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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 materials are free 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.
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Unit 9: Carbon Cycle and Ocean Chemistry

David Bice, Department of Geosciences, Penn State
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Initial Publication Date: September 15, 2017 | Reviewed: March 2, 2023


In this module, students first review some background material on the terrestrial, marine, and anthropogenic processes involved in the storage and transfer of carbon in the Earth system. The students then build a simple carbon cycle of their own in STELLA and after a few experiments with the basic version, they download a more complex version that includes a fairly complete representation of the carbonate chemistry of seawater, and is coupled to a simple climate model.

In working with their numerical model of the carbon cycle, students learn about the consequences of highly variable residence times on the behavior of a system. They also see how, by comparing the model output with the historical record of CO2, we gain confidence in the model's ability to the point where we can do some meaningful modeling with IPCC emissions scenarios for future carbon emissions.

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Learning Goals

On completing this module, students are expected to be able to:

  • Explain the different components and processes of the marine and terrestrial carbon cycle.
  • Explain how the carbonate chemistry of seawater dictates how much CO2 can be absorbed by the oceans, and how it determines the pH.
  • Explain variations in atmospheric CO2 as a consequence of carbon cycle processes.
  • Evaluate the residence times within the carbon cycle and explain what these mean in terms of the behavior of the system.
  • Apply systems thinking concepts to develop hypotheses about how the carbon cycle will respond to a range of changes.
  • Explain how human activities affect the carbon cycle.
  • Evaluate the model's performance by comparing calculations with the actual historical record.
  • Use a model to predict how the amount and rate of future carbon emissions will impact the climate and the chemistry of the oceans.

This module addresses several of the guiding principles of the InTeGrate program. In particular, it helps develop their systems thinking toolbox, develops students' abilities to use numerical modeling to generate and test geoscientific hypotheses, uses data on the history of carbon emissions and the history of atmospheric CO2 in order to test the capabilities of the carbon cycle model, and addresses a grand challenge facing society by introducing students to a tool for exploring the climatological consequences of future carbon emissions scenarios.

Context for Use

We intend this module to be used in a three- to four-hour class period that meets once a week (or two shorter periods in the same week). It can be used as part of this modeling course or it can be adapted as a lab exercise for a course in biogeochemistry or climate change. We assume that the students will have a basic understanding of biology and chemistry, although there is background reading material that covers the biology and chemistry of relevant processes. For this module, students should come to class prepared to take a short quiz on the assigned reading. Thereafter they will be led through a series of prompts designed to help them create and experiment with a number of simple models using the iconographic box modeling software STELLA (see for different options for purchasing student or computer lab licenses of STELLA or for downloading a trial version). After building and working with a simple model, the students download and experiment with a more complex version of the model that includes the carbonate chemistry needed to calculate ocean pH and the oceanic pCO2. This more sophisticated model can be "validated" against the historical record and then used to run a range of IPCC emissions scenarios for the future.

For those learning to use STELLA, we suggest the online "play-along" tutorials from isee systems. You can find them here: isee Systems Tutorials.

Description and Teaching Materials

In preparation for the exercise, students should read the following rather lengthy overview: Unit 9 Student Reading

Students should take the following quiz prior to coming to class to ensure they have done the assigned reading:carbon pre-lab quiz (Microsoft Word 2007 (.docx) 47kB Aug11 16). The instructor's key to the quiz is here:


In class, students should be provided with the exercise found here: carbon cycle exercise (Microsoft Word 2007 (.docx) 580kB Jan12 17).

An answer key for the exercise can be found here:

. It contains answers to the different questions, strategies instructors can use to guide students through the exercise, and information on typical stumbling blocks.

Instructors can download the simple carbon cycle model by clicking on this link: Simple Carbon Cycle (Stella Model (v10 .stmx) 15kB Aug11 16) . The model was created using STELLA Professional. If you are using an earlier version of STELLA, the complete model graphic and equations can be found in the answer key so that you can reconstruct the model yourself. The second part of this exercise deals with a more complex version of the carbon cycle model that is coupled to a simple climate model Complex Carbon Cycle Model (Stella Model (v10 .stmx) 67kB Aug11 16).

Teaching Notes and Tips

We generally post the readings and assignments for students to an LMS site (e.g. Moodle, Blackboard, Canvas). Students can open the assignment in Microsoft Word on the same computer they are using to construct the STELLA model and then answer the questions by typing directly into the document. Students can then either print a paper copy to hand in to the instructor or email their modified file to the instructor. It is straightforward to copy graphs and model graphics out of STELLA and to paste them into Word. Simply select the items to be copied, hit copy in STELLA, and paste into Word. There is no need to export graphics to jpg.

We teach the course in a three- to four-hour block once a week because we have found that models require a lot of uninterrupted time to construct. If students have a 50- or 75-minute class period several times a week, they spend time trying to figure out where they left off, making this inefficient. However, we also know that sustaining attention for this length of time can be difficult. We therefore recommend allowing students the freedom to take breaks throughout the modeling session to get snacks or coffee.

A typical 4-hour class session might be broken up into the following sections:

  • 20-minute discussion of the reading to ensure all the students are familiar with the construction of the system.
  • 0.5 hr to build the simple carbon cycle model to hour to build the model
  • 0.5 hr to conduct experiments with the simple model
  • 0.5 hr to download, study, and test the more sophisticated carbon cycle model
  • 1.5+ hrs to carry out the experiments with the more sophisticated carbon cycle model

For instructors who have more limited contact hours with their students, we suggest that the model construction parts of this exercise be assigned as a pre-lab to be handed in a day or two before class along with the completed STELLA model itself. This would allow the instructor to determine whether students' models are working correctly and to provide feedback to address errors in construction that might lead to spurious model behavior. Class time could then be devoted to running experiments and analyzing the results.


Answers to exercise questions are located in the answer keys for this unit (see Description and Teaching Materials section above). Instructors may download an assessment rubric for the modeling exercise here: Assessment rubric (Microsoft Word 2007 (.docx) 121kB Jan8 15). Rather than assign a point value to every question in the exercise, we employ a holistic approach that determines the extent to which a student has correctly built the model, supplied appropriate documentation of equations and units, thoroughly answered questions throughout the assignment, and provided appropriately labeled graphs and figures in answering questions.

References and Resources

The background reading material included with this module are extensive enough that additional outside readings are not recommended, although there is of course an extensive literature on modeling the global carbon cycle. Parts of the model developed here are based on work by:

Broecker, W.S., and Peng, H.-S., 1993, Greenhouse Puzzles, New York, Eldigio Press, 251 p.

Kwon, O.-Y., and Schnoor, J.L., 1994, "Simple global carbon model: the atmosphere-terrestrial biosphere - ocean interaction," Global Biogeochemical Cycles, v. 8, p. 295–305.

Siegenthaler, U., and Sarmiento, J.L., 1993, "Atmospheric carbon dioxide and the ocean," Nature, v. 365, p. 119–125.

Walker, J.C.G., 1991, Numerical Adventures with Geochemical Cycles, Oxford, Oxford University Press, 192 p.

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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 »