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Unit 2: The Carbon Cycle

Callan Bentley (Northern Virginia Community College)

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Students will explore the different aspects of the carbon cycle on Earth. This includes the original source of all the carbon on our planet, the near ubiquity of carbon, the six principle reservoirs of carbon in the Earth system, and the movement (flux) of carbon between reservoirs. Students will approach the chemical history of carbon by personifying the "journey" of specific carbon atoms throughout geologic time.

The unit emphasizes the grand challenges of energy resources and climate change by grounding these issues in a solid understanding of carbon from a systems thinking perspective. The point here is for students to gain a more robust appreciation for the movement of carbon between atmosphere and geosphere, between hydrosphere and biosphere. The unit provides dynamic understanding of how perturbations to one sphere or changes in the amount of carbon in a given reservoir can have implications throughout the Earth system.

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Learning Goals

  1. Students will be able to describe the ultimate source of all carbon in the universe.
  2. Students will be able to quantify movements (fluxes) associated with carbon reservoirs on Earth.
  3. Students will be able to create an illustration of Earth's carbon cycle that clearly shows and distinguishes reservoirs and movements (fluxes). 

Context for Use

This unit is designed to be applicable in an introductory level college Earth science course or environmental science course. This unit consists of classroom activities, discussions, and an out-of-class project. It would be appropriate for classes ranging in size from 5–50. In larger introductory courses, students could play in teams, or the activities could be done piecemeal during lab sections.

It requires dice (1 die per student or team), scissors, and string. It should take 50 minutes (not including the out-of-class assessment project). The activities are designed to be flexible and adaptable for use in other circumstances.

Description and Teaching Materials

The overall flow of this unit is as follows:

Pre-class preparation

Students watch a series of videos about the carbon cycle. (20 min, before the day of the lesson)
Optional: There is an activity to accompany the video which instructors may assign. (an additional 25 min before the day of the lesson)

In-class lesson components:

  1. Engagement activity: Students brainstorm where carbon is found on Earth. (5 min)
  2. The origin of carbon: Instructor presents information on nucleosynthesis (fusion) in stars. (10 min)
  3. Carbon cycle game: Students explore the concepts of reservoirs and fluxes with a game of chance. (15 min)
  4. Sizing up the reservoirs: Students quantify the size of principle carbon reservoirs in the Earth system by drawing circles. (20 min)

    During class, this comprehensive PowerPoint can help guide instructors through the sequence of activities: Comprehensive Unit 2 PowerPoint (PowerPoint 2007 (.pptx) 8.7MB Aug11 16)

Post-class assessment

  1. Homework: Students complete draw/create a graphical representation of the carbon cycle as a homework assignment. (20–60 min, after class)
  2. Summative assessment: Quiz: can be administered at the start of the next class, or as part of a larger whole-module assessment. (10 min)

All of the following activities can take place during class time, with the exception of the pre-class preparation (NPR "Climate Connections" videos) and the post-class assessment (carbon cycle diagram/Prezi).

Pre-class preparation:

Before the day of the lesson, students will be introduced to the subject via a series of short videos. In the context of climate change, the videos examine why carbon is such an important element, and who it bonds with and why. To do this, students will be assigned to watch some or all of the five videos in the "Climate Connections" series by National Geographic and National Public Radio. There is helpful information in the instructor's guide to the activity that assists instructors in deciding which of the videos to assign (link below). If instructors wish, they can use the worksheet to assign students the activity. The students should use the series' cartoon character carbon, hydrogen, and oxygen atoms to show bonding arrangements, and then to tell small stories about how bonds form or break (photosynthesis, respiration, geological carbon sequestration, and fossil fuel burning). Instructors could offer bonus points for added details to make the stories dramatic and memorable. As written, the students turn in the worksheet as a homework assignment to be collected by the teacher, but it could also be assigned to be performed in class — if there were enough time for the performance. (Time estimate: 20 min before class; 45 min total with activity option)

Link to videos on YouTube:

Episode 1 of 5 — Introduction

Episode 2 of 5 — Making Carbon Bonds

Episode 3 of 5 — Breaking Carbon Bonds

Episode 4 of 5 — How Carbon Makes Us Rich

Episode 5 of 5 — How Carbon Makes Us Warm

Student worksheet (simplified version):

