Explainer: Ocean acidification | Science News for Students

Explainer: Ocean acidification

Here’s what happens when seawater is forced to absorb too much carbon dioxide
Dec 5, 2012 — 3:00 pm EST

When carbon dioxide dissolves in ocean water, a process begins that uses up carbonate molecules. So more carbon dioxide means less carbonates for shell building by swimming snails and other creatures.


Every day, the oceans, which cover 72 percent of the planet, remove about 22 million tons of carbon dioxide from the air. That’s fortunate, since too much carbon dioxide in the air will cause Earth’s atmosphere to become overly warm. That process is already underway. Climate scientists attribute this global warming to the greenhouse effect. And that explains why scientists refer to carbon dioxide, or CO2, as a greenhouse gas.

Of every 10 tons of CO2 added to the atmosphere, two or three tons end up in the water. And these growing emissions of carbon dioxide that human activities have been adding to the air have begun changing the chemistry of the oceans. It’s making them more acidic, a process called ocean acidification.

Acids include liquids like vinegar and lemon juice that taste sour. These materials react with bases ― substances, such as ammonia or lye, that feel slippery — to form salts. Distilled water is neutral, which means it’s neither an acid nor a base. Scientists measure acidity using the pH scale; acids have a pH between 0 and 7, and bases a pH between 7 and 14. (Neutral water has a pH of 7.0.)

Ocean water tends to be slightly basic, with a pH of about 8.1. But that number has been falling as the amount of carbon dioxide in ocean water has been rising. That means seawater is becoming more acidic. It’s happening now, and it’s happening fast.

By the year 2100, if people are still adding the same amount of carbon dioxide into the air that they are adding now, the oceans will be more than twice as acidic as they were before the Industrial Revolution. (The Industrial Revolution began more than 200 years ago and describes the rapid growth of industry.)

“We don’t really give it much thought,” says Peter Neill, “but [ocean acidification] may be one of the most important aspects of what’s going on [with climate change]. And it may be the one that lasts the longest.” Neill directs the World Ocean Observatory in Boothbay Harbor, Maine.

When carbon dioxide dissolves into the sea, it starts a chemical reaction. The carbon dioxide and water molecules combine with calcium carbonate to produce molecules called bicarbonates. And that poses a problem for all of the animals that rely on seawater’s calcium carbonate to make shells or skeletons.

Even though bicarbonate sounds like carbonate, it’s useless to sea animals, from snails and clams to sea butterflies. These animals can’t build shells from bicarbonates. And without their protective shells, such animals can die or will certainly be eaten in greater numbers. As greater amounts of carbon dioxide enter the ocean, more and more carbonates transform into bicarbonates.

But if people reduce the amount of carbon dioxide they put in the air, then less will end up in the ocean. The International Panel on Climate Change, a large group of climate scientists, has concluded that to avoid major destruction to the world’s oceans, by 2050 people must cut carbon dioxide emissions in half. (The IPCC was established by the United Nations and pulls together the latest science to try and determine how climate change will affect people and Earth’s ecosystems.)

Power Words

acid A chemical that increases the level of hydrogen ions (H+) in a solution. Acids have a pH of between 0 and 7. Vinegar is made from acetic acid. Citrus fruits contain an acid known as citric acid.

acidification The process of becoming more acidic. In the ocean, acidification occurs when seawater absorbs carbon dioxide. This triggers chemical reactions that reduce not only the pH of that water, but also the concentration of calcium carbonate, a mineral that many sea creatures use in making their shells.

base  On a pH scale, bases are alkaline materials that have a pH of between 7 and 14. What makes them alkaline is the presence of hydroxide ions (OH-).

calcium carbonate The main chemical compound in limestone, a rock made from the tiny shells of ancient marine organisms. Its formula is CaCO3 (meaning it contains one calcium atom, one carbon atom and three oxygen atoms).

carbon dioxide  A gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. This colorless, odorless gas also is released when organic matter (including fossil fuels like oil or gas) is burned. Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.

distilled (as in water)  Boiling a liquid, such as water, then allowing it to later condense from the steam. This process can be used to remove impurities in the starting material.

global warming The gradual increase in the overall temperature of Earth’s atmosphere due to the greenhouse effect. This effect is caused by increased levels of carbon dioxide, chlorofluorocarbons and other gases in the air, many of them released by human activity.

greenhouse effect  A greenhouse is a building made of glass. As sunlight streams through the windows, it turns into heat, which becomes trapped inside. Because many gases in Earth's atmosphere also let in light but trap heat, scientists have come to call the phenomenon the “greenhouse effect.” Although the greenhouse effect works somewhat differently from an actual greenhouse, the name stuck anyway.

greenhouse gas  A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.

Further Reading

S. Ornes. “Sea changes.” Science News for Students. April 7, 2011.

S. Ornes. “Worm glue.” Science News for Students. Sept. 9, 2009.

S. Perkins. “Cool Jobs: Repellent chemistry.” Science News for Students. Sept. 7, 2013.

E. Sohn. “Super sticky power.” Science News for Students. Aug. 3, 2007.

Learn more about ocean acidification from the National Oceanic and Atmospheric Administration