Modeling the carbon cycle of the anthropocene
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.
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
Description and Teaching Materials
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
Teaching Notes and Tips
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
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.