Ocean Acidification: pH Drop, Dissolving Shells, and Coral Calcification

Thirty Percent More Acidic Since 1750

The surface ocean has become approximately 30% more acidic since the start of the Industrial Revolution — not in terms of pH units, but in hydrogen ion concentration, which is what acidity actually measures. Ocean surface pH averaged approximately 8.2 in pre-industrial times; it now averages approximately 8.1. That 0.1-unit difference seems small. It is not: pH is a logarithmic scale, meaning a drop of 0.1 represents a 26% increase in hydrogen ion concentration. A drop from 8.2 to 8.0 — which researchers project may occur before 2100 under high-emissions scenarios — would represent a 58% increase from pre-industrial levels. The ocean has not experienced pH this low in at least 800,000 years, based on ice core records, and possibly 14–35 million years, based on boron isotope proxies in marine sediments.

The Chemistry: CO₂ Becomes Carbonic Acid

The chemistry of ocean acidification is straightforward. When carbon dioxide dissolves in seawater, it reacts with water to form carbonic acid (H₂CO₃). Carbonic acid is unstable and quickly dissociates into bicarbonate ions (HCO₃⁻) and hydrogen ions (H⁺). It is these excess hydrogen ions that lower pH and create the acidification effect. The hydrogen ions then react with carbonate ions (CO₃²⁻) in the water, converting them to more bicarbonate — reducing the pool of free carbonate ions available for marine organisms to build calcium carbonate shells and skeletons.

• CO₂ + H₂O → H₂CO₃ → HCO₃⁻ + H⁺ (primary acidification reaction)
• H⁺ + CO₃²⁻ → HCO₃⁻ (carbonate consumption reaction — reduces available carbonate)
• The ocean absorbs approximately 30% of all human CO₂ emissions annually — about 10 billion metric tons per year.
• Without ocean CO₂ absorption, atmospheric CO₂ concentrations would be approximately 55 ppm higher than they currently are.

Aragonite Saturation: The Threshold That Matters

Calcium carbonate exists in two mineral forms relevant to marine biology: calcite and aragonite. Aragonite is more soluble than calcite and is the form used by corals, pteropods, and many mollusks. The aragonite saturation state (Ωarag) of seawater — a measure of how supersaturated water is with aragonite — determines whether organisms can build and maintain aragonite structures. When Ωarag falls below 1.0, aragonite dissolves spontaneously in seawater. Tropical surface waters historically had Ωarag of approximately 3.5; current values have declined to approximately 3.0 in many tropical regions and are already below 1.0 in parts of the Arctic and Antarctic oceans, where cold temperatures increase CO₂ solubility and accelerate acidification. Southern Ocean Ωarag is projected to fall below 1.0 in winter throughout the region by 2050 under moderate emissions scenarios.

Pteropods: The Canary in the Acidified Ocean

Pteropods — free-swimming sea snails whose aragonite shells are among the thinnest and most vulnerable to dissolution — have become the primary biological indicator of ocean acidification impacts. Research published in Nature Geoscience in 2012 by Nina Bednaršek and colleagues documented severe shell dissolution in pteropods (Limacina helicina) collected from the California Current off the U.S. West Coast, in waters already upwelling corrosive deep water with Ωarag below 1.0. Electron microscopy revealed that shells were pitted and dissolving in ways that would weaken structural integrity and impair survival. Pteropods are not merely indicators — they are foundational prey for salmon, herring, whales, and seabirds in both the North Pacific and Southern Ocean. Disrupting pteropod populations cascades through food webs that humans depend on for fisheries worth billions of dollars annually.

• Pteropods constitute up to 50% of the diet of juvenile pink salmon in the Gulf of Alaska.
• Antarctic krill (Euphausia superba), the bedrock of the Southern Ocean food web, may also face shell dissolution risk as Ωarag drops, though krill have more calcite than aragonite in their exoskeleton.
• Corrosive upwelled water along the U.S. West Coast has already caused significant oyster hatchery failures in Washington and Oregon, with industry losses exceeding $100 million.

Coral Calcification and Reef Structure

Coral reefs are built from calcium carbonate skeletons secreted by coral polyps — primarily aragonite. As ocean pH drops and carbonate ion concentration decreases, the rate at which corals can deposit calcium carbonate declines. Research has shown that coral calcification rates fall approximately 15–40% for every 0.1-unit drop in pH, depending on species. Declining calcification produces thinner, weaker skeletons that are more susceptible to physical damage from storms, bioerosion by sponges and urchins, and dissolution during bleaching events when the coral polyp stops actively secreting carbonate. The combination of acidification with warming (which drives bleaching) and physical damage creates compounding stressors that individually might be manageable but together exceed coral recovery capacity.

Rate of Change: The Speed Problem

The rate of contemporary ocean acidification is as significant as the absolute magnitude. Past ocean acidification events in Earth's history — such as the Paleocene-Eocene Thermal Maximum (PETM) 56 million years ago — occurred over thousands of years, allowing evolutionary adaptation by marine species. Current ocean pH is changing at a rate estimated at 10 to 100 times faster than any acidification event in the past 300 million years. Marine organisms' evolutionary toolkit — the capacity to develop new calcification strategies, shift shell mineralogy, or adjust physiology — operates on timescales of thousands of generations, not decades. The speed of the current change is what makes it genuinely unprecedented as a biological threat.