Word format: "Climate Connections" video worksheet (Microsoft Word 233kB Aug11 16)

PDF format: "Climate Connections" video worksheet (Acrobat (PDF) 257kB Aug11 16)

Student worksheet (more advanced version):

Word format: Climate Connections Chemistry Worksheet (Microsoft Word 2007 (.docx) 235kB Aug11 16)

PDF format: Climate Connections Chemistry Worksheet (Acrobat (PDF) 277kB Aug11 16)

Instructor's guide to activity:

Word format: Instructor’s Guide to Climate Connections Chemistry Worksheet (Microsoft Word 2007 (.docx) 67kB Aug11 16)

PDF format: Instructor’s Guide to Climate Connections Chemistry Worksheet (Acrobat (PDF) 136kB Aug11 16)

2. Engagement activity:

Students identify the many substances in the Earth system and in everyday life that contain appreciable amounts of carbon. Discussion occurs in the "think/pair/share" format, or simply "pair/share" for the sake of a more expedited learning activity. As instructors facilitate the final large-group discussion, they can encourage students to compare and contrast some of the listed components [for instance, soda pop vs. seltzer water: only the soda contains sugar (a carbon compound), but both are carbonated with dissolved CO2]. The results of this activity can serve as a formative assessment for the instructor. (Time estimate: 5 min)

Engagement activity handout:

Word format: Engage: Where's the carbon? (Microsoft Word 2007 (.docx) 27kB Jun3 14)

PDF format: Engage: Where's the carbon? (Acrobat (PDF) 68kB Jun3 14)

Instructor's key:

Word format:

PDF format:

3. The origin of carbon:

Where does carbon come from? A brief instructional presentation on stellar nucleosynthesis — how carbon forms from the thermonuclear fusion of smaller atoms. Guiding questions address issues such as how carbon got from the middle of a star to Earth: i.e., the nebular theory of solar system formation. The presentation is available as a PowerPoint slideshow, or as a video (Quicktime or YouTube formats available). (Time estimate: 10 min)

PowerPoint presentation (Note that these slides are already included in the "Comprehensive Unit 2" PowerPoint, if you downloaded that above):

Steller nucleosynthesis of carbon by thermonuclear fusion (PowerPoint 8.1MB Aug11 16)

PDF version of slideshow:

Steller nucleosynthesis of carbon by thermonuclear fusion (Acrobat (PDF) 15.1MB Aug11 16)

presenter notes (PDF):

Presenter notes: Steller nucleosynthesis of carbon by thermonuclear fusion (Acrobat (PDF) 2.2MB Aug1 14)

Video of presentation:

Windows Media Player version to download and play on PCs: Video of Stellar C nucleosynthesis presentation ( 7.9MB Jun2 16)

YouTube video link:

The instructor may wish to invoke a short discussion at this point to emphasize that chemists and biologists (and often geologists) tend to think of atoms as eternal unchanging entities, but in reality, atoms are constructed from smaller building blocks. While thermonuclear fission is generally well appreciated as being the cause of radioactivity, thermonuclear fusion is less intuitive, given how different the interior of a star is from our everyday experience. Atoms can be created, and destroyed, and we need to allow our minds the latitude to appreciate that if we are to understand carbon.

Next up is an exploration of the major reservoirs and fluxes of carbon in the Earth system. This will be accomplished with two activities.

4. Carbon cycle game:

The first activity is a game wherein students play carbon atoms that move through the Earth system between reservoirs via various fluxes. The game builds small student-generated dramas of carbon's epic journey. In the game, students roll dice, and depending on which number comes up, they follow a path from one reservoir to another through geologic time. The six reservoirs modeled are: mantle, crust, ocean, atmosphere, vegetation & soils, and fossil fuels. Instructors introduce the game with a definition of "reservoir" and "flux." Students can play (journey between reservoirs via fluxes) as long as the instructor sees fit. The random influence of the dice will generate countless possible pathways through the carbon cycle. Students can "report out" at the conclusion of the game to tell the most interesting "twists" in their carbon atom's personal voyage through the carbon cycle. Instructors should debrief the activity by emphasizing again the idea of carbon as a system, with places of residence (reservoirs) and processes of transfer (fluxes). An optional extension would be to explore which of these fluxes occur rapidly, and which are more likely to occur at slow rates. (Time estimate: 15 min)

