Ocean acidification refers to the ongoing decrease in the pH of the Earth's oceans caused by the uptake of carbon dioxide from the atmosphere. For the last 200 years, the burning of fossil fuels — coal, oil, natural gas — for energy, cement production, and deforestation pumps carbon dioxide or CO₂ into the atmosphere. The ocean has absorbed about 1/3 of this CO₂, which when combined with water, forms a weak acid. The drop in pH increases the hydrogen ion concentration in the ocean thereby making the oceans less alkaline. The impacts of ocean acidification are an urgent issue because of the potential global-scale effects they present across a broad spectrum of marine life.

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 corals, leaving reef structures vulnerable to storm damage, and may affect economically important shellfish species such as oysters, scallops, mussels, clams, sea urchins, crabs, and lobsters.

Some organisms could benefit from ocean acidification, while others are negatively impacted, and the impacts may differ from one life stage to another (i.e., adults, eggs, larvae, juveniles, etc.). Overall, the net effect is likely to disrupt the normal functioning of many marine and coastal ecosystems. But scientists are currently unable to predict the net impacts on most marine ecosystems or the services they provide such as fisheries and coastline protection.

Other calcifying organisms like tiny sea snails known as pteropods are also affected by the chemistry changes. Shelled pteropods are an important food source for salmon, mackerel, herring, cod, and even whales. So, if pteropods are adversely impacted, salmon could be too, and then other fish could move in and make it hard for salmon to come back. Bottom line, if calcifying organisms are unable to sustain their populations, many other species are likely to be affected.