Since the Industrial Revolution in the late 1700s, we have been mining and burning coal, oil and natural gas for energy and transportation. These processes release carbon dioxide (CO2) into the atmosphere. It is well established that the rising level of CO2 in our atmosphere is a major cause of global warming. However, the increase in CO2 is also causing changes to the chemistry of the ocean. The ocean absorbs some of the excess atmospheric CO2, which causes what scientists call ocean acidification. And ocean acidification could have major impacts on marine life.





Click through the arrows to see how ocean acidification is affecting some shelled marine organisms.

What exactly is ocean acidification? Acidity is a measure of the concentration of hydrogen ions (H+) in a given solution. When CO2 from the air mixes with ocean water it combines and forms carbonic acid (H2CO3). The carbonic acid then dissociates into hydrogen (H+) and bicarbonate (HCO3) ions.

CO2 (carbon dioxide) + H2O (water) → H2CO3 (carbonic acid)

H2CO3 (carbonic acid) → H+ (hydrogen ions) + HCO3 (bicarbonate)

This chemical reaction causes an increase in hydrogen ions in the seawater, thus increasing ocean acidity. The acidity of a solution is often reported in terms of pH. The pH scale ranges from 0 to 14. A value below 7 is acidic and a value above 7 is basic. The more acidic a solution, the higher the concentration of hydrogen ions and the lower the pH. There are many different ways to measure pH, ranging from litmus paper to sophisticated sensors.

Over the last several decades scientists have been measuring the levels of CO2 and pH throughout the ocean. Data from these types of measurements show that as the concentration of CO2 in the atmosphere increases, levels of CO2 in the ocean also increase, increasing the levels of hydrogen ions and thus decreasing the pH of the ocean. This suggests that atmospheric CO2 is contributing to the acidification of the ocean. It’s important to note that the pH of the ocean is still basic (around a pH of 8), but scientists use the term ocean acidification to indicate the increase in hydrogen ions in the ocean, which lowers the ocean’s pH.

Ocean acidification could have major impacts on the health and biodiversity of the ocean. For example, the increase in hydrogen ions in the ocean is making it harder for some marine invertebrates like corals, oysters and tiny sea snails to form their shells and grow. This has implications that extend throughout the marine food web.

Discussion Questions

  • What does the term ocean acidification mean?
  • Why is the ocean becoming more acidic?
  • If the pH of the ocean is still around 8, why do scientists use the term ocean acidification?
  • How does a decrease in ocean pH affect the ability of some marine organisms to build their shells?
  • What do you think are some possible effects of ocean acidification?

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