Pre-game terminology definitions by instructor: very brief lecture notes to define "reservoir" and "flux" prior to the start of the game. (Time estimate: 4 min)

Instruction before the Carbon Cycle game (Microsoft Word 440kB Aug11 16)

Instruction before the Carbon Cycle game (Acrobat (PDF) 459kB Aug11 16)

"Game rolls" handout

Carbon cycle dice game (Microsoft Word 201kB Jun12 17)

Carbon cycle dice game (Acrobat (PDF) 314kB Aug6 18)

5. Sizing up the reservoirs and calculating the magnitude of fluxes:

The second activity is to quantify the size of the six principle carbon reservoirs. Here, students scale a series of circles to various sizes commensurate with estimates of the size of the carbon load of that reservoir. Depending on the math abilities of the students, the instructors can opt to provide the radii of the circles to be created, or ask the students to calculate the radii from the size of the reservoir. A follow-up activity quantifies the size of the fluxes between reservoirs, and students will be able to rank the amount of change in the carbon in each reservoir as a percentage of total. This information can be used to predict the growth of the carbon proportion of the atmospheric reservoir at several hypothetical points in the future — an introduction to the crudest form of climate modeling. (Time estimate: 20 min)

"Reservoirs and fluxes" activity handout

Reservoirs and fluxes activity (Microsoft Word 149kB Aug11 16)

Reservoirs and fluxes activity (Acrobat (PDF) 206kB Aug11 16)

Instructor's copy (with answers)

If instructors want to make students do their own calculations, they can modify this Excel spreadsheet for distribution:

Area gigatonne calculation (Excel 2007 (.xlsx) 11kB Jun3 14)

If instructors want to provide the reservoir circles pre-drawn, here is a PDF showing their relative size:

Sizing Up Reservoir Graphic (Acrobat (PDF) 322kB Mar14 16)
(This is a PDF with the reservoirs drawn to scale. Be forewarned: some are tiny! To read the lower right corner more easily, remember that you can select your level of zoom on a PDF.)

The unit concludes with two assessments — see the Assessment section below.

Teaching Notes and Tips

Emphasis should be put on the temporary residence of a single given carbon atom in any reservoir, and the more or less eternal nature of the carbon atom (14C's radioactive decay aside). The same carbon that powers our bodies today was part of minerals and ancient life in the past. It has been part of the air, and the crust, and dissolved in the ocean. Ultimately, it, like everything but hydrogen, was forged in incredibly powerful reactions in the heart of long-dead stars.

The video version of the "carbon nucleosynthesis" presentation could be used as an optional pre-class assignment if instructors want more flexibility during class.

If the classroom lacks actual dice for the carbon cycle game, there is a free dice-rolling app that could substitute for students with iPads or iPhones.

For the reservoirs & fluxes activity, you will need a wide open space and a big piece of paper â€" the biggest circle is 2.6 meters in diameter! If the instructor is short on time or materials, the instructor could prepare the circles before class. Successful deployments of this activity used four sheets of butcher paper taped together (prepared in advance), unfolded cardboard shipping boxes, or the back side of old topographic maps. Alternatively, a parking lot and chalk could work, or partial circles drawn on a chalkboard or whiteboard.

Three possible extensions include:

  • Elemental abundance activity:

(optional) Expanding on the relationship between carbon in the universe and carbon in our bodies, students will engage in an elemental abundance comparison. In this activity, they will use an Excel spreadsheet of elemental abundances in the Milky Way galaxy and compare them with the elements that make up people, coming to the conclusion (through a discussion facilitated by the instructor) that people (and life in general) are anomalously enriched in carbon relative to galactic background levels. It is suggested that students work on the initial calculations in pairs or small groups (determined by number of computers available). (Time estimate: 10 min)

Elemental abundance worksheet:

Elemental abundance comparison activity (Microsoft Word 2007 (.docx) 29kB Aug11 16)

Elemental abundance comparison activity (Acrobat (PDF) 61kB Aug11 16)

Elemental abundance spreadsheet:

Elemental abundance comparison spreadsheet (Excel 2007 (.xlsx) 22kB Jun3 14) (student copy for distribution)

(instructor's copy with answers)

  • The Suess Effect

A discussion of the Suess effect (isotopically light carbon as a signature of fossil fuel sources). The atmosphere has become progressively more enriched in 14C- and 13C-depleted CO2 since the Industrial Revolution. Corals have incorporated this light carbon into their skeletal material in a measurable way. If the population of students in the class is sufficiently advanced as to read primary literature, discussing The 13C Suess effect in scleractinian corals mirror changes in the anthropogenic CO2 inventory of the surface oceans (Swart, et al., 2010) would be a good place to start.

