### Unbalancing the carbon cycle

Purpose: After completing this activity, students will better understand where carbon is stored in the Earth, how carbon moves through Earth’s various spheres and how humans are impacting that carbon flow.

Procedural overview: Students will answer questions related to the Science News article “Here’s where Earth stores its carbon” (Readability: 15.1) and then explore the carbon cycle as a class. Finally, students will graph historical atmospheric carbon dioxide levels and temperature anomalies to understand how humans are affecting the carbon cycle.

Approximate class time: 1 class period to complete the activity questions, calculations and graphing.

Supplies:
Unbalancing the Carbon Cycle student activity guide
Calculators
Graph paper or computers with graphing software
Online access to research information about the carbon cycle and impacts of carbon dioxide in the atmosphere
A projector for introducing the activity (optional)

Directions for teachers:

Carbon storage
For the first part of this activity, have students read the Science News article “Here’s where Earth stores its carbon” and answer the questions that follow about where Earth’s carbon is found.

1. Based on the Science News article “Here’s where Earth stores its carbon,” where is the majority of carbon on Earth stored? How much is stored there?

The majority of carbon on Earth is stored inside the planet in the lower mantle — about 1,500,000,000 billion metric tons.

2. How does the amount of carbon stored underneath the surface compare to that above the surface? Round to the correct number of significant figures.

Underneath the surface: 1,845,000,000 billion metric tons

Above the surface: 43,490 billion metric tons

(1,845,000,000 billion metric tons)/(43,490 billion metric tons) = 42,420

There is about 42,420 times more carbon underneath the surface of the Earth than there is above the surface of the Earth.

3. Above the surface, where is most of the carbon stored? Where is the least amount of carbon stored?

Above the surface, most of the carbon (37,000 billion metric tons) is stored in the deep ocean. The least amount of carbon (about 590 billion metric tons) is stored in the atmosphere.

4. What percentage of above-surface carbon is found in the atmosphere?

Total amount of above-surface carbon = 43,490 billion metric tons

Amount of above-surface carbon in the atmosphere = 590 billion metric tons

Percentage of above-surface carbon in the atmosphere =

590 billion metric tons/43,490 billion metric tons x 100% = 1.4%

About 1.4% of the above-surface carbon is found in the atmosphere.

5. How does carbon naturally enter the atmosphere?

Carbon can naturally enter the atmosphere through volcanic eruptions and impacts from asteroids, and from mid-ocean ridges, naturally seeping through volcanic vents, soil and bodies of water.

6. What is the largest contributor to the amount of carbon in the atmosphere today? How much carbon does this contributor add to the atmosphere yearly?

Humans are the largest contributor of carbon to the atmosphere, contributing about 10 billion metric tons of carbon into the atmosphere each year.

Carbon cycle
For the second part of this activity, discuss the general information about the carbon cycle that follows, and then have students answer the questions as a class or in small groups.

Explain to the class that carbon is stored in all of Earth’s spheres. Life-forms on Earth are “carbon-based,” meaning carbon is a key ingredient in our cells. Carbon dioxide is a greenhouse gas that helps the planet retain heat, making life on Earth possible. Carbon can even be found dissolved in water. Carbon moves among Earth’s spheres through a variety of methods as part of the carbon cycle.

7. Give an example of how carbon shows up in the biosphere, the geosphere, the hydrosphere and the atmosphere?

Biosphere: Animals (including humans) and plants, as well as all other known life on Earth, are carbon-based.

Geosphere: Carbon can be found in rocks on the surface, the continental and oceanic crust and in the mantle.

Hydrosphere: Carbon is dissolved in the ocean.

Atmosphere: Carbon is found in the form of carbon dioxide in the atmosphere.

8. Carbon is the basis for life on Earth — people are 18 percent carbon and plants are 45 percent carbon, for example. The element is considered key to Earth’s habitability. What are three examples that show how important carbon is for life on Earth?

