How Ocean Acidification Dissolves Marine Shells and Reefs

Shells Dissolving in 45 Days

Pteropods — free-swimming sea snails about the size of a lentil — form a critical link in polar food webs, consumed by salmon, herring, and whales. In 2008, researchers from NOAA collected pteropods (Limacina helicina) from the Southern Ocean and observed their shells under scanning electron microscopy after 45 days in water with the pH projected for 2100. The shells showed severe pitting, erosion, and partial dissolution. The animals were alive but their protective structures were degrading in water that was not yet as acidic as end-of-century projections indicate. The observation was among the first direct photographic evidence of ocean acidification dissolving living organisms.

Ocean acidification is the ongoing decrease in seawater pH caused by the ocean's absorption of atmospheric CO₂. Since the Industrial Revolution, the ocean has absorbed approximately 30% of all CO₂ emitted by human activity — about 165 billion tonnes of carbon. This absorption has reduced mean ocean surface pH from 8.21 to 8.10, a change of 0.11 units. Because pH is a logarithmic scale, this represents approximately a 26% increase in hydrogen ion concentration — the operational measure of acidity. The rate of acidification is at least 100 times faster than any natural acidification event in the past 55 million years.

The Carbonate Chemistry of Acidification

When CO₂ dissolves in seawater, it reacts with water to form carbonic acid (H₂CO₃), which rapidly dissociates into bicarbonate (HCO₃⁻) and hydrogen ions (H⁺). The increased H⁺ concentration drives down pH. Critically, these excess hydrogen ions react with carbonate ions (CO₃²⁻) already present in seawater, converting them to bicarbonate and reducing carbonate ion concentration.

Carbonate ions are the building blocks that marine calcifiers — corals, mollusks, echinoderms, certain algae — use to construct shells and skeletons of calcium carbonate (CaCO₃). Two forms of calcium carbonate are biologically important: calcite and aragonite. Aragonite, the form used by corals and pteropods, is approximately 50% more soluble than calcite. As carbonate ion concentration decreases, seawater becomes less saturated with respect to aragonite — and eventually undersaturated, meaning aragonite spontaneously dissolves in contact with that water. Parts of the Arctic and Southern Oceans are already seasonally undersaturated with respect to aragonite.

Biological Impacts by Organism Group

• Corals — reduced calcification rates at lower pH; bleached and weakened reefs less able to maintain accretion against erosion; projected 70–90% decline of coral reefs at 2°C warming
• Pteropods and other mollusks — shell dissolution at aragonite undersaturation; reduced survival and behavior alterations documented at pH 7.9
• Oysters and bivalves — larval settlement failure under acidified conditions; Pacific oyster larvae mortality increased 25–35% in naturally acidified upwelling waters off Oregon and Washington
• Sea urchins and sea stars — reduced sperm motility, lower fertilization success, thinner spines at elevated CO₂
• Fish — sensory and behavioral disruption via CO₂ effects on olfaction and acid-base balance; clownfish cannot locate settlement habitat in acidified water

The Saturation State Problem: A Chemical Threshold

The aragonite saturation state (Ω_arag) is the ratio of actual dissolved carbonate and calcium ion concentrations to the solubility product of aragonite. When Ω_arag exceeds 1, aragonite is stable and organisms can build shells. When it falls below 1, aragonite spontaneously dissolves. Current mean ocean surface Ω_arag is approximately 2.5, down from pre-industrial 3.4. Tropical coral reefs require Ω_arag above roughly 3.3 for optimal calcification; they are experiencing measurably reduced calcification rates at current values.

Arctic surface waters are projected to become seasonally aragonite-undersaturated (Ω_arag below 1) by 2030–2040 under business-as-usual emission scenarios. Antarctic surface waters follow by 2050. Once undersaturation becomes year-round, shell-building organisms in those regions face net dissolution pressure every moment they live in surface waters. The transition from a calcification-favorable to a dissolution-promoting ocean is not gradual for organisms — it is a threshold crossing.

Coral Reefs: Ecosystem Collapse in Acid

Coral reefs occupy less than 0.1% of ocean area but harbor approximately 25% of all described marine species. They are built almost entirely from the calcium carbonate skeletons of coral animals, supplemented by coralline algae and invertebrate contributions. The three-dimensional structure of a reef is not static; it is maintained by continuous calcification that outpaces bioerosion by sponges, urchins, and physical waves. As acidification reduces calcification rates and increases bioerosion, reefs shift from net accretion to net erosion — from reef-building to reef-dissolving mode.

A 2018 review in Science found that live coral cover on the Great Barrier Reef declined by approximately 50% between 1995 and 2017, driven by bleaching events, crown-of-thorns starfish outbreaks, and storm damage. Ocean acidification works synergistically with warming — each stressor weakening reef resilience to the other. Reefs projections under 2°C global warming show 70–90% decline. Under 1.5°C, 70–90% still decline. The window for preserving functional coral reef ecosystems is effectively closed at any emission trajectory consistent with current policy.

The Pace of Change as the Critical Variable

Marine organisms have survived acidification events before in Earth's history — during the PETM (Paleocene-Eocene Thermal Maximum) 56 million years ago, ocean pH dropped by 0.15–0.3 units. But that event unfolded over at least 10,000 years. The current acidification is occurring over decades — too fast for evolutionary adaptation in species with generation times of years to decades. Corals that reproduce slowly, pteropods whose generation time is one year, oysters that take years to mature — none can evolve fast enough to genetically adapt to a 0.3–0.4 unit pH decline within the lifetime of the organisms currently alive. The problem is not the direction of change; it is the speed.

A Problem Shared Between Ocean and Atmosphere

Ocean acidification and climate change are not separate problems with separate solutions. They have the same cause — CO₂ emissions — and the only effective response to both is the same: reducing CO₂ concentrations. Localized buffering strategies — adding alkaline minerals to seawater, protecting seagrass meadows that locally raise pH through photosynthesis — may help specific reef systems survive but cannot alter ocean-wide chemistry at scale. The ocean absorbed human emissions for decades as a service. That service has a cost, and the bill has come due in dissolving shells.