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Unit 3: The Interconnected Nature of the Atmosphere, Hydrosphere, and Biosphere

Sam Donovan (University of Pittsburgh), with contributions from LeeAnna Chapman (North Carolina State University), Cailin Huyck Orr (SERC), Camille Holmgren (SUNY Buffalo State), and Rebecca Teed (Wright State University)
Initial Publication Date: April 20, 2017 | Reviewed: May 10, 2019


Using a systems dynamics approach, students will work in groups to conceptualize and construct a model of the global carbon cycle considering five major Earth systems: atmosphere, hydrosphere, geosphere, cryosphere, and biosphere. The models will draw on information from the pre-class activity and invoke system features such as boundaries, stocks, flows, and control variables. Using a scenario describing a global, catastrophic event, the students will consider how new conditions change the behavior of carbon cycling in their model world. Students will use the model to explain changes in environmental variables such as permafrost cover, atmospheric gases, and global temperature, as well as feedbacks within the system.

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Learning Goals

Unit 3 Learning Outcomes

Students will:

  1. Identify important processes in the global carbon cycle.
  2. Discuss how changes in the tundra biome would influence the carbon cycle.
  3. Create a systems diagram to illustrate reservoirs, feedbacks, and fluxes within a system composed of the atmosphere, biosphere, and permafrost.
  4. Modify the systems diagram to account for perturbations in the system resulting from a global catastrophic event.

Overarching Module Goals

  • Interdisciplinary and system-based problem-solving: Students will explain and interpret the global carbon cycle using systems thinking concepts including boundaries, inputs and outputs, drivers, and feedbacks.

Context for Use

This unit can be used as a stand-alone lesson or within the broader context of the Changing Biosphere Module. This unit could fit into Earth science, environmental science, physical geography, or environmental biology courses. It will be helpful if students have basic understanding of processes that influence carbon cycling on Earth such as photosynthesis, respiration, and dissolution in water.

The unit is designed to provide a basic introduction to system dynamics and systems thinking in the context of an Earth system model. These concepts should be transferable to other types of systems, so the unit could work as an introduction to the consideration of other Earth cycles (water, energy, elements, etc.). It could also precede a computational modeling exercise where the conceptual model serves as the base for a numeric model.

The unit was developed with the expectation that there would be peer interactions and real-time instructor support for the lesson. This could be achieved in a distance-learning environment with screen sharing and a computer-based drawing program, but will still require real-time interaction between students and with the instructor.

Description and Teaching Materials

Materials for this unit include a pre-class assignment with a video, reading, and questions, a PowerPoint that steps through the classroom activity, and a handout describing a scenario with discussion questions. The PowerPoint slides are meant to be used to guide the classroom activity, with large breaks for students to work together in groups between slides.

Pre-class Assignment

This unit begins with a short (~30 min.) pre-class assignment designed to orient students to the tundra biome, characteristics of permafrost, and its role in the global carbon cycle. The assignment is to watch a short video, complete a one-page reading assignment and respond to several questions. You may decide to collect the questions for points.

In-class Materials

Before class, instructors should prepare for the group activity by deciding how students will be split into groups of ~4 members and providing drawing materials for each group. Dividing into groups should be done quickly at the beginning of the period. Students need to sit together so they can all see and reach the drawing materials. In a face-to-face classroom, this could include a large piece of paper and markers or a large area of a chalkboard or whiteboard. In a distance learning environment, this could be done through a shared screen and any drawing program that will allow students to make and label circles and arrows and edit them quickly. However, students will also need to be able to see lecture slides. You may want to structure the room so students can work without being able to see other groups' work, and then periodically share their work with each other or the class to gain feedback. This approach would be particularly helpful in a large class setting where interaction with the instructor is limited.

Systems thinking and the modeling activity (5 min)

Introduce students to the goals of the unit and outline the activity. Explain the idea of a system and why it is useful to think about Earth as a system, that is, a series of interconnected parts.

Slides 1–4 Setting the context for the day

These slides contain the learning objectives, an outline of activities for the period, and additional information about carbon storage in permafrost. Emphasize that students will work together to build their understanding and that the goals are related to understanding and using the process or framework.

Slides 5–6 Introduction to systems thinking

Introduce the idea of a system and why it is valuable to learn to use system thinking.