Answers may include: Plants take in carbon dioxide to make food and release oxygen for humans and other animals to breathe. Carbon dioxide in the atmosphere allows temperatures on Earth to be at the right range for life to flourish. Carbon forms the center of amino acids, which are the building blocks of proteins and are found in all living things. Carbohydrates and lipids also require carbon. Carbon is a key element in seashells, which protect the sea life that lives inside them. Carbon forms nearly 10 million compounds due to its naturally high ability to bond, and many of these compounds are used for fuel, pharmaceuticals and in industry.

9. How can carbon in the geosphere enter the biosphere? And how does carbon move from the biosphere back into the geosphere?

Geosphere to biosphere: The weathering or erosion of rocks or events such as volcanic eruptions allow carbon to move from rock to soil where it is absorbed by plants, which are then eaten by humans and other animals. Also, the carbon from rocks can end up in the ocean, where it can be used by photosynthetic plankton or to form new seashells.

Biosphere to geosphere: Dead organic material gets buried and pushed deep into the Earth where it can be incorporated into rocks and, through heat and pressure over time, become fossil fuels such as coal and oil.

10. The carbon cycle shows how carbon moves through all four spheres of Earth. Sketch some of the interactions that connect the movement of carbon through the spheres. Describe and/or label the interactions.

Answers should include a quick sketch that may be similar to the one shown in the Visual Answer Key for Teachers and should include examples of how carbon moves across each of the four spheres.

11. What does it mean when we say that the carbon cycle is a closed cycle?

Though carbon can move between Earth’s spheres, the total amount of carbon in all four spheres remains constant.

Humans and the carbon cycle
For the third part of this activity, have students consider the effects that human-released carbon dioxide is having on the planet.

12. If the carbon cycle is closed, where is the additional carbon dioxide that is entering the atmosphere coming from?

Carbon is being taken out of the geosphere, hydrosphere and biosphere and being put into the atmosphere in the form of carbon dioxide.

13. How have human activities increased the amount of carbon dioxide in the atmosphere?

The burning of fossil fuels for any number of activities — such as energy use, driving, flying and factory production — have increased the amount of carbon dioxide being added to the atmosphere. Additionally, deforestation has reduced the amount of carbon dioxide being removed from the atmosphere by trees.

14.If carbon is vital for life, why is increasing the amount of carbon dioxide in the atmosphere problematic?

Carbon dioxide is a greenhouse gas, and greenhouse gases can trap heat near Earth’s surface. This is good up to a point, but humans have evolved and adapted to a certain temperature range. More trapped heat means higher temperatures. Too much warming could dramatically alter our lifestyles and the lifestyles of other species.

15. Describe three potential outcomes of increased carbon dioxide in the atmosphere.

More carbon dioxide in the atmosphere leads to increases in global temperatures. Higher temperatures can lead to more ground-level ozone and more allergens in the air, which can affect human breathing. If there is more carbon dioxide in the atmosphere, the ocean can absorb that carbon dioxide to offset the atmospheric increase, which can alter the acidity of the water and can impact marine life.

Data analysis and graphing
Explain to your students that scientists agree that humans are causing climate change. Carbon dioxide in the atmosphere has increased since the start of the Industrial Revolution, and rising temperatures have tracked the increase in carbon dioxide.

The table below shows the amount of carbon dioxide in the atmosphere from 1750 (about 10 years before the start of the Industrial Revolution) to 2019. It also shows the temperature anomalies for each of those years — that is, how much the temperature differed from the baseline, or average temperature, for a period of time (in this case, the average global temperature between the years 1000 and 2019). Help students understand what a temperature anomaly is by reviewing the table and questions 16 to 18 together. Then ask students to create their own graphs and answer the remaining questions.

Atmospheric CO2 and Temperature Over Time
Sources: www.co2levels.org and www.temperaturerecord.org

 Year Atmospheric CO2 level (pp,m) Temperature anomaly (degrees C) 1750 227 -0.3 1760 277.6 -0.16 1770 278.6 -0.31 1780 280.1 -0.31 1790 281.6 -0.3 1800 282.9 -0.28 1810 283.8 -0.26 1820 284.2 -0.49 1830 284.4 -0.28 1840 283.4 -0.35 1850 284.7 -0.31 1860 286.2 -0.11 1870 287.5 -0.06 1880 290.7 -0.21 1890 294.2 -0.25 1900 295.8 -0.13 1910 299.7 -0.39 1920 303 -0.31 1930 307.2 -0.22 1940 310.4 0.12 1950 310.7 -0.09 1960 319.4 -0.05 1970 325.7 -0.03 1980 337.8 0.22 1990 356.7 0.39 2000 372 0.4 2010 391.2 0.69 2019 414.2 0.92

