Ocean acidification refers to the ongoing change in the chemistry of the ocean caused primarily by the ocean's absorption of carbon dioxide from the atmosphere. For the last 250 years, the burning of fossil fuels — coal, oil, natural gas — for energy, cement production, and deforestation has been pumping carbon dioxide or CO₂, into the atmosphere. The ocean absorbs about 1/4 of this excess CO₂ released into the atmosphere every year. This addition of CO₂ into the ocean is changing the chemistry of seawater by increasing the acidity and lowering the seawater's naturally occurring carbonate ion. CO₂, when combined with water, forms a weak acid, which increases the hydrogen ion concentration in the ocean, lowering the pH and making the oceans less alkaline or more acidic. As the ocean becomes less alkaline there is a reduction in the amount of carbonate ions and calcium carbonate minerals, biologically important building blocks for many marine organisms.

Since 1750, the average acidity of the ocean has increased about 30 percent. The current rate of acidification is nearly 10 times faster than any time in the past 50 million years, outpacing the ocean’s capacity to restore oceanic pH and carbonate chemistry. The rapid pace of change gives marine organisms, marine ecosystems, and humans less time to adapt, evolve, or otherwise adjust to the changing circumstances.

Many life processes, including photosynthesis, growth, respiration, recruitment, reproduction, and behavior are sensitive to carbon dioxide and pH. As a result, ocean acidification has the potential to affect a wide range of organisms, from marine invertebrates like shellfish, to photosynthesizers including phytoplankton and seagrasses, as well as vertebrates including fish in many different ways.

The absorption of excessive amounts of CO₂ from the atmosphere is changing the chemistry of seawater by increasing the acidity and lowering the seawater's naturally occurring carbonate ion, a building block of the calcium carbonate required of many marine organisms to grow their shells and skeletons. Ocean acidification reduces calcification rates in shell-forming organisms such as corals and shellfish. In coastal areas with coral reefs, reef structures impacted by ocean acidification are weaker and less able to protect coastal communities from storm damage. Economically important shellfish species such as oysters, scallops, mussels, and clams are negatively impacted by reduced calcification rates brought on by ocean acidification, particularly in larval stages shell building. Other calcifying organisms like tiny sea snails known as pteropods are affected by the chemistry changes. Shelled pteropods are a critical food source for salmon, mackerel, herring, cod, and even whales.