How to build a model (10 min)

Glossary for System Thinking (Microsoft Word 28kB Jan13 17)

In this section you will provide the tools and context students need to build their own model. Make sure to present the idea of a model as a simplified representation of the real world. A glossary of systems terminology is provided above and may be useful for students new to systems thinking. In this activity, students will introduce and use a set of symbols for general model components. These components can be used to represent any system that has stocks, flows, and controlling variables. You can use the example of a "book cycle" to help reinforce the idea that anything with stocks and flows can be part of a system and to guide students to begin brainstorming how outside forcing could influence system dynamics.

Slides 7–8 Model components

These slides introduce the basic components of a stock and flow model using components that are used in systems dynamics models. The slides explain the building blocks of a model and show a simple example.

Slides 9–10

These slides contain an oversimplified example of an everyday system that you can use to reinforce the ideas of reservoirs, fluxes, and controlling variables. Encourage students to think about what might control the flow of books between the different reservoirs, and how extreme conditions might change the size of the fluxes and the resulting size of the reservoirs. This could be structured as a whole-class discussion, or students could discuss with a partner (for example a think pair share ) and then you could mediate a whole-class report out session. The goal of the discussion is to ensure students understand how reservoirs and flows work in a conceptual model before moving on to building their own model.

Building a carbon cycling system model (20 min)

Working in groups, students step through building their own conceptual model by starting with major reservoirs, adding the the direction of carbon fluxes between the reservoirs and naming those fluxes, and finally adding additional variables that influence the rate of flux and the direction of that influence. Resulting models from this exercise will vary in their components and complexity.

Slides 11–16 Building a conceptual model

Students are stepped through working together to construct a conceptual model of the carbon cycle on Earth based on their reading and prior knowledge. The resulting models will vary by group, but will have common elements important to the exercise. You may want to review the models or have groups compare their work to catch major missing variables before moving on to the next step.

Slides 17–19 Model behavior

Students will add variables that influence the rates of fluxes between reservoirs. They can refer to what they learned in their pre-class work and the lecture slides. Encourage group discussion about how these variables should be added to the model and what the resulting model behavior would be.

Applying systems thinking to a new scenario (15 min)

Slides 23–24 A new scenario

Ask students to continue to work in their groups and use their model to consider changes to the carbon cycle immediately following a fictional, catastrophic perturbation to the system such as a meteor impact or massive volcanic explosion, similar to events discussed in Unit 2. First students will identify how the influencing variables they added to the model would change following the event. Students will also determine the direction these changes would have on fluxes and will use the model to trace the overall impact on carbon fluxes between the reservoirs. Students will use their model to justify their answers.

Teaching Notes and Tips

Additional notes, resources and discussion prompts are included in the notes section of the lecture slides.

This activity is structured to build in complexity and understanding over the class period. The introduction lecture is intentionally short and provides just enough information for students to explore model building and behavior in groups. Students do not need to know a great deal about carbon cycling, the Earth system or modeling prior to starting the activity. Most of the class period should be focused on students working together to construct a model that fits their collective understanding of the system, and practicing using the model to explain their reasoning. Notes from Unit 1 on handling discussion are relevant here also.

The materials and activities in this unit address both a challenging topic (understanding characteristics of permafrost and tundra) and a potentially novel way of reasoning (using a systems perspective to diagram complex relationships). Students may not be comfortable making conjectures about the relationships between the system components or interpreting how changes may influence the system behavior. It is important to emphasize that a model is a tool that can be useful for organizing our understanding, but it is always an incomplete representation of the "real world" and there are no perfect, right answers. You can emphasize the utility of using a systems approach to characterize and study complex sets of interdependencies and to understand the ways that seemingly disparate factors can, in fact, have an influence on other parts of a complex system.

The approach presented in this activity could be thought of as the first, or conceptualization, stage of building a system dynamics model. The key steps to this stage are all represented in this activity: defining the purpose of the model, defining boundaries and variables, describing behavior, and diagramming basic mechanisms or model behavior. Conceptual models are used for a variety of purposes including building common understanding, anticipating surprises in a system, and as a basis for numeric modeling. More information about using system dynamics with students is available in the references and resources section.


This unit has two assessment opportunities: a pre-class worksheet and an in-class activity, which could both be graded. The in-class activity could be completed as a group or individual activity.

References and Resources

There are a variety of existing SERC resources that address systems thinking and using an Earth systems approach including:

The National Snow and Ice Data Center (NSDIC) has a wonderful collection of educational and research resources including:

There are also resources on building and using models and system dynamics models:

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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 »