oceanservice.noaa.gov 26

Activity 7: Why Should I Care?

What You Will Do: Show how increased carbon dioxide makes the ocean more acidic

I

ncreased carbon dioxide in the atmosphere 

is leading to many changes that have serious 

consequences for Earth’s ecosystems, including 

humans. Climate change is causing ice sheets 

and glaciers to melt, and sea level rises as the 

melt water flows into Earth’s ocean. Seawater 

is beginning to contaminate sources of fresh 

water. Rising sea level is submerging coastal 

marshes that help protect the shore from 

severe storms, and bringing structures on land 

closer to the edge of the ocean. These changes 

make it more likely that these structures will 

be damaged by coastal storms, which are 

expected to increase as additional heat in Earth’s 

atmosphere changes the patterns of winds and 

precipitation. Heat waves and droughts are 

also expected to increase, and bring additional 

threats to human lives.

Increased atmospheric carbon dioxide is also 

causing serious changes in the chemistry of Earth’s ocean. The 

ocean absorbs about a quarter of the carbon dioxide humans 

release into the atmosphere every year, and this additional 

carbon dioxide in seawater is causing Earth’s ocean to become 

more acidic. Scientists use a measurement called pH to describe 

how acidic or basic a solution is. A pH of 7 is considered neutral. 

Acidic solutions, such as vinegar or lemon juice, have a pH less 

than 7. The more acidic a solution is, the lower the pH number. 

Basic solutions, such as milk or baking soda dissolved in water, 

have a pH greater than 7.

In addition to other impacts of global climate 

change, ocean acidification poses potentially 

serious threats to the health of Earth’s ocean 

and its ecosystems. The impact on individual 

species is expected to vary. A more acidic 

environment has a dramatic effect on 

some species that build calcium carbonate 

(limestone) shells, such as oysters. When shelled 

organisms are at risk, the entire food web 

also is at risk. For example, pteropods are an 

important food source for salmon. According 

to some research reports, a 10 percent drop 

in pteropod production could result in a 20 

percent drop in the mature body weight 

of pink salmon (Oncorhynchus gorbuscha). 

These impacts are happening right now, and 

are affecting marine food webs that provide 

important sources of food for humans as well as 

ocean species.

How It Works

Red cabbage contains chemicals that change color depending 

upon pH. These types of chemicals are called pH indicators. 

In basic solutions, these chemicals from red cabbage are light 

blue, but they change to pink-purple in acidic solutions. At the 

beginning of your demonstration, you will put a very small 

amount of baking soda into the container of tap water to be sure 

that it is slightly basic (seawater is normally slightly basic, with 

a pH of about 8.2). So, when you add the red cabbage indicator 

solution to the container of tap water, the indicator will have a 

light blue color.

What You Will Need

Adult partner

5 – Clear glass jars or plastic cups, 

about 250 ml (8 oz) capacity

1 – Graduated cylinder or measuring 

cup, about 250 ml (8 oz) capacity

1 – Measuring spoon, about 1 ml (1/4 

tsp) capacity

1 – Measuring spoon, about 5 ml (1 

tsp) capacity

1 – Small pot, about 1 liter (1 qt) 

capacity

1 – Funnel, about 7 cm (3 in) diameter

Safety glasses

Cheesecloth, about 30 cm (12 in) 

square

500 ml (2 c) finely chopped red 

cabbage

Drinking straw

Boiling water, about 500 ml (2 c)

Baking soda

White vinegar

Limacina helicina, a free-swimming planktonic snail. 

These snails, known as pteropods, form a calcium 

carbonate shell and are an important food source in 

many marine food webs. As levels of dissolved CO2 in sea 

water rise, skeletal growth rates of pteropods and other 

calcium-secreting organisms will be reduced due to the 

effects of dissolved CO2 on ocean acidity. Image credit: 

Russ Hopcroft, UAF/NOAA.

Big Idea

Climate change will have 

consequences for the Earth system 

and human lives. 

(Climate Science Principle 7)

Discover Your Changing World with NOAA
27

7. Why Should I Care?

Climate Science Principle 7. Climate change will have consequences for the Earth system and human lives.

When you exhale, the air from your lungs contains 

more carbon dioxide than the air in the atmosphere. 

Blowing through a straw into the container of tap 

water bubbles carbon dioxide through the liquid. 

Some of this carbon dioxide dissolves to form a 

weak acid (carbonic acid). When this happens, the 

red cabbage indicator changes to a pink-purple 

color, showing that the pH has changed and the 

liquid has become more acidic.

How to Do It

1. Put the chopped red cabbage into a small pot, 

and pour in about 500 ml (2 c) boiling water to 

cover the cabbage. Let the mixture rest for about 

30 minutes.

2. Fold the cheesecloth so that it is at least four 

layers thick, but still covers the opening of the 

funnel. Strain the cabbage mixture through the 

cheesecloth into one of the glass jars or plastic 

cups. You will have some liquid left over in the pot. 

Save this in case you need it later.

3. Pour about 50 ml (1/4 c) of tap water into another 

glass jar or plastic cup. Add 5 ml (1 tsp) of the red 

cabbage indicator solution to the jar. The solution 

should have a pale blue color. The pH of tap water 

varies from place to place, so if the solution is 

not pale blue, add a pinch of baking soda, and 

gently swirl the container so that the baking soda 

dissolves. Repeat if necessary until the solution 

has a pale blue color.

4. Pour about 50 ml (1/4 c) of tap water into another 

glass jar or plastic cup. Add 5 ml (1 tsp) of the red 

cabbage solution to the jar, then add 5 ml (1 tsp) 

of white vinegar. The solution should have a pinkpurple color.

5. Put about 1 ml (1/4 tsp) of baking soda into 

another glass jar or plastic cup, fill the container 

with tap water, and gently swirl the container so 

that the baking soda dissolves.

6. Pour about 50 ml (1/4 c) of tap water into the 

last glass jar or plastic cup. Add 1 ml (1/4 tsp) of 

the baking soda solution from Step 5 solution to 

the jar, then add 5 ml (1 tsp) of the red cabbage 

solution to the jar. The solution should have a pale 

blue color.

7. Put on a pair of safety glasses. Blow gently 

through the straw into the solution prepared in 

Step 6. Keep blowing for several minutes, until the 

color of the solution changes from pale blue to 

pink-purple. You have just shown how dissolved 

carbon dioxide can make a solution more acidic! 

This is what is happening to Earth’s ocean because 

of carbon dioxide that has been added to Earth’s 

atmosphere by human activities.

Want To Do More?

This Web page has more information about ocean 

acidification: http://www.pmel.noaa.gov/co2/story/

What+is+Ocean+Acidification%3F. Images on this page 

show what happens to a pteropod (pronounced 

“TARE-oh-pod”) shell when it is exposed to seawater 

with a pH that is lower than normal. After 45 days, 

the shell slowly dissolves. Earth’s ocean is not this 

acidic yet, but is predicted become this acidic by the 

year 2100 if humans continue the present pattern of 

adding carbon dioxide to the atmosphere.

For more information about ocean acidification, 

check out these slide shows and videos:

http://coralreef.noaa.gov/education/oa/presentationvideos.html

Left: Red cabbage solution; 

Right: Blue cabbage solution