Ocean Acidification Calculator

Ocean acidification is the ongoing decrease in the pH of Earth's oceans, caused by the absorption of atmospheric CO₂. When CO₂ dissolves in seawater, it forms carbonic acid (H₂CO₃), which then dissociates into bicarbonate and hydrogen ions — lowering pH and increasing acidity. Since the Industrial Revolution, average ocean pH has dropped from approximately 8.18 to 8.07.

Before the Industrial Revolution (around 1750), atmospheric CO₂ was approximately 280 ppm and average surface ocean pH was about 8.18. This calculator uses 280 ppm as the baseline for calculating pH change, so you can see exactly how much acidification has occurred at any given CO₂ level.

Warmer water holds less dissolved CO₂ (gas solubility decreases with temperature), which tends to raise pH slightly. However, warmer oceans also affect carbonate chemistry equilibria. In practice, ocean warming and acidification are separate but interacting stressors, and this calculator accounts for the temperature dependence of dissociation constants.

Aragonite saturation state (Ω) measures whether seawater is supersaturated or undersaturated with respect to aragonite, a form of calcium carbonate used by corals, mollusks, and other marine organisms to build shells and skeletons. A value above 1 means conditions favor shell formation; below 1, shells and reefs begin to dissolve. Pre-industrial surface waters had Ω values of about 3.5; acidification is pushing this lower.

Salinity affects the ionic strength of seawater, which in turn influences the activity coefficients of dissolved species and the equilibrium constants for carbonate chemistry. Higher salinity generally increases buffering capacity slightly. Coastal and polar regions often have lower salinity due to freshwater input, making them more vulnerable to acidification.

Under high-emission scenarios (RCP 8.5), atmospheric CO₂ could reach 800–1000 ppm by 2100. This calculator lets you explore those scenarios directly — at 900 ppm, ocean pH could drop to around 7.95, a level not seen for millions of years. Under aggressive mitigation (RCP 2.6), concentrations may stabilize near 450 ppm.

This calculator uses a simplified carbonate equilibrium model based on the Henry's Law constant (K₀) for CO₂ solubility and the first and second dissociation constants (K₁, K₂) of carbonic acid, all adjusted for temperature and salinity using standard oceanographic parameterizations (Weiss 1974; Mehrbach et al. as refitted by Dickson & Millero). Aragonite saturation is estimated from carbonate ion concentration and the temperature-dependent solubility product.

The pH scale is logarithmic, so a drop of 0.1 pH units represents a roughly 26% increase in hydrogen ion concentration. Even small shifts alter the availability of carbonate ions, disrupting the ability of organisms like oysters, sea urchins, pteropods, and coral reefs to build and maintain their calcium carbonate structures. It also affects fish behavior and larval development at the cellular level.