial Revolution, the ocean has absorbed approximately 30% of anthropogenic CO₂ emissions, leading to a 26% increase in acidity (a pH drop of 0.1 unit). If current trends continue, oceans could become 150% more acidic by 2100.

The Science of Ocean Acidification

Chemistry

• CO₂ + H₂O → H₂CO₃ (carbonic acid)
• H₂CO₃ → H⁺ + HCO₃⁻ (bicarbonate) + 2H⁺ + CO₃²⁻ (carbonate ions)
• Increased H⁺ concentration = lower pH = more acidic ocean
• Excess H⁺ binds with carbonate ions (CO₃²⁻), reducing their availability
• Key consequence: Less carbonate means calcifying organisms struggle to build shells/skeletons

Carbonate Saturation

• Aragonite saturation state (Ω): Measures availability of carbonate ions
• Ω < 1 = corrosive water (shells begin to dissolve)
• Southern Ocean and parts of the Arctic already experience seasonal undersaturation
• Tropical coral reefs are under threat as Ω declines toward critical thresholds

vs Ocean Warming