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Unit 1.3 - Systems Models

Timothy White (Pennsylvania State University)
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  1. 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.

  2. This activity was selected for the On the Cutting Edge Reviewed Teaching Collection

    This activity has received positive reviews in a peer review process involving five review categories. The five categories included in the process are

    • Scientific Accuracy
    • Alignment of Learning Goals, Activities, and Assessments
    • Pedagogic Effectiveness
    • Robustness (usability and dependability of all components)
    • Completeness of the ActivitySheet web page

    For more information about the peer review process itself, please see

This page first made public: May 15, 2017


The term "Earth system science" is typically used to describe the science (especially quantitative modeling) of the interactions between the atmosphere, hydrosphere, and cryosphere, and biosphere---the addition of lithosphere to that list provides all of the main generalized components ("spheres") of the Critical Zone.

In this lesson, students will consider basic concepts of system science (studying complex systems), specifically as it can be applied to Critical Zone science. Students will engage in developing a qualitative systems model graphic of the Critical Zone. The knowledge gained here will be applied later in the semester to more in-depth systems thinking of the Critical Zone.

Learning Goals

By the end of this lesson students should be able to:

  • Define the term "system" as it pertains to the natural world, and describe the difference between quantitative and qualitative system modeling.
  • Design a qualitative Critical Zone system model.
  • Explain how (and when) humans have altered global erosion rates.
  • Recognize some of the consequences of human domination of ecosystems.
  • Discuss how human-induced climate change is expected to alter the hydrologic cycle.
  • Describe what global-scale, human-induced changes can be observed in soils, the role of agriculture in these changes, and some of the consequences of changes to our soils.
  • Formulate and evaluate any adaptive actions humanity can take to lessen negative impacts to the Critical Zone and soils.

Context for Use

In this single 75-minute class, students will be introduced to some of the basic concepts of system modeling---the interdisciplinary study of the use of models to conceptualize and construct systems---in this case, the complex environmental system named the Critical Zone. They will accomplish this through a series of readings and an activity. The readings will introduce students to the basics of system modelling and some of the issues associated with human impacts to the biosphere, atmosphere, hydrosphere, lithosphere, and soil (i.e., the Critical Zone), a topic that will be explored in more detail later in the semester. The final task for the module is to create a qualitative Critical Zone system model.

Description and Teaching Materials

Earth Systems

Systems (in this case, the Critical Zone) consist of components (the "spheres" described earlier in this course). System components are not isolated, but instead typically interact in complex ways; systems may also interact with other systems. These interactions, or linkages, are called couplings in Earth system science vocabulary.

  • Positive couplings mean a change in one component, whether positive or negative, causes a change in the same direction in a linked component, whereas negative couplings mean the linked component undergoes change in the opposite direction.
  • Feedback loops are circuits of change and response to change: negative feedback loops typically diminish the effects of change, whereas positive feedback loops usually amplify the change.

The state of a system is described using the characteristics of the system at a particular time. Changes to the state of a system are caused by:

  1. Interactions between other systems
  2. Interactions among the components within a system

An equilibrium state will not change unless the system is disturbed. Temporary disturbances to a system are called perturbations, whereas persistent disturbance is called forcing. When slight disturbances carry a system further from equilibrium it is said to be an unstable system.

Examples of Simple Earth System Diagrams

Learning Goal: Introduction to the panoply of human interactions and impacts with the various components ("spheres") of the Critical Zone, and preparation for an in-class discussion of this information.

Before Class

  • Pre-class Reading (everyone):
    • Brantley, S. L., Goldhaber, M. B., & Ragnarsdottir, K. V. (2007). Crossing disciplines and scales to understand the critical zone. Elements, 3(5), 307–14. Focus on how the Critical Zone acts like a system. (Primary link is at: Elements Magazine). Alternative source:
    • Mann, C. (2008). Our Good Earth. National Geographic, 214(3), 80-106. Notice how excessive and adaptive actions by human society can lead to soil compaction, erosion, and loss of nutrients and organic matter. This article provides some good general information to consider regarding what human actions might be included in the Critical Zone system model to be assigned at the end of this class. (Link may require a FREE registration on the NGM page; Note: this is a link to their NEW beta site so may only be temporary.)
  • Pre-class Reading (group specific, see below. Class should be broken into 5 reading groups before this class.):
    1. Atmosphere and hydrosphere (climate and hydrology):
      • IPCC, 2008, Climate Change and Water (7 Mb, slow load)
      • Read the executive summary and browse the rest of this important report from the Intergovernmental Panel on Climate Change.
    2. Lithosphere and erosion:
    3. Biosphere:
    4. Soil:
    5. Soils—The Final Frontier"
      • Sugden, A., Stone, R., & Ash, C. (2004). Ecology in the Underworld. Science, 304(5677), 1613–1615. See also the interactive version of the map from that article.
      • Kaiser, J. (2004). Wounding Earth's Fragile Skin. Science, 304(5677), 1616–1618.
      • Stokstad, E. (2004). Defrosting the Carbon Freezer of the North. Science, 304(5677), 1618–1620.
      • McNeill, J. R., & Winiwarter, V. (2004). Breaking the Sod: Humankind, History, and Soil. Science, 304(5677), 1627–1629.

