Ocean Acidification: pH and the Ocean's Balance

Essential Question

What does a shift in the ocean's pH mean for marine organisms?

Activity Summary

About a quarter of the carbon dioxide (CO2) that humans release into the atmosphere by burning fossil fuels ends up in the ocean. Carbon dioxide is known as an "acid gas," and when it dissolves in seawater, it makes the ocean more acidic. Scientists measure this using pH, a scale that shows how acidic or basic (alkaline) a liquid is—the lower the pH, the more acidic the liquid.

While pH is only one part of the complex chemistry behind ocean acidification, learning the pH scale is a great first step toward understanding how the ocean's waters stay balanced. Although ocean water isn't actually acidic overall, even a small shift toward a lower pH can have a big impact on marine life—much like a small chemical shift can seriously affect the human body.

Learning Objectives

Students will be able to:

• Recognize that pH is one way scientists measure ocean acidification
• Understand the pH scale, and what acids and bases are
• Understand that a small change in pH actually represents a much larger shift in ocean chemistry

Key Messages

• The ocean is a vital part of Earth's ecosystem and home to marine life that supports human well-being.
• Human activity is changing the chemical balance of the ocean.
• Small shifts in pH can seriously affect the health of marine organisms. (For comparison, in humans, a drop of just 0.05 in blood pH causes a dangerous condition called acidosis.)

Core Ideas

• Interconnection: Both marine organisms and humans depend on a delicate pH balance to stay healthy.
• Simplified models: A small change on the pH scale represents an exponentially larger shift in ocean chemistry—not a simple, one-to-one change.
• Cause and effect: Burning fossil fuels releases CO2 → the ocean absorbs this CO2 → because CO2 is an "acid gas," it makes the ocean more acidic → even a small pH drop (as little as 0.1 units) can tip marine organisms from healthy to harmed, just as it would affect the human body.

Solutions

The less "acid gas" (CO2) humans release into the atmosphere, the less ends up in the ocean over time. Many innovations in energy efficiency and transportation can be adopted—not just by individual households, but by whole communities and cities.

• Look into what options are available in your area and get involved:
• Organize a community event to make a home (yours or a neighbor's) more energy-efficient with better appliances, windows, or insulation.
• Use public transportation, bike, or walk when possible.
• If public transit or bike lanes aren't available in your area, talk to a local government representative and start that conversation in your community.

Activity Outline

Preparation: Before the activity, write the numbers 1 through 14 on separate sheets of paper (one number per sheet). Lay the sheets out in order from left to right, like a number line, on a table or the floor. Write "8.1" on an index card and place it in the upper right corner of the "8" sheet.

Steps

1. Introduce acids and bases.
Ask students if they know what an acid or a base is, and whether they'd like to learn more. Introduce the pH scale, which runs from 0 to 14:

• Anything above 7 is a base (alkaline substance); anything below 7 is an acid. A pH of exactly 7 is neutral—neither acidic nor basic.
• 0 represents the strongest acid. Acids are usually sour, can sting if touched, and can react with or wear away metal.
• 14 represents the strongest base. Bases tend to taste bitter, feel slippery, and can dissolve fats and oils.

2. Explore everyday acids and bases.
Point out that many foods and household items we use daily are acidic or basic. Have students choose a household item (or hand them one) and guess where it falls on the pH scale. Use the reference list of common pH values below. You can guide them with hints like "hotter" or "colder" as they move closer to or further from the correct spot, or by asking questions like "Does it taste sour?"

• When a student places an item correctly, congratulate them! Explore as many items as time allows, saving the "ocean" item for last.

• When you hand the student the object representing the ocean, explain how special the ocean is, and consider offering a small prize for placing it correctly on the scale.
• Discuss how the ocean's pH has dropped over the last century: 100 years ago, the ocean's pH was 8.1; today, it's about 8.0. That may not sound like much of a difference, but consider this—
• A similar-sized shift in the human body (a drop of just 0.05 in blood pH, half as much as the ocean's change) can cause headaches, confusion, fatigue, trembling, drowsiness, and impaired brain function. This condition is called acidosis (healthy blood pH is about 7.45; acidosis occurs around 7.4). Scientists have observed similar effects in ocean fish—for example, clownfish (like Nemo) have shown confusion in more acidic water, sometimes even swimming toward predators instead of away from them.

3. Discuss the cause of changing ocean pH.
Ask students if they know what's causing this shift. Explain that carbon dioxide is an "acid gas" that makes the ocean more acidic (shown as a drop in pH). This CO2 comes from human activities such as:

• Driving cars, boats, planes, and trains
• Using electricity at home
• Cutting down forests (deforestation), since trees normally store carbon dioxide in their leaves and prevent it from reaching the ocean

Follow-Up Discussion

• Ask students: Can you think of anything you could do to help reduce the carbon dioxide released into the atmosphere—and eventually into the ocean?
• Discuss possible solutions (some community-level ideas are listed above), which work well for families and middle/high school students.
• For younger audiences, simpler individual actions may be more appropriate, such as:
• Turning off lights when leaving a room
• Unplugging electronics that draw power even when off, like computers and chargers ("vampire electronics")

Materials: Approximate pH of Everyday Household Objects

• Car battery acid: 2.2
• Vinegar: 2.8
• Coca-Cola: 2.5 (classic); up to 4.2 for other brands
• 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 6.5–8 depending on brand)
• Human blood: 7.4
• Ocean water: 8.1 historically, now about 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.