Oceans and seawater

The oceans hold roughly 1.338 billion cubic kilometres of water — 96.5% of the planetary total. For water intelligence they matter in four ways: they are the source of all precipitation, the eventual destination of all river flow, the feedstock for desalination, and the receiving environment for most pollution.

Composition

Average seawater contains about 35 grams of dissolved salts per kilogram. Remarkably, the proportions between the major ions are almost constant everywhere in the world ocean — the principle of constant proportions. Total salinity varies between regions; the ratios barely do.

Why salinity varies

Salinity is set locally by the balance of evaporation, which concentrates, against precipitation and river inflow, which dilute. Enclosed seas show this most clearly: the Mediterranean is saltier than the Atlantic because it evaporates more than it receives, and the Baltic is barely brackish because it receives enormous river inflow into a nearly closed basin.

Circulation

Density differences drive the deep ocean circulation, and density depends on temperature and salinity together — hence thermohaline. Cold, salty water in the North Atlantic and around Antarctica sinks and flows along the ocean floor, upwelling elsewhere over roughly a thousand years. This circulation moves an enormous amount of heat and is why north west Europe is far milder than its latitude implies.

Freshwater input from melting ice reduces surface salinity and therefore density, which is the mechanism by which ice loss could weaken the circulation. This is an area of active research and genuine uncertainty rather than settled prediction.

Acidification

The ocean has absorbed roughly a quarter to a third of the carbon dioxide emitted since industrialisation. Dissolved CO₂ forms carbonic acid, and surface ocean pH has fallen by about 0.1 units — which, because the scale is logarithmic, is around a 30% increase in hydrogen ion concentration.

The mechanism is the same carbonate system that governs hardness and limescale in fresh water. Lower pH reduces carbonate ion availability, which makes it harder for corals, molluscs and some plankton to build calcium carbonate structures.