The ongoing decrease in the pH of the Earth's oceans, caused by the absorption of carbon dioxide from the atmosphere, is called:

Ocean acidification is the gradual decrease in the pH of sea water caused mainly by the oceans absorbing carbon dioxide (CO2) released into the atmosphere by human activities, such as burning fossil fuels and cutting forests.

The chemistry:

• When carbon dioxide dissolves in sea water, it reacts with water to form carbonic acid (H2CO3).
• Carbonic acid releases hydrogen ions, which lower the pH, making the water more acidic (less alkaline).
• The extra hydrogen ions also combine with carbonate ions, reducing the amount of carbonate available in the water.

How much has changed:

• The oceans absorb about a quarter to a third of the carbon dioxide released by human activities.
• Since the start of the Industrial Revolution, the average pH of surface ocean water has fallen from about 8.2 to about 8.05 to 8.1.
• Because the pH scale is logarithmic, this means the oceans have become about 30 percent more acidic.
• Sea water is still alkaline (pH above 7), but it is becoming less alkaline.

Effects on marine life:

Many marine organisms, such as corals, shellfish (oysters, clams, mussels), sea urchins and some plankton, build shells or skeletons of calcium carbonate.

With less carbonate available, they find it harder to build and maintain these structures. In very acidic water, their shells can even dissolve.

Coral reefs, already threatened by warming (bleaching), become weaker.

Relationship with climate change:

• Ocean acidification is sometimes called "the other CO2 problem", because it is caused by the same emissions that cause global warming.
• By absorbing CO2, the oceans slow down global warming but pay a price in acidification.
• Solution: cutting carbon dioxide emissions is the main remedy; UN Sustainable Development Goal 14.3 aims to minimise ocean acidification.

Why not the other options?

• ✗Eutrophication is the excessive growth of algae due to nutrient pollution.
• ✗Ocean stratification is the layering of water by temperature and salinity.
• ✗Desalination is the removal of salt from sea water.