Ocean Acidification Calculator

Surface ocean pH tracks atmospheric carbon dioxide because the two reservoirs exchange gas until the partial pressures equilibrate. Dissolved CO2 hydrates to carbonic acid, which dissociates and releases hydrogen ions, so a rising atmospheric mixing ratio drives pH down. The logarithmic form used here, pH equal to 8.18 minus 0.85 times log10 of the ratio of current CO2 to the pre-industrial 280 ppm, is a compact fit to that behavior: doubling CO2 costs roughly 0.26 pH units rather than a fixed amount per ppm, because seawater buffers itself. Because pH is itself logarithmic, the drop from 8.18 to about 8.07 already represents close to a 30 percent rise in hydrogen ion concentration. The second output, aragonite saturation state, matters more to organisms than pH alone. It compares the in-situ product of calcium and carbonate ion concentrations against the solubility product of aragonite, the metastable calcium carbonate polymorph that corals and pteropods precipitate. A value above 1 is thermodynamically favorable, but reef accretion in practice needs about 3.0 to 3.5, and most healthy tropical reef water sits between 3.5 and 4.0. The temperature and salinity corrections applied here, on the order of 0.005 pH units per degree C and 0.002 per practical salinity unit, are first-order adjustments only. A rigorous treatment solves the full carbonate system from two measured variables such as total alkalinity and dissolved inorganic carbon, and coastal waters with river input, upwelling or strong biological drawdown can depart from any CO2-only estimate by several tenths of a unit.

The chemistry predates the concern. Niels Bjerrum worked out the carbonate equilibria in the 1910s, and in 1957 Roger Revelle and Hans Suess at Scripps Institution of Oceanography showed that seawater buffering limits how much fossil carbon the ocean can absorb, introducing what is now called the Revelle factor. Charles David Keeling began continuous CO2 measurements at Mauna Loa in 1958, giving the atmospheric side of the ledger the long record that acidification estimates lean on. Through the 1970s and 1980s the emphasis stayed on the ocean as a carbon sink rather than on what absorbing that carbon did to the water. The framing changed in 2003, when Ken Caldeira and Michael Wickett published a short Nature paper titled Anthropogenic Carbon and Ocean pH projecting pH declines larger than anything in the geological record of the past several hundred million years; the phrase ocean acidification entered general use from that period. A 2005 Royal Society report brought it to policymakers. Sustained observations gave the trend empirical footing: the Hawaii Ocean Time-series and the Bermuda Atlantic Time-series Study, both beginning in 1988, plus the European ESTOC station, documented measurable pH decline at fixed points. The Global Ocean Acidification Observing Network, launched in 2012, now coordinates moorings, Argo floats with biogeochemical sensors and repeat hydrographic sections into a single international framework.