Ocean Acidification


More than a hundred thousand marine species built their bodies using calcium carbonate (CaCO3), including snails, oysters, sea stars, coral, and plenty of planktonic animals. This incredible diversity of life evolved over millions of years as animals figured out ways to pull calcium (Ca2+) and carbonate (CO32-) ions from the water to build shells and skeletons so robust that they remain intact long after the animals perish. But all of this is changing. Our addiction to fossil fuels and the billions of tons of carbon dioxide (CO2) we're pumping into the atmosphere each year may be undoing millions of years of evolution in a geological blink of time.

In the 200-plus years since the industrial revolution began, the concentration of carbon dioxide in the atmosphere has increased due to human actions. About 30% of the world's anthropogenic carbon dioxide is then absorbed by the oceans. This excess dissolved CO2 reacts with seawater (H2O) and creates carbonic acid (H2CO3), a weak acid that breaks (or "dissociates") into hydrogen ions (H+) and bicarbonate ions (HCO3-). As a result, the free hydogen ions lowers the water's pH level and makes the water more acidic. During thie past 200+ years, the pH of surface ocean waters has fallen by 0.1 pH units. This might not sound like much, but the pH scale is logarithmic, so this change represents approximately a 30 percent increase in acidity.

Living in more acidic waters is bad enough for shell-building animals, but CO2 adds another problem. Animals need both calcium and carbonate to build their skeletons. But the extra hydrogen ions in the high CO2 water bind carbonate, reducing the amount available for animals to build their shells. If the pH gets too low, shells and skeletons can even begin to dissolve. And, if these organisms that form the base of the ocean's food web are unable to form their shells or survive, that will lead to a reduction in food for organisms higher up the food web, such as marine mammals, sharks, sea turtles, and humans.

Ocean acidification is currently affecting the entire ocean, including coastal estuaries and waterways. Billions of people worldwide rely on food from the ocean as their primary source of protein. Many jobs and economies in the U.S. and around the world depend on the fish and shellfish that live in the ocean. Ocean acidification is one aspect of global climate change. Anything we do to mitigate climate change today will benefit the future of the ocean as well.

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National Park Service

Ocean Acidification

National Oceanic and Atmospheric Administration

Ocean Acidification

Pacific Marine Environmental Laboratory's Carbon Program

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    Multimedia Presentations

    From 2007 to 2012, Pacific Coast Science and Learning Center Science Communication Interns produced a series of podcasts, videos, and audio-slide shows exploring science from Bay Area national parks. One of these The Natural Laboratory multimedia products focused on the causes and impacts of ocean acidification. View the video or listen to the podcast below. Visit our Multimedia Presentations: The Natural Laboratory page for additional videos and podcasts.



    Gruber, Nicolas, Dominic Clement, Brendan R. Carter, Richard A. Feely, Steven van Heuven, Mario Hoppema, Masao Ishii, et al. 2019. "The oceanic sink for anthropogenic CO2 from 1994 to 2007." Science 363, no. 6432 (March 15, 2019): 1193-1199. Available at https://doi.org/10.1126/science.aau5153 (accessed 30 June 2020)

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    Last updated: June 30, 2020

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