EarthLabs > Climate and the Carbon Cycle: Unit Overview > Lab 7: Ocean Acidification > 7A: C02 and Ocean pH - What's the Connection?

Ocean Acidification

Part A: CO2 and Ocean pH - What's the Connection?

Oyster farmers sound the alarm about ocean acidification

Oyster farmers have been on the front lines of ocean acidification. In Washington and Oregon, oysters farms are in coastal Pacific waters where upwelling currents are bringing up cold, deep water with higher amounts of CO2 and a more acidic pH. Watch and listen to two oyster farmers from Taylor Shellfish Farms in Washington state talk how about ocean acidification is impacting their young oysters.


Why is ocean acidification important to oyster farmers?

Surface ocean acidity has increased by 30% since pre-industrial times

Recent estimates by scientists ( Le Quere et al., 2012) have calculated that approximately 26% of all CO2 emitted from human-related activity was absorbed by the oceans during the decade 2002 - 2012. That's 2.5 billion gigatons of excess carbon that moved from the atmosphere into the ocean each year during that one decade. Some of this excess CO2 ends up in deep ocean currents that eventually upwell along coastal areas bringing all that excess CO2with them and to oyster farms.

Scientists have also observed that the oceans have become more acidic since the beginning of the Industrial Revolution. They project that this trend will continue in this century as indicated in the visualization pictured on the right. Scientists are concerned that ocean acidificationa gradual acidification of seawatercould have negative consequences for marine organisms, marine food webs, and entire ecosystems. To better understand what ocean acidification is and why it happens, you will:

Understanding pH Chemistry

You have probably used pH paper and a pH scale in past science classes to determine how acidic or basic (alkaline) a substance is. Whether a substance is an acid or a base depends on the concentration of hydrogen (H+) ions an atom or group of atoms that carries a positive or negative electric charge as a result of having lost or gained one or more electrons. compared to the concentration of hydroxide (OH-) ions in solution. A pH scale is used to measure the concentration of (H+) ions.

  1. Click to enlarge the pH scale image on the right.
  2. Take a few minutes to examine the relationship between the pH value (1-14) and the concentration of (H+) ions compared to the concentration of hydroxide (OH-) ions. Also pay attention to the logarithmic scale on the left which illustrates how much the concentration of H+ ions changes relative to the neutral pH of 7. Key points to know about the pH scale and acidity:
    • The lower the pH value, the more acidic the solution.
    • The higher the pH value, the more basic (alkaline) the solution.
  3. The pH scale is based on a logarithmic scale (powers of ten). For example, pH 7 is ten times more acidic than pH 8. pH 6 is 100 times (10 X 10) more acidic than pH 8. This means that even a small change in pH can significantly change the concentration of H+ ions in seawater.
  4. The term "acidity" does not mean the same thing as a solution being an "acid." For example, if a pH value of a substance changes from pH 10 to pH 9, the concentration of H+ ions increases making the substance more "acidic" but not making the solution an acid. To be an acid, the pH value has to be less than 7. This concept will become important in understanding ocean acidification.
  5. NOTE: If you have no prior knowledge of pH, acids and bases, you may want to watch this video: Acids, Bases & pH — bozemanscience

Checking In

  • Based on your understanding of the pH scale, which of the following statements are TRUE?

Does a change in dissolved CO2 cause a change in pH? What's the evidence?

So far, you have learned that the concentration of CO2 in the atmosphere is increasing and some of this extra CO2is dissolving in the oceans. But, can an increase in dissolved CO2 change the pH of sea water? Try this experiment to find out! With your lab group or partner, choose one of the following hypotheses:

  1. Increased CO2 will cause the pH to become more acidic
  2. Increased CO2 will cause the pH to become less acidic and instead, become more basic (alkaline).
  3. Increased CO2 will have no effect on pH at all.

To test your hypothesis, you will use a chemical called bromothymol blue (BTB). This chemical is commonly used in many laboratory experiments to test for a change in pH. You will add CO2 to plain tap water. The source of CO2 you will use in this experiment will be the CO2 that you exhale.

Materials you will need for your group:

Follow this procedure:

  1. Pour 50 ml of BTB solution into the beaker. Make note of its color.

    Indicators are special dyes that "indicate" the presence of a specific chemical by turning color. When the Bromothymol Blue (BTB) has a neutral pH, the solution will be slightly green. When the BTB has a slightly acidic pH, the solution will appear yellow. When the solution is slightly basic (alkaline), the BTB solution will appear blue.

  2. Exhale your CO2 through the straw into the beaker. Make sure you don't "suck up" any of the BTB solution into your straw and mouth.
  3. When a color change has occurred, stop exhaling CO2 into the straw. Compare your color change to the image below.


  • Describe the results of your CO2/pH experiment.
  • Which hypothesis is supported by the results? Explain why.
  • Your experiment was a small scale experiment completed at a lab bench. Can the results of this experiment be extrapolated to understanding the effect of increased CO2 on pH in the oceans? Explain why or why not?

Comparing time series data of atmospheric CO2, sea surface CO2 and sea water pH can uncover trends and causal relationships

The image on the right shows ocean researchers using a rosette device to collect seawater samples to measure pH and dissolved CO2 in addition to other biological, chemical and physical parameters. By sampling in the same location for several years, researchers can generate times series data that can identify trends in a changing ocean chemistry.

Using the interactive below, you can compare three different time series data sets collected from Station Mauna Loa and Station Aloha in the Pacific Ocean.

As you click forward in the interactive, note that each data set shows considerable variability in yearly measurements but when this variability is averaged over time, definite trends can be observed. Make note of these trends and what these trends might tell you about causality between increased atmospheric CO2 and ocean acidity.

This content is available in flash format only

To view this interactive on an iPad, use this link to download/open the free TERC EarthLabs App.


  • Describe the trend for each data set.
  • Does comparing the trends from these three time series data sets provide evidence for a cause and effect relationship between increasing CO2 emissions and ocean acidification. Explain why or why not.

CO2, ocean acidification and the ocean's carbonate chemistry system

In Lab 6A, you learned that CO2 can dissolve in seawater and become part of the ocean's carbonate chemistry system. As the oceans absorb increasing amounts of CO2, the ocean's pH chemistry becomes more acidic. This change in acidity is causing changes in the ocean's carbonate system. Ocean acidification, or "OA" for short, is the term given to these chemical changes in the ocean as a result of increasing carbon dioxide emissions.

How is the chemistry of the ocean carbonate system changed by a more acidic pH? Find out by watching the short video below on Ocean Chemistry by the Alliance for Climate Education (ace). As you watch the video, make note of what happens to the amounts of:

Stop and Think

Throughout this module, you have learned that a change in one part of the carbon cycle can cause changes in other parts of the carbon cycle. Keeping this important theme in mind, explain how an increase in CO2 fossil fuel emissions can alter the ocean.

Optional Extensions

Want to find out more about ocean acidifcation and ocean chemistry? Check out these resources:

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