16. A temperature anomaly is the difference between the current recorded temperature and a long-term average temperature, known as a baseline temperature. If the recorded temperature in a room is 25° C but the baseline temperature for that room is 20° C, what is the temperature anomaly in that room?

recorded temperature – baseline temperature = temperature anomaly

25° C – 20° C = 5 degrees C

The temperature anomaly in the room is 5 degrees Celsius.

17. What does a positive temperature anomaly indicate? What does a negative temperature anomaly indicate?

A positive value indicates that the recorded temperature is greater than the baseline, and a negative value indicates that the recorded temperature is lower than the baseline.

18. What does a temperature anomaly of zero indicate?

An anomaly of zero indicates that the recorded temperature was equal to the average temperature.

19. What trend do you notice about the numbers in the atmospheric carbon dioxide column of the table? What does this trend indicate?

The numbers in this column increase as time passes. This indicates that decade after decade there is more carbon in the atmosphere.

20. Graph the data presented in the table. Graph the atmospheric carbon dioxide levels on one graph and the temperature anomalies on another. Refer to your teacher’s instructions for where to create your graphs.

See Visual Answer Key for Teachers for example graphs.

The graphs indicate that both the amount of CO2 in the atmosphere and the temperature anomalies are increasing. The amount of atmospheric CO2 seems to increase fairly smoothly and is rising faster and faster each decade. The temperature anomalies show more variability, yet they also follow an upward trend.

22. Does the line for the temperature anomalies cross the x-axis? What does this indicate?

The line for temperature anomalies crosses from below the x-axis on the left to above the x-axis on the right. This means that the anomalies went from negative values to positive values. In 1930 and before, the temperatures were always below the average temperature, and in 1980 and after, they have always been above the average temperature.

23. If the trend keeps going in the same pattern, what might atmospheric CO2 levels look like in 10 years? 50 years?

Since the graph grows steeper as the decades pass, the levels will be much higher than they are today. They will be exponentially higher in 50 years compared with 10 years.

24. If the trend keeps going in the same pattern, what might the temperature anomalies look like in 10 years? 50 years?

The overall temperature anomaly trend is toward higher temperatures each decade, so the temperature anomaly will be higher in 10 years and then much higher in 50 years.

25. Discuss some impacts on Earth’s spheres that may occur based on your understanding of the carbon cycle and on the carbon dioxide trend you observe in your graph.

The hydrosphere may be affected as the ocean absorbs more carbon from the atmosphere, which can increase its temperature and lower its pH. The biosphere may be affected as animals and other organisms shift their ranges (and humans move their homes) to deal with temperature changes and rising seas. The geosphere locks carbon away over long time periods and is thus less susceptible to short-term changes in atmospheric carbon dioxide, but humans are releasing that locked away carbon into the atmosphere by burning fossil fuels.

26. What can people do to limit the amount of CO2 released into the atmosphere?

People can make changes to their lives to decrease the amount of carbon dioxide they are adding to the atmosphere. Using fewer fossil fuels and getting most of our energy from solar, wind or other renewable sources would help decrease the amount of CO2 added to the atmosphere. People could use travel methods that don’t have such a large carbon footprint, such as by foot, bike, public transportation or electric car charged by solar power.

27. Some estimates suggest that reducing carbon dioxide entering the atmosphere will not be enough to curb the effects of global warming — people will also have to remove some of the carbon dioxide they have already pumped into the atmosphere. Based on your understanding of the carbon cycle, what possible approaches could remove carbon dioxide from the atmosphere?

People could find a way to capture carbon dioxide released by power plants and store that carbon in the geosphere. People could limit deforestation and plant more trees, so those trees store more carbon in the biosphere. Seeding the oceans with iron could encourage the growth of organisms that would absorb carbon dioxide from the atmosphere.