Ocean acidification

Carbon dioxide from the atmosphere enters seawater and sets off chemical reactions that raise the concentration of hydrogen ions. A higher hydrogen-ion concentration is exactly what a lower pH means: the water shifts toward the acidic side.

The same reactions make carbonate ions relatively less abundant. This is the part with direct biological consequences: marine organisms need calcium carbonate to build their shells and skeletons, and with less carbonate available they grow more slowly and build weaker shells.

NOAA notes that atmospheric CO₂ has risen for more than 200 years since the industrial revolution, through the burning of fossil fuels and land use change, and that the ocean absorbs about 30 percent of the CO₂ released into the atmosphere (NOAA's figure, checked September 2026 — other agencies put it at about a quarter, so read it as roughly a quarter to a third).

Note the wording: "acidification" describes the DIRECTION in which pH is moving, not a claim that seawater has become an acid. Over the last 200 years ocean pH has fallen from 8.2 to 8.1 — still above 7, still on the basic side of the scale.