What is ocean acidification?
The fall in ocean pH caused by seawater absorbing carbon dioxide from the atmosphere — sometimes called "the other carbon problem", because it is a direct chemical consequence of emissions that has nothing to do with temperature.
The chemistry, which is straightforward. CO₂ dissolves in seawater and reacts to form carbonic acid, which releases hydrogen ions, lowering pH. Those hydrogen ions also react with carbonate ions, reducing their availability — and carbonate is what marine organisms use to build shells and skeletons.
The scale. Ocean surface pH has fallen by roughly 0.1 units since the industrial revolution, which sounds trivial and is not: pH is logarithmic, so that represents an increase in acidity of around 30%. The rate of change is faster than anything in the geological record for millions of years, which matters because adaptation takes time.
Why "acidification" is technically imprecise. The ocean remains alkaline, at around pH 8.1. It is becoming less alkaline — moving toward acid rather than becoming acidic. The term describes the direction, and critics who point this out are correct about the chemistry and wrong that it makes the problem smaller.
What is actually affected:
Calcifying organisms — corals, oysters, mussels, sea urchins and the tiny pteropods and coccolithophores at the base of many food webs. Building shells becomes energetically more expensive, and in sufficiently corrosive water shells dissolve.
Shellfish aquaculture, where hatcheries have already experienced failures attributed to acidified upwelling water and now monitor and treat intake water.
Coral reefs, already stressed by warming, where weaker skeletons compound bleaching.
Fish behaviour and sensory function, with evidence that is real and contested in magnitude.
Why it is a separate problem from warming. Reducing warming through solar geoengineering would do nothing for acidification, because the cause is the CO₂ itself. Only reducing emissions addresses both.
The ocean has absorbed roughly a quarter of emitted CO₂, which has slowed warming considerably — acidification is the price of that service.