Earth Science Week Classroom Activities

Ocean Acidification

Activity Source:

Adapted with permission by National Oceanic and Atmospheric Administration.

The ocean is a “carbon sink,” which means that it removes CO2 (carbon dioxide) from the atmosphere. The ocean currently absorbs about one-third of the CO2 released by the burning of fossil fuels.

However, beyond a certain level of atmospheric CO2, the ocean can no longer act as a carbon sink without it having a negative impact on marine life. When CO2 dissolves in seawater, it leads to decreased pH levels. The ocean becomes less alkaline. This is referred to as ocean acidification.

Earth’s oceans are naturally slightly alkaline. The pH of surface seawater is around 8.1. Ocean pH levels can naturally fluctuate due to seasonal temperatures or extreme weather events.

Ocean acidification refers to the observed, ongoing process by which the pH in Earth’s oceans is gradually decreasing, with the result that the water is becoming less alkaline. In this instance, “acidification” means that it the pH is becoming increasingly less alkaline and moving toward the acid end of the pH scale. The water in Earth’s oceans will never actually become acidic (pH less than 7).

Scientists are not able to measure pH over vast expanses of ocean. Instead, they have created models based on satellite-imaging data to look at complementary data that allows them to better understand changes in pH over large areas of ocean. Results of these models are then checked with hands-on data collected by researchers on ships or from scientific instruments on buoys distributed within a study area.


  • Computer with internet connection


  1. These false-color maps* represent ocean surface water pH using colors. The key on the right correlates pH values to map colors. Use these maps to check your understanding.
  2. Discuss:
  • What is the range of pH values in the ocean area shown for January 2008?

  • What is the range of pH values in the ocean area shown for August 2008?

  • Comparing the two maps, has the ocean area become more or less alkaline from January to August?

  • A false-color map is a map that uses colors to represent difference in measured values. The color is “false” in that the land, water, or other surface being shown is not really that color.