# Modeling the carbon cycle of the anthropocene

#### Summary

This computer lab exercise allows students to investigate the consequences of fossil fuel burning and land use change on the amount of carbon dioxide in the atmosphere. Students work with a simple numerical model of the carbon cycle which is rendered in Excel, and conduct a set of different sensitivity tests with different amounts and rate of C additions, and then graph and discuss their results. In the recommended approach, the model is provided to students without the biosphere and in class the formulas to integrate this module are typed into Excel simultaneously by instructor and students, helping students understand how the larger model is set up.

## Learning Goals

Students recognize the redistribution of fossil fuel carbon between the ocean, biosphere, and atmosphere, and distinguish the consequences of rapid vs slow rates of addition of fossil fuel CO2 and the reasons for this difference.

Students become familiar with the use of formulas in Excel and working with a large (300 rows, 20 columns) worksheet and gain competence in graphical representation of multiple scenarios. Students learn to appreciate the power and limitations of numerical models of complex cycles, the concept of inverse and forward models and sensitivity tests.

Finally, students learn the importance of quantitative checks of "reasonable" hypotheses in inverse modeling (ie that a reasonable hypothesis, may be "reasonable" but still not quantitatively sufficient to explain observations).

Students become familiar with the use of formulas in Excel and working with a large (300 rows, 20 columns) worksheet and gain competence in graphical representation of multiple scenarios. Students learn to appreciate the power and limitations of numerical models of complex cycles, the concept of inverse and forward models and sensitivity tests.

Finally, students learn the importance of quantitative checks of "reasonable" hypotheses in inverse modeling (ie that a reasonable hypothesis, may be "reasonable" but still not quantitatively sufficient to explain observations).

## Context for Use

The exercise is best suited for upper level courses for Geoscience, Environmental Science, or Oceanography classes or first year Masters class exercise. The exercise works best for a laboratory session of 2-3 hours, followed by independent work by students to complete the exercise and an oral or written presentation of main results by students. Depending on student level, a second scheduled lab work period could be employed. Each student or pair of students requires access to computer with Excel, and the optimal group size is 12-15 students (with more students it is very difficult to attend to individual Excel troubleshooting questions without excessive wait times). Ideally the instructor computer has the ability to project to screen to illustrate formulas initially.

## Description and Teaching Materials

Content with which students should be familiar before this activity:

Before this activity, students should be familiar with the surface earth carbon cycle and the main reactions which convert C between CO2 and biologically fixed carbon (photosynthesis-respiration CO2+H2O <-> CH2O + O2), between CO2 and the ocean (CO2+H2O+CO3-2 <-> 2HCO3-). The treatment in Kump, Kasting and Crane (The Earth System) is especially well suited to the exercise. For the second part of the exercise, it is useful for students to understand the significance of the carbonate ion in the surface ocean for absorbing atmospheric CO2, and recognize that the carbonate ion in the surface ocean is limited, and when "consumed" by this reaction it is renewed at a slow rate by mixing of deep ocean water into the surface (and at a much much slower rate by rivers, not incorporated in the model).

In preparation I deliberately avoid discussing the role of changes in land use in the carbon cycle so that this arises from discovery during the lab exercise.

Material needed by students

Before this activity, students should be familiar with the surface earth carbon cycle and the main reactions which convert C between CO2 and biologically fixed carbon (photosynthesis-respiration CO2+H2O <-> CH2O + O2), between CO2 and the ocean (CO2+H2O+CO3-2 <-> 2HCO3-). The treatment in Kump, Kasting and Crane (The Earth System) is especially well suited to the exercise. For the second part of the exercise, it is useful for students to understand the significance of the carbonate ion in the surface ocean for absorbing atmospheric CO2, and recognize that the carbonate ion in the surface ocean is limited, and when "consumed" by this reaction it is renewed at a slow rate by mixing of deep ocean water into the surface (and at a much much slower rate by rivers, not incorporated in the model).

In preparation I deliberately avoid discussing the role of changes in land use in the carbon cycle so that this arises from discovery during the lab exercise.

Material needed by students

- Instruction/question sheet (Microsoft Word 56kB Dec13 12)
- Excel file (Excel 222kB Dec13 12) and access to computer with MS Excel.

## Teaching Notes and Tips

Additional information for instructors is given in the:

- Annotated copy of instruction/question sheet (Microsoft Word 61kB Dec13 12)
- Worked version of the Excel model (Excel 1.3MB Dec13 12)

## Assessment

Students may be required to present orally the 4-5 key graphs for the exercise with a discussion/explanation. Alternatively or in addition, students may submit the graphs/tables and a 1 page summary of the results from the model.

I find it useful to have all students submit to me (upload) their graphs a few days before a final written summary is due. I use those to compile a composite powerpoint of 8-10 graphs (all with correct results) and in class we comment on the strengths of presentation in these and ways to improve them. In this way, all students have a chance to check that their calculations are correct and if needed complete revisions before turning in the final version for grading.

I find it useful to have all students submit to me (upload) their graphs a few days before a final written summary is due. I use those to compile a composite powerpoint of 8-10 graphs (all with correct results) and in class we comment on the strengths of presentation in these and ways to improve them. In this way, all students have a chance to check that their calculations are correct and if needed complete revisions before turning in the final version for grading.

## References and Resources

Kump, Kasting, and Crane. The Earth System. Prentice Hall (2009 3rd edition, 1999, 1st edition).

Orr, V. and 26 others. Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature 427, p 681-683, 2005.

Zachos, J. and 11 others. Rapid Acidification of the Ocean During the Paleocene-Eocene Thermal Maximum. Science 308, 1611-1614, 2005.

Orr, V. and 26 others. Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature 427, p 681-683, 2005.

Zachos, J. and 11 others. Rapid Acidification of the Ocean During the Paleocene-Eocene Thermal Maximum. Science 308, 1611-1614, 2005.