During Class (75 min total)

  • Group Reporting Instructions (5x 5-6 min=30 min): Each group should come to class prepared to present a 5-minute synopsis of their reading focused on key issues associated with human interaction and impact in their group's "sphere" of the CZ - their presentation and in-class discussion should address those unit learning goals that are relevant to their disciplinary focus (based on reading groups) so that all of the students leave the discussion with the knowledge required to address all of the unit learning goals.
    • Other questions the groups should address or be ready to discuss include:
      • What are the main ideas in the documents?
      • What research techniques were used?
      • What kinds of models or assumptions were applied to this research?
  • Class Discussion (~25 min) - The Critical Zone as a systems model, and human impacts and threats.
  • Lecture (~20 min) - Description of systems modeling
    • Based on: Few (1986) System Behavior and System Modeling pages 1–19.
    • Consider using this Systems Modeling presentation (PowerPoint 2007 (.pptx) 663kB Dec23 16) to guide your lecture.

Homework Assignment

Each individual student should produce a detailed drawing of a simple Critical Zone system diagram. Due next class.

Teaching Notes and Tips

The final goal of this module is to create a qualitative Critical Zone model, recognizing that the Critical Zone is a very complex system with natural and human components.

Students should be pre-assigned readings based on their placement within five reading groups. The groups will come to class prepared to present a five-minute summary of their readings including the use of terms like positive and negative couplings or feedbacks, forcing, and perturbation. To get the students to focus, it is good to limit the number of slides presented per group. Some of the possible tasks that the group might assign to its members include: reading synopsis, systems view of this topic, human impacts, ways this "sphere" might link with other "spheres", local ties or perspectives on the topic. The five five-minute presentations will be followed by a 30-minute in-class discussion of the readings, focused on the CZ as a system and human impacts on that system, thus a "think-pair-share" approach to metacognition will have been implemented. The final task of the class is for the instructor to present a 15-20 minute long description of systems and system modelling to prepare the students for their homework assignment - to read a basic guided tutorial on developing a generalized systems model and use this guide to develop a qualitative graphic model of the Critical Zone. The information covered in this class will help students to consider the interdisciplinary nature of Critical Zone science through the remainder of the semester and provide some basic background knowledge for learning in the modules entitled Methods in Critical Zone science and Human Impacts in the CZ.

Have the students hold questions until after all of the presentations have been completed. Have them take notes to remember questions that may arise. Once the final presentation has been made, open the class for a question and answer session, followed by a discussion of the information presented. The discussion should begin with a conversation of the various impacts and threats to the CZ and gradually should focus on the concepts associated with modelling the CZ including the appropriate human components.

The class will culminate with a 15-20 minute presentation on system modelling based on pages 1–19 in System Behavior and System Modeling with an assignment for the students to create a qualitative CZ system model.

  1. Suggestions:
    1. Skip the preface and any mentions of the Stella II computer model unless you intend to use it for this exercise. Also skip the exercises on pp. 6, 10, and 19.
      • Focus in on the text that describes: (1) the identification of system components and interactions of a system with other systems; (2) the identification of interactions between the components of a system; and, (3) qualitative and quantitative modeling.
      • Work through the exercise described beginning on p. 2 of "System Behavior and System Modeling," using an outline format, adding arrows, labels, etc., as you progress deeper into a model. Consider working through the example for atmospheric carbon dioxide as a "thought" exercise if you choose, but the point of the overall effort should be to outline a qualitative system definition of the Critical Zone.
      • Continue through the exercise by building a conceptual model with a stylized drawing (p. 5). The drawing should include reservoirs, flows, valves, branches, and interconnections where appropriate (p. 11).
      • Challenge students to plug numbers or equations into their conceptual model to create a quantitative model - perhaps for extra credit. For the highly motivated students in your class: have them assess potential perturbations or forcings and the future effect of these on the Critical Zone.
      • Highlight that this activity will position the students to better consider the many connections between the spheres and the various different processes to be studied through the remainder of the semester.

Optional Materials

For more resources directed at Developing Student Understanding of Complex Systems in the Geosciences visit the following url.


Students should enthusiastically and knowledgeably engage in an in-class discussion and demonstrate the capacity to repeat, translate, interpret, organize and debate information derived from their disciplinary-specific readings as well as information presented and described by the other disciplinary groups. The "Class Discussion Rubric" listed under course-level assessment is applicable here.

Students should also be able to organize and manage their knowledge of the Critical Zone to create a qualitatitve CZ system model.

References and Resources

  • Brantley, S. L., Goldhaber, M. B., & Ragnarsdottir, K. V. (2007). Crossing disciplines and scales to understand the critical zone. Elements, 3(5), 307–14.
  • Mann, C. (2008). Our Good Earth. National Geographic, 214(3), 80–106.

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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 »