Ocean Acidification: The Basics

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Animation created by Sarah Cooley, Woods Hole Oceanographic Institution, showing a computer recreation of surface ocean pH from 1895 to the present and a forecast of ocean pH between now and 2100 under current emission rates. The pH scale is shown on the right.

Carbon dioxide is naturally present in tiny amounts in the atmosphere; however, since humans started burning fossil fuels at the beginning of the industrial age, carbon dioxide levels have increased dramatically. Carbon dioxide is a gas that plants combine with energy from the sun to create food. It is also a greenhouse gas: it traps the heat of the sun and keeps it from reflecting back into space. Today the oceans absorb about 25 percent of the CO2 released every year. Initially, scientists focused on the benefits of removing the greenhouse gas from the atmosphere, but in the last decade research has shown that the excess CO2 absorbed by the oceans is changing the chemistry -- the pH -- of the seawater in a process called ocean acidification.

Scientists use the pH scale to measure the concentration of hydrogen (H) ions in a solution on a scale of 0 to 14. A pH of 7 is considered neutral, numbers smaller than 7 indicate increased acidity, and numbers greater than 7 indicate increased alkalinity. The pH scale is logarithmic, so one pH unit corresponds to a tenfold increase in the concentration of H ions.

Ocean acidification takes place when excess CO2 from the atmosphere dissolves in the ocean and combines with seawater to form carbonic acid. Carbonic acid breaks up into hydrogen ions and bicarbonate ions -- the increase in hydrogen ions reduces the pH of the seawater, causing it to become more acidic, and interferes with the uptake of calcium carbonate by shelled ocean organisms. Many shelled organisms are affected by a decrease in pH, including oysters, scallops, abalone, and even the smallest plankton that makes up an important part of the ocean food web. When there is less carbonate available, shells become thinner and growth slows down. Organisms have to spend more energy building their shells, which means less energy for growth. Larval stages are especially at risk because they need to grow shells before they start to eat. Shelled organisms provide food for a multitude of other creatures in the ocean, and so their decline has the potential to affect the entire ocean ecosystem.

One of the major concerns of ocean acidification is the drastic rate at which it is happening. Scientists have determined the most rapid CO2 increase in Earth’s history occurred 55 million years ago, and that change was likely not as rapid as the change we see today. It is uncertain whether marine organisms will be able to adapt to the rapidly changing chemistry in such a short time. As organisms are compromised, the biodiversity of the ocean may decrease, as only certain species will be able to adapt to the changing chemistry.


Scientists studying ocean acidification have concluded that in order to keep the ocean environment sustainable, we need to reduce CO2 emissions. Reducing the burning of fossil fuels is necessary for the health of the ocean as well as Earth’s climate.

Pre-discussion Questions

  • What do you know about the ocean?
  • Is the ocean valuable to humans?
  • What is a greenhouse gas?
  • How are greenhouse gases created?

Post-discussion Questions

  • How have humans contributed to ocean acidification?
  • How is ocean acidification affecting organisms in the ocean?
  • Why is the rate of ocean acidification alarming to scientists?
  • What can you do to reduce the effects of ocean acidification?

Extension Activities

  • Carbon Cycle Game, Centers for Ocean Sciences Education Excellence (COSEE). This digital game lets students take on the role of a carbon atom moving through the carbon cycle. Understanding the carbon cycle is essential to understanding the causes and consequences of climate change.
  • GEMS Alien Juice Bar Game, Lawrence Hall of Science. An engaging game for students in grades 4 through 8 explores the properties of acids and bases using red cabbage juice. The game helps students to understand the pH scale.

 Links to Learn More

  • Ocean Acidification, Pacific Marine Environmental Lab, NOAA. This content- and media-rich website is by scientists studying ocean acidification. The site contains research, background information, videos, and infographics.
  • Our Acidifying Ocean, Stanford University. This interactive slideshow about ocean acidification is directed at a middle-school audience. The slideshow contains text, images, and games.
  • We Are Aware, Are You?, Suquamish Tribe. This video was made by four high school students from the Suquamish Tribe in Washington State to highlight the impact of ocean acidification in their community.

Next Generation Science Standards

Performance Expectation: Apply scientific principles to design a method for monitoring and minimizing a human impact on the environment. MS-ESS3-3

Disciplinary Core Idea: Human activities have significantly altered the biosphere, sometimes damaging or destroying natural habitats and causing the extinction of other species. But changes to Earth’s environments can have different impacts (negative and positive) for different living things. ESS3.C Human Impacts on Earth’s Systems

Crosscutting Concept: Cause and effect, stability and change

Science and Engineering Practices: Constructing explanations and designing solutions, asking questions and defining problems