  • Read and discuss CO2 Rising by Tyler Volk

A reading of (portions of) Tyler Volk's book CO2 Rising, and ensuing discussion. This book provides a terrific, accessible approach to the carbon cycle by chronicling the journeys of "Dave," a single carbon atom. Over time, Dave is incorporated into a limestone, the ocean, the atmosphere (and passes through the measurement apparatus at Mauna Loa), a wheat plant, a beer, and the author, Tyler Volk, who exhales Dave back into the atmosphere again. The book is recommended as background "perspective" reading for anyone contemplating teaching this unit, or any curriculum about the carbon cycle. I think students would find it engaging and useful for visualizing the complexity of the carbon cycle in the Earth system.


There are several ungraded (formative) and two graded (summative) means of assessment for this unit. (Ungraded assessments include keeping track of correct and insightful observations and statements during discussions of unit material and activities, as well as incorrect or "off-base" comments. In particular, the initial "engagement" activity should provide baseline formative assessment for the instructor.) If time and student achievement level permits, the optional graded homework assignment (#6, below) could be assigned as a summative assessment. It could also work as an in-class group activity, if time permits.

6. Post-class homework summative assignment (optional):

Students will provide their instructors a formative assessment as they create a graphical representation of the carbon cycle, as a paper poster, a PowerPoint presentation, or the dynamic, zoomable medium called a Prezi. Synthesizing material from each of the previous activities into something visual that can be understood "at a glance" will provide instructors with a "read" on whether students are assimilating unit lessons. Based on the accuracy and quality of these graphic representations (scored with a rubric provided with the activity), instructors can address any misconceptions before moving on. (Time estimate for this optional assessment: 30 min; to be assigned as out-of-class work)


Unit 2 assessment: graphic representation of the carbon cycle (Microsoft Word 2007 (.docx) 19kB Aug6 18)

Unit 2 assessment: graphic representation of the carbon cycle (Acrobat (PDF) 74kB Jun3 14)

Scoring rubric:

Unit 2 assessment grading rubric (Microsoft Word 34kB Aug11 16)

Unit 2 assessment grading rubric (Acrobat (PDF) 80kB Aug18 16)

7. Summative assessment (in-class quiz):

The summative assessment for this unit, which can also function as a "pre-test" or a part of a larger, whole-module exam, is a quiz covering key aspects of the carbon cycle. Some of these questions are basic recall, while others ask students to apply their understanding. (Time estimate: 10 min)


Instructor's key for scoring the quiz:

References and Resources

"Climate Connections" video series by National Geographic and National Public Radio, featuring Robert Krulwich with cartoons by Odd Todd.

CO2 Rising by Tyler Volk. The MIT Press (September 24, 2010) ISBN-13: 978-0262515214. An excellent overview of the carbon cycle, with an emphasis on the transformations experienced by an individual carbon atom ("Dave") that is at various points part of the oceans, a limestone, the air, a wheat plant, a beer, and the author's body, then exhaled back into the air, and so on. It also does a great job incrementally building up an understanding of pre-historic atmospheric carbon variations with a series of additive graphs. It could make an excellent book for students to read and discuss. At the very least, it is a good conceptual/philosophical background for instructors who see climate change as a big issue, but want a better grounding on the basics of carbon reservoirs and fluxes. See a more detailed review of it.

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These materials are part of a collection of classroom-tested modules and courses developed by InTeGrate. The materials engage students in understanding the earth system as it intertwines with key societal issues. The collection is freely available and ready to be adapted by undergraduate educators across a range of courses including: general education or majors courses in Earth-focused disciplines such as geoscience or environmental science, social science, engineering, and other sciences, as well as courses for interdisciplinary programs.
Explore the Collection »