In simple terms
Ocean acidification is the ongoing decrease in the pH of the ocean, meaning the water is becoming gradually more acidic, caused mainly by seawater absorbing carbon dioxide from the atmosphere. [1] It is a direct chemical consequence of rising carbon dioxide levels, separate from the warming that the same gas also causes.
How it works
The ocean has absorbed a large share of the carbon dioxide released by human activity since the industrial revolution. [1] When carbon dioxide dissolves into seawater, it reacts chemically to form carbonic acid, which lowers the water’s pH and reduces the availability of carbonate ions that many marine organisms need. Average ocean surface pH has fallen by roughly 0.1 units since the industrial revolution began, a change that corresponds to about a 30 percent increase in the water’s acidity, because the pH scale is logarithmic. [1]
Organisms that build shells or skeletons from calcium carbonate, including corals, oysters, clams, and some types of plankton, rely on carbonate ions being reasonably abundant in seawater. As acidity rises and carbonate becomes scarcer, building and maintaining those structures becomes harder, and in some cases existing shells can begin to dissolve. [2]
Why it matters
Coral reefs and shellfish fisheries support enormous numbers of other marine species and, in many coastal regions, entire local economies. Scientists sometimes describe ocean acidification as climate change’s “other” carbon dioxide problem, because it threatens those systems through ocean chemistry directly, independent of temperature change, which means reducing it requires cutting carbon dioxide emissions themselves rather than only managing warming. [2]
Where you’ll see it
- Coral reef decline studies and reef monitoring programs
- Oyster and shellfish hatcheries along the U.S. Pacific coast, some of which have already reported production losses
- Ocean and climate research tracking seawater chemistry over time
- International climate assessments and ocean policy discussions
Example
Oyster hatcheries in the Pacific Northwest have at times struggled with larval die-offs linked to more acidic, corrosive seawater reaching their coastal intake water.
Often confused with
Ocean warming is a separate effect of greenhouse gases, caused by heat being trapped in the atmosphere and absorbed by the ocean; acidification is a direct chemical reaction between dissolved carbon dioxide and seawater, and the two processes, though linked at the source, affect marine life through different mechanisms.
Key facts
- The ocean has absorbed a substantial share of the carbon dioxide humans have released since the industrial revolution, and that absorbed carbon dioxide reacts with seawater to form carbonic acid.1
- Average ocean surface pH has fallen by about 0.1 units since the start of the industrial revolution, which corresponds to roughly a 30 percent increase in acidity.1
- More acidic water makes it harder for shell- and skeleton-building organisms, such as corals, oysters, and some plankton, to form and maintain calcium carbonate structures.2
- Ocean acidification is sometimes called climate change's "other" carbon dioxide problem, because it is a direct chemical effect of carbon dioxide rather than a warming-driven effect.2
Quick checkWhy is ocean acidification considered a separate problem from ocean warming, even though both are tied to carbon dioxide?Show answer
Acidification is a direct chemical reaction between absorbed carbon dioxide and seawater, changing its pH, while warming is caused by heat trapped by greenhouse gases; the two happen through different mechanisms even though they share the same root cause.
Sources
- National Oceanic and Atmospheric Administration. Ocean Acidification. Undated (accessed 1 November 2026)
- International Atomic Energy Agency. What is Ocean Acidification?. Undated (accessed 1 November 2026)
Editorially reviewed by Last reviewed November 1, 2026Researched and drafted with AI assistanceReport an issue