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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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These materials have been reviewed for their alignment with the Next Generation Science Standards as detailed below. Visit InTeGrate and the NGSS to learn more.


In this unit, students develop both a simple model and a more complex model of the marine and terrestrial carbon cycles. They hypothesize about how the cycle will respond to perturbations, then conduct experiments to test their hypotheses.

Science and Engineering Practices

Using Mathematics and Computational Thinking: Use simple limit cases to test mathematical expressions, computer programs, algorithms, or simulations of a process or system to see if a model “makes sense” by comparing the outcomes with what is known about the real world. HS-P5.4:

Using Mathematics and Computational Thinking: Create and/or revise a computational model or simulation of a phenomenon, designed device, process, or system. HS-P5.1:

Planning and Carrying Out Investigations: Make directional hypotheses that specify what happens to a dependent variable when an independent variable is manipulated. HS-P3.5:

Obtaining, Evaluating, and Communicating Information: Critically read scientific literature adapted for classroom use to determine the central ideas or conclusions and/or to obtain scientific and/or technical information to summarize complex evidence, concepts, processes, or information presented in a text by paraphrasing them in simpler but still accurate terms. HS-P8.1:

Developing and Using Models: Develop, revise, and/or use a model based on evidence to illustrate and/or predict the relationships between systems or between components of a system HS-P2.3:

Developing and Using Models: Develop and/or use multiple types of models to provide mechanistic accounts and/or predict phenomena, and move flexibly between model types based on merits and limitations. HS-P2.4:

Developing and Using Models: Develop and/or use a model (including mathematical and computational) to generate data to support explanations, predict phenomena, analyze systems, and/or solve problems. HS-P2.6:

Developing and Using Models: Design a test of a model to ascertain its reliability. HS-P2.2:

Constructing Explanations and Designing Solutions: Apply scientific reasoning, theory, and/or models to link evidence to the claims to assess the extent to which the reasoning and data support the explanation or conclusion. HS-P6.4:

Cross Cutting Concepts

Scale, Proportion and Quantity: Time, space, and energy phenomena can be observed at various scales using models to study systems that are too large or too small. MS-C3.1:

Systems and System Models: When investigating or describing a system, the boundaries and initial conditions of the system need to be defined and their inputs and outputs analyzed and described using models. HS-C4.2:

Systems and System Models: Models can be used to predict the behavior of a system, but these predictions have limited precision and reliability due to the assumptions and approximations inherent in models. HS-C4.4:

Systems and System Models: Models (e.g., physical, mathematical, computer models) can be used to simulate systems and interactions—including energy, matter, and information flows—within and between systems at different scales. HS-C4.3:

Structure and Function: Investigating or designing new systems or structures requires a detailed examination of the properties of different materials, the structures of different components, and connections of components to reveal its function and/or solve a problem. HS-C6.1:

Stability and Change: Change and rates of change can be quantified and modeled over very short or very long periods of time. Some system changes are irreversible. HS-C7.2:

Cause and effect: Changes in systems may have various causes that may not have equal effects. HS-C2.4:

Cause and effect: Cause and effect relationships can be suggested and predicted for complex natural and human designed systems by examining what is known about smaller scale mechanisms within the system. HS-C2.2:

Energy and Matter: Changes of energy and matter in a system can be described in terms of energy and matter flows into, out of, and within that system. HS-C5.2:

Disciplinary Core Ideas

Earth’s Materials and Systems: All Earth processes are the result of energy flowing and matter cycling within and among the planet’s systems. This energy is derived from the sun and Earth’s hot interior. The energy that flows and matter that cycles produce chemical and physical changes in Earth’s materials and living organisms. MS-ESS2.A1:

Global Climate Change: Through computer simulations and other studies, important discoveries are still being made about how the ocean, the atmosphere, and the biosphere interact and are modified in response to human activities. HS-ESS3.D2:

Chemical Reactions: The fact that atoms are conserved, together with knowledge of the chemical properties of the elements involved, can be used to describe and predict chemical reactions. HS-PS1.B3:

Chemical Reactions: In many situations, a dynamic and condition-dependent balance between a reaction and the reverse reaction determines the numbers of all types of molecules present. HS-PS1.B2:

Performance Expectations

Earth's Systems: Develop a quantitative model to describe the cycling of carbon among the hydrosphere, atmosphere, geosphere, and biosphere. HS-ESS2-6:

This material was developed and reviewed through the InTeGrate curricular materials development process. This rigorous, structured process includes:

  • team-based development to ensure materials are appropriate across multiple educational settings.
  • multiple iterative reviews and feedback cycles through the course of material development with input to the authoring team from both project editors and an external assessment team.
  • real in-class testing of materials in at least 3 institutions with external review of student assessment data.
  • multiple reviews to ensure the materials meet the InTeGrate materials rubric which codifies best practices in curricular development, student assessment and pedagogic techniques.
  • review by external experts for accuracy of the science content.

This page first made public: Sep 15, 2017


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.

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:

carbon pre-lab quiz key

This file is only accessible to verified educators. If you are a teacher or faculty member and would like access to this file please enter your email address to be verified as belonging to an educator.


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:

carbon cycle exercise key

This file is only accessible to verified educators. If you are a teacher or faculty member and would like access to this file please enter your email address to be verified as belonging to an educator.

. 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 »