What is Ocean Acidification?

What is Ocean Acidification?

Ocean acidification is the ongoing decrease in seawater pH caused by the absorption of atmospheric carbon dioxide (CO₂). When CO₂ dissolves in seawater, it forms carbonic acid, which dissociates into bicarbonate and hydrogen ions — increasing acidity and reducing the availability of carbonate ions that marine organisms need to build shells and skeletons. Since the Industrial Revolution, ocean surface pH has dropped from approximately 8.2 to 8.1, representing a 26% increase in acidity.

Why It Matters

The ocean has absorbed roughly 30% of anthropogenic CO₂ emissions since 1750, buffering atmospheric warming but fundamentally altering marine chemistry. This is not a marginal change — the current rate of acidification is faster than any natural event in at least 300 million years, including the Paleocene-Eocene Thermal Maximum that triggered mass marine extinctions.

The ecological consequences are cascading. Calcifying organisms — corals, mollusks, echinoderms, and certain plankton species — struggle to form and maintain calcium carbonate structures in more acidic water. Coral reefs, which support roughly 25% of all marine species despite covering less than 1% of the ocean floor, are particularly vulnerable. Pteropods, tiny sea snails that form a critical base of Arctic and Antarctic food webs, show shell dissolution in waters that have already acidified.

Economic exposure is substantial. Global fisheries and aquaculture generate over $400 billion annually, and shellfish industries are among the most directly threatened. The US Pacific Northwest oyster industry experienced severe larval die-offs in the mid-2000s directly linked to upwelling of acidified water — a $110 million industry nearly collapsed before hatcheries adapted intake water chemistry. These are early signals of systemic risk.

Ocean acidification interacts with warming and deoxygenation in a triple threat to marine ecosystems. Warmer water holds less oxygen and amplifies metabolic stress on organisms already weakened by acidification. These compounding stressors make ecosystem responses nonlinear and difficult to predict, increasing the importance of precautionary management and aggressive emissions reduction.

How It Works / Key Components

The chemistry is straightforward. CO₂ + H₂O → H₂CO₃ (carbonic acid), which rapidly dissociates into H⁺ + HCO₃⁻ (bicarbonate). The additional hydrogen ions lower pH while simultaneously reacting with carbonate ions (CO₃²⁻) to form more bicarbonate. This reduces carbonate ion concentration — the mineral building block that corals, shellfish, and calcifying plankton use to construct their structures. Below certain saturation thresholds (the aragonite and calcite saturation horizons), calcium carbonate structures actively dissolve.

Acidification is not uniform. Polar and high-latitude waters acidify faster because cold water absorbs more CO₂. Upwelling zones along continental margins bring naturally CO₂-rich deep water to the surface, creating acidification hotspots. Coastal waters face additional acidification from nutrient runoff that fuels algal blooms — when these blooms decompose, the respiration process releases CO₂ locally, compounding ocean-atmosphere absorption.

Monitoring relies on a global network of ocean observation systems. The Global Ocean Acidification Observing Network (GOA-ON) coordinates pH, dissolved inorganic carbon, and alkalinity measurements across open ocean moorings, coastal stations, and ship-based surveys. Autonomous profiling floats equipped with pH sensors — deployed through programs like Biogeochemical Argo — have dramatically expanded spatial coverage since 2015.

Mitigation is primarily an emissions story. Because ocean acidification is driven by CO₂ concentration, the only systemic solution is reducing atmospheric CO₂. Local interventions — alkalinity enhancement, seagrass and kelp restoration to locally draw down CO₂, and managed relocation of sensitive species — can provide partial, place-based relief but cannot reverse the global chemical shift without parallel emissions reductions.

Council Fire's Approach

Council Fire integrates ocean acidification science into climate resilience planning for coastal communities, marine industries, and conservation organizations. Our advisory work spans vulnerability assessment for fisheries and aquaculture operations, adaptation strategy for ocean-dependent economies, and policy engagement on ocean-climate linkages within UNFCCC and regional governance frameworks.

Frequently Asked Questions

Can ocean acidification be reversed?

Only through significant reduction of atmospheric CO₂ concentrations. Even if emissions ceased immediately, ocean chemistry would take centuries to return to pre-industrial conditions due to the slow mixing between surface and deep ocean waters. This makes aggressive near-term emissions reduction critical — every additional ton of CO₂ absorbed further shifts the chemical baseline.

How does ocean acidification affect food security?

Shellfish, coral reef fisheries, and plankton-dependent fish stocks are all at risk. The FAO estimates that over 1 billion people rely on fish as their primary protein source, with the highest dependence in tropical coastal communities least equipped to adapt. Aquaculture operations can partially buffer by managing water chemistry, but wild fisheries have no such option.

Is ocean acidification the same as coral bleaching?

No. Coral bleaching is primarily driven by elevated water temperatures that cause corals to expel symbiotic algae. Ocean acidification weakens coral skeletons by reducing carbonate availability. Both processes damage corals, and they interact synergistically — acidified corals are less resilient to thermal stress, and bleached corals recover more slowly in acidified water.