Ocean Acidification: pH and the Ocean's Balance

Essential Question

What does a shift in the ocean's pH mean for marine organisms (living things in the ocean)?

Activity Summary

About a quarter of the carbon dioxide (a gas released when we burn fuels like coal, oil, and gas) that goes into the atmosphere ends up in our ocean. Carbon dioxide is called an "acid gas," and it makes the ocean more acidic. Scientists measure this using something called pH—a scale that shows how acidic or basic (also called alkaline) a liquid is. Learning about the pH scale is a great first step toward understanding how the ocean stays balanced. Even though ocean water isn't acidic overall, a small shift in its pH can make a big difference to marine life—just like small changes in pH can affect the human body.

Learning Objectives

Students will be able to:

• Recognize that pH is one way scientists measure ocean acidification (the ocean becoming more acidic)
• Understand the pH scale, and what acids and bases are
• Understand that a small shift on the pH scale actually represents a much bigger change in ocean chemistry

Key Messages

• The ocean is a vital part of Earth's ecosystem (the network of living things and their environment). It's home to marine life that helps keep humans healthy, too.
• Human actions are changing the balance of the ocean's chemistry.
• Small shifts in pH can make a big difference in the health of marine organisms. (In humans, a drop of just 0.05 in blood pH causes a condition called acidosis, which makes people sick.)

Values

• Everything is connected: Both marine organisms and humans need to keep a very precise pH balance to stay healthy.
• Simple models can be powerful: A small increase or decrease on the pH scale actually means a much larger change in ocean chemistry.
• Cause and effect chain: Burning fossil fuels releases CO₂ → the ocean absorbs this CO₂ → CO₂ acts as an "acid gas" → this makes the ocean more acidic → even a small drop in pH (just 0.1 units) can tip marine organisms from healthy to unhealthy, similar to what happens in the human body.

Solutions

The less acid gas (CO₂) humans release into the atmosphere, the less ends up in the ocean. There are many new inventions in alternative and efficient energy and transportation that individuals, neighborhoods, and even whole cities can use.

• Look into what options exist in your town and get involved:
• Organize a group event to make a home (yours or a neighbor's) more energy-efficient with better appliances, windows, or insulation.
• Use public transportation, or bike or walk when you can.
• If your area doesn't have good public transportation or bike lanes, talk to your local government representatives to start that conversation in your community.

Activity Outline

Preparation: Before the activity, write the numbers 1 through 14 on separate sheets of paper. Lay the sheets out in order from left to right (on a table or on the floor). On a small index card, write "8.1" and place it in the upper right corner of the paper marked "8."

Steps

1) Start with a question: Do you know what an acid is? What about a base? Would you like to find out?

• Introduce the pH scale, which runs from 0 to 14.
• Anything above 7 is basic (also called alkaline). Anything below 7 is acidic. The number 7 itself is neutral—neither acid nor base.
• 0 = the strongest acid. Acids are usually sour, can sting if touched, and can react with or wear away metal.
• 14 = the strongest base. Bases taste bitter, feel slippery, and can dissolve fats and oils.

2) Ask: Did you know there are acidic and basic substances we use—and even eat—every day?

• Have the student pick a household item (or hand them one) and guess where it falls on the pH scale. Use the "pH values of everyday objects" list below to check. You can guide them by saying they're getting "warmer" or "cooler," or by asking questions like "Does it taste sour?"
• When they guess correctly, celebrate! Explore as many items as time allows, saving the "ocean" item for last.

• When you hand the student the item representing the "ocean," explain how special the ocean is, and consider offering a small prize if they place it correctly on the scale.
• Discuss how the ocean's pH has changed over the last hundred years. A century ago, the ocean's pH was 8.1. Today, it's 8.0. That may not sound like much, but consider this:
• If your blood's pH changes by just half that amount (a drop of 0.05), it can cause headaches, confusion, tiredness, shakiness, sleepiness, and trouble thinking clearly. (Healthy blood pH is about 7.45; a condition called "acidosis" happens when blood pH drops to 7.4.) Scientists have observed similar effects in ocean fish—for example, clownfish (like Nemo) may become confused and swim toward predators when ocean water becomes more acidic.

3) Ask: Do you know what's causing this change in the ocean's pH?

• Carbon dioxide is an "acid gas" that makes the ocean more acidic (shown as a drop on the pH scale).
• Carbon dioxide pollution comes from things like:
• Driving cars, boats, planes, and trains
• Using electricity in our homes
• Cutting down trees (deforestation)—trees normally store carbon dioxide in their leaves, keeping it out of the ocean

Follow-Up Discussion

• Can you think of anything you could do to help lower the amount of carbon dioxide released into the atmosphere—carbon dioxide that eventually ends up in the ocean?
• Discuss solutions (some community-level ideas are listed above) with families and middle or high school students.
• For younger students, individual actions may work better, such as:
• Turning off lights when leaving a room
• Unplugging "vampire electronics" (devices like computers and chargers that use electricity even when not in use)

Materials: Approximate pH of Everyday or Household Objects

• Large battery = 2.2
• Vinegar = 2.8
• Coca-Cola (classic) = 2.5 (other brands range up to 4.2)
• Lemon = 2.3
• Grapefruit = 3.0–3.3
• Orange = 3.0–4.0
• Apple = 3.5
• Banana = 5.0
• Milk = 6.5
• Bottled water = 7 (ranges from 6.5–8 depending on brand)
• Human blood = 7.4
• Ocean = 8.1 in the past, now 8.0
• Baking soda = 8.4
• Tums (antacid) = 10.5
• Bleach = 12.6
• Drain cleaner = 14

Resources

• http://www.cisanctuary.org/ocean-acidification/
• http://oceanacidification.noaa.gov/

Credit: Original lesson created by NOAA Ocean Acidification Program. Email noaa.oceanacidification@noaa.gov with any questions.