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How can rising carbon dioxide levels make ocean water more acidic?

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World map shaded to show annual carbon dioxide emissions by country in 2022, with China, the United States, and India highlighted as top emitters in dark red. A bar chart below lists the top ten emitting countries by million metric tons. — Greenhouse gas Map of annual carbon dioxide emissions by country in 2014.

Rising carbon dioxide ( $C O_{2}$ ) levels in the atmosphere lead to ocean acidification through a direct chemical reaction when the gas dissolves into seawater.1

When $C O_{2}$ is absorbed by the ocean, it reacts with water to form carbonic acid ( $H_{2} C O_{3}$ ). This process liberates hydrogen ions ( $H^{+}$ ) into the water. Because pH is defined as the negative logarithm of the hydrogen ion concentration, an increase in the concentration of these ions results in a lower pH value, which signifies higher acidity.12

As acidity increases, these excess hydrogen ions react with carbonate ions ( $C O_{3}^{2 -}$ ) to form bicarbonate ( $H C O_{3}^{-}$ ). This chemical shift reduces the availability of carbonate ions, which are essential building blocks for marine organisms—such as coral, shellfish, and other calcifiers—to create and maintain their shells and skeletons. Consequently, the absorption of human-produced $C O_{2}$ has caused a measurable decline in seawater pH, threatening the stability of marine ecosystems and food chains.1