Ocean Acidification: The Other CO₂ Problem

Ocean acidification — the ongoing decrease in ocean pH resulting from the absorption of atmospheric CO₂ — is one of the most consequential and least publicly understood consequences of greenhouse gas emissions. The ocean absorbs approximately 25-30% of all human CO₂ emissions — approximately 10 billion tonnes of CO₂ per year — buffering atmospheric CO₂ concentrations but in the process fundamentally altering ocean chemistry. Since the Industrial Revolution, ocean surface pH has declined from approximately 8.2 to approximately 8.1 — a change of 0.1 pH units that represents a 26% increase in hydrogen ion concentration (acidity), because pH is a logarithmic scale. At current emissions trajectories, ocean pH could reach 7.9 or below by 2100 — a level not seen in the ocean for over 20 million years.

25-30%

of human CO₂ absorbed by the ocean

0.1 pH

decline since pre-industrial times

26%

increase in ocean acidity since 1750

20M yrs

since ocean pH was this low

The Chemistry of Acidification

When CO₂ dissolves in seawater, it forms carbonic acid (H₂CO₃), which rapidly dissociates to produce hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻). The increased hydrogen ion concentration — reduced pH — reacts with carbonate ions (CO₃²⁻) already present in the seawater, reducing their concentration. This reduction in carbonate ion concentration is the primary mechanism by which ocean acidification threatens marine life: many marine organisms — including corals, oysters, mussels, sea urchins, and certain plankton — build their shells and skeletons from calcium carbonate (CaCO₃), either in the aragonite or calcite crystal form. As carbonate ion concentrations decline, the seawater becomes increasingly undersaturated with respect to these minerals, making it thermodynamically unfavourable to precipitate calcium carbonate and, below the "saturation horizon," actively corrosive to existing carbonate structures.

"Ocean acidification is not a future threat — it is a present reality. The pteropods we are catching in the Southern Ocean already have shells that are dissolving in the water around them. This is happening now, at current CO₂ concentrations, to foundational components of the marine food web." — NOAA Ocean Acidification Programme

Ocean water chemistry showing acidification impact on marine carbonate chemistry and coral reefs

Impacts on Marine Life

The biological impacts of ocean acidification vary dramatically among species and life history stages, but the organisms most directly threatened are those that build calcium carbonate structures — corals, molluscs, echinoderms, and certain plankton groups. Laboratory experiments show that many calcifying species produce thinner, weaker shells and skeletons at elevated CO₂ concentrations, with energetic costs of calcification increasing as carbonate saturation decreases. Pteropods — small free-swimming molluscs that are a critical food source for salmon, herring, mackerel, and whales in polar and sub-polar oceans — are showing shell dissolution in Southern Ocean waters that are already approaching aragonite undersaturation. Oyster larvae are particularly sensitive: larval mortality in Pacific oyster hatcheries along the US Pacific Coast was linked to ocean acidification as early as 2007, representing the first documented economic impact of acidification on commercial fisheries.