Ocean acidification

Ocean acidification is a gradual shift in seawater chemistry driven by the rise of carbon dioxide in the atmosphere since the Industrial Revolution, chiefly from burning fossil fuels and clearing forests. The ocean takes up a large share of this carbon dioxide, roughly a quarter to a third of what people emit. Once dissolved, carbon dioxide reacts with water to form carbonic acid, which releases hydrogen ions, and these hydrogen ions lower the pH. Surface seawater is still slightly alkaline, at about pH 8.1, so acidification means becoming less basic rather than actually turning acidic.

Since the 1800s, surface ocean pH has fallen by about 0.1 units. Because the pH scale is logarithmic, that small change means roughly 30 percent more hydrogen ions. The extra hydrogen ions also bind carbonate ions, leaving fewer of them for organisms that build shells and skeletons of calcium carbonate. Corals, oysters, clams, sea urchins, tiny swimming snails called pteropods and chalky plankton must spend more energy to build their hard parts, and in more corrosive water their shells can grow slowly or even dissolve. Larval shellfish are especially vulnerable, and some oyster hatcheries have already suffered heavy losses.

The effects spread through food webs that rely on shell-building species, and acidification adds to the stress that warming already places on